Use new type Estring to avoid cloning &'static str

`ui.label("static string")` is a very common use case,
and currently egui clones the string in these cases.

This PR introduces a new type:

``` rust
pub enum Estring {
    Static(&'static str),
    Owned(Arc<str>),
}
```

which is used everywhere text is needed, with
`impl Into<Estring>` in the API for e.g. `ui.label`.

This reduces the number of copies drastically and speeds up
the benchmark demo_with_tessellate__realistic by 17%.

This hurts the ergonomics of egui a bit, and this is a breaking change.

For instance, this used to work:

``` rust
fn my_label(ui: &mut egui::Ui, text: &str) {
    ui.label(text);
}
```

This must now either be changed to

``` rust
fn my_label(ui: &mut egui::Ui, text: &str) {
    ui.label(text.to_string());
}
```

(or the argument must be changed to either
`text: &'static str` or `text: String`)

egui/src/containers/collapsing_header.rs

@@ -152,7 +152,7 @@ impl CollapsingHeader {
     /// If the label is unique and static this is fine,
     /// but if it changes or there are several `CollapsingHeader` with the same title
     /// you need to provide a unique id source with [`Self::id_source`].
-    pub fn new(label: impl ToString) -> Self {
+    pub fn new(label: impl Into<Estring>) -> Self {
         let label = Label::new(label).wrap(false);
         let id_source = Id::new(label.text());
         Self {

egui/src/containers/combo_box.rs

@@ -23,7 +23,7 @@ use epaint::Shape;
 pub struct ComboBox {
     id_source: Id,
     label: Option<Label>,
-    selected_text: String,
+    selected_text: Estring,
     width: Option<f32>,
 }
 
@@ -56,9 +56,8 @@ impl ComboBox {
     }
 
     /// What we show as the currently selected value
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn selected_text(mut self, selected_text: impl Into<Estring>) -> Self {
-    pub fn selected_text(mut self, selected_text: impl ToString) -> Self {
+        self.selected_text = selected_text.into();
-        self.selected_text = selected_text.to_string();
         self
     }
 
@@ -143,11 +142,10 @@ impl ComboBox {
     }
 }
 
-#[allow(clippy::needless_pass_by_value)]
 fn combo_box<R>(
     ui: &mut Ui,
     button_id: Id,
-    selected: impl ToString,
+    selected: impl Into<Estring>,
     menu_contents: impl FnOnce(&mut Ui) -> R,
 ) -> InnerResponse<Option<R>> {
     let popup_id = button_id.with("popup");
@@ -158,9 +156,9 @@ fn combo_box<R>(
         let full_minimum_width = ui.spacing().slider_width;
         let icon_size = Vec2::splat(ui.spacing().icon_width);
 
-        let galley =
+        let galley = ui
-            ui.fonts()
+            .fonts()
-                .layout_delayed_color(selected.to_string(), TextStyle::Button, f32::INFINITY);
+            .layout_delayed_color(selected, TextStyle::Button, f32::INFINITY);
 
         let width = galley.size.x + ui.spacing().item_spacing.x + icon_size.x;
         let width = width.at_least(full_minimum_width);

egui/src/containers/popup.rs

@@ -194,7 +194,7 @@ fn show_tooltip_at_avoid<R>(
 ///     egui::show_tooltip_text(ui.ctx(), egui::Id::new("my_tooltip"), "Helpful text");
 /// }
 /// ```
-pub fn show_tooltip_text(ctx: &CtxRef, id: Id, text: impl ToString) -> Option<()> {
+pub fn show_tooltip_text(ctx: &CtxRef, id: Id, text: impl Into<Estring>) -> Option<()> {
     show_tooltip(ctx, id, |ui| {
         ui.add(crate::widgets::Label::new(text));
     })

egui/src/containers/window.rs

@@ -37,9 +37,8 @@ impl<'open> Window<'open> {
     /// The window title is used as a unique [`Id`] and must be unique, and should not change.
     /// This is true even if you disable the title bar with `.title_bar(false)`.
     /// If you need a changing title, you must call `window.id(…)` with a fixed id.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(title: impl Into<Estring>) -> Self {
-    pub fn new(title: impl ToString) -> Self {
+        let title = title.into();
-        let title = title.to_string();
         let area = Area::new(&title);
         let title_label = Label::new(title).text_style(TextStyle::Heading).wrap(false);
         Self {

egui/src/layout.rs

@@ -1,4 +1,4 @@
-use crate::{egui_assert, emath::*, Align};
+use crate::{egui_assert, emath::*, Align, Estring};
 use std::f32::INFINITY;
 
 // ----------------------------------------------------------------------------
@@ -737,7 +737,7 @@ impl Layout {
         painter: &crate::Painter,
         region: &Region,
         stroke: epaint::Stroke,
-        text: impl ToString,
+        text: impl Into<Estring>,
     ) {
         let cursor = region.cursor;
         let next_pos = self.next_widget_position(region);

egui/src/lib.rs

@@ -352,7 +352,7 @@ pub use emath as math; // historical reasons
 pub use emath::{lerp, pos2, remap, remap_clamp, vec2, Align, Align2, NumExt, Pos2, Rect, Vec2};
 pub use epaint::{
     color, mutex,
-    text::{FontDefinitions, FontFamily, TextStyle},
+    text::{Estring, FontDefinitions, FontFamily, TextStyle},
     ClippedMesh, Color32, Rgba, Shape, Stroke, Texture, TextureId,
 };
 
@@ -378,7 +378,9 @@ pub use {
 };
 
 pub mod text {
-    pub use epaint::text::{Galley, LayoutJob, LayoutSection, TextFormat, TAB_SIZE};
+    pub use epaint::text::{
+        Galley, LayoutJob, LayoutJobBuilder, LayoutSection, TextFormat, TAB_SIZE,
+    };
 }
 
 // ----------------------------------------------------------------------------

egui/src/menu.rs

@@ -64,7 +64,7 @@ pub fn bar<R>(ui: &mut Ui, add_contents: impl FnOnce(&mut Ui) -> R) -> InnerResp
 /// Returns `None` if the menu is not open.
 pub fn menu<R>(
     ui: &mut Ui,
-    title: impl ToString,
+    title: impl Into<Estring>,
     add_contents: impl FnOnce(&mut Ui) -> R,
 ) -> Option<R> {
     menu_impl(ui, title, Box::new(add_contents))
@@ -104,13 +104,12 @@ pub(crate) fn menu_ui<'c, R>(
     })
 }
 
-#[allow(clippy::needless_pass_by_value)]
 fn menu_impl<'c, R>(
     ui: &mut Ui,
-    title: impl ToString,
+    title: impl Into<Estring>,
     add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
 ) -> Option<R> {
-    let title = title.to_string();
+    let title = title.into();
     let bar_id = ui.id();
     let menu_id = bar_id.with(&title);
 

egui/src/painter.rs

@@ -5,7 +5,7 @@ use crate::{
 };
 use epaint::{
     mutex::Mutex,
-    text::{Fonts, Galley, TextStyle},
+    text::{Estring, Fonts, Galley, TextStyle},
     Shape, Stroke,
 };
 
@@ -193,17 +193,10 @@ impl Painter {
 
 /// ## Debug painting
 impl Painter {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn debug_rect(&mut self, rect: Rect, color: Color32, text: impl Into<Estring>) {
-    pub fn debug_rect(&mut self, rect: Rect, color: Color32, text: impl ToString) {
         self.rect_stroke(rect, 0.0, (1.0, color));
         let text_style = TextStyle::Monospace;
-        self.text(
+        self.text(rect.min, Align2::LEFT_TOP, text.into(), text_style, color);
-            rect.min,
-            Align2::LEFT_TOP,
-            text.to_string(),
-            text_style,
-            color,
-        );
     }
 
     pub fn error(&self, pos: Pos2, text: impl std::fmt::Display) -> Rect {
@@ -211,15 +204,14 @@ impl Painter {
     }
 
     /// text with a background
-    #[allow(clippy::needless_pass_by_value)]
     pub fn debug_text(
         &self,
         pos: Pos2,
         anchor: Align2,
         color: Color32,
-        text: impl ToString,
+        text: impl Into<Estring>,
     ) -> Rect {
-        let galley = self.layout_no_wrap(text.to_string(), TextStyle::Monospace, color);
+        let galley = self.layout_no_wrap(text, TextStyle::Monospace, color);
         let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size));
         let frame_rect = rect.expand(2.0);
         self.add(Shape::Rect {
@@ -332,16 +324,15 @@ impl Painter {
     /// [`Self::layout`] or [`Self::layout_no_wrap`].
     ///
     /// Returns where the text ended up.
-    #[allow(clippy::needless_pass_by_value)]
     pub fn text(
         &self,
         pos: Pos2,
         anchor: Align2,
-        text: impl ToString,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         text_color: Color32,
     ) -> Rect {
-        let galley = self.layout_no_wrap(text.to_string(), text_style, text_color);
+        let galley = self.layout_no_wrap(text, text_style, text_color);
         let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size));
         self.galley(rect.min, galley);
         rect
@@ -353,7 +344,7 @@ impl Painter {
     #[inline(always)]
     pub fn layout(
         &self,
-        text: String,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         color: crate::Color32,
         wrap_width: f32,
@@ -367,7 +358,7 @@ impl Painter {
     #[inline(always)]
     pub fn layout_no_wrap(
         &self,
-        text: String,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         color: crate::Color32,
     ) -> std::sync::Arc<Galley> {

egui/src/placer.rs

@@ -262,7 +262,7 @@ impl Placer {
 }
 
 impl Placer {
-    pub(crate) fn debug_paint_cursor(&self, painter: &crate::Painter, text: impl ToString) {
+    pub(crate) fn debug_paint_cursor(&self, painter: &crate::Painter, text: impl Into<Estring>) {
         let stroke = Stroke::new(1.0, Color32::DEBUG_COLOR);
 
         if let Some(grid) = &self.grid {

egui/src/response.rs

@@ -1,8 +1,7 @@
 use crate::{
     emath::{lerp, Align, Pos2, Rect, Vec2},
-    CursorIcon, PointerButton, NUM_POINTER_BUTTONS,
+    CtxRef, CursorIcon, Estring, Id, LayerId, PointerButton, Sense, Ui, NUM_POINTER_BUTTONS,
 };
-use crate::{CtxRef, Id, LayerId, Sense, Ui};
 
 // ----------------------------------------------------------------------------
 
@@ -373,14 +372,14 @@ impl Response {
     /// For that, use [`Self::on_disabled_hover_text`] instead.
     ///
     /// If you call this multiple times the tooltips will stack underneath the previous ones.
-    pub fn on_hover_text(self, text: impl ToString) -> Self {
+    pub fn on_hover_text(self, text: impl Into<Estring>) -> Self {
         self.on_hover_ui(|ui| {
             ui.add(crate::widgets::Label::new(text));
         })
     }
 
     /// Show this text when hovering if the widget is disabled.
-    pub fn on_disabled_hover_text(self, text: impl ToString) -> Self {
+    pub fn on_disabled_hover_text(self, text: impl Into<Estring>) -> Self {
         self.on_disabled_hover_ui(|ui| {
             ui.add(crate::widgets::Label::new(text));
         })

egui/src/style.rs

@@ -919,7 +919,7 @@ impl DebugOptions {
 fn slider_vec2<'a>(
     value: &'a mut Vec2,
     range: std::ops::RangeInclusive<f32>,
-    text: &'a str,
+    text: &'static str,
 ) -> impl Widget + 'a {
     move |ui: &mut crate::Ui| {
         ui.horizontal(|ui| {

egui/src/ui.rs

@@ -979,7 +979,7 @@ impl Ui {
     /// Shortcut for `add(Hyperlink::new(url))`
     ///
     /// See also [`Hyperlink`].
-    pub fn hyperlink(&mut self, url: impl ToString) -> Response {
+    pub fn hyperlink(&mut self, url: impl Into<Estring>) -> Response {
         Hyperlink::new(url).ui(self)
     }
 
@@ -991,7 +991,7 @@ impl Ui {
     /// ```
     ///
     /// See also [`Hyperlink`].
-    pub fn hyperlink_to(&mut self, label: impl ToString, url: impl ToString) -> Response {
+    pub fn hyperlink_to(&mut self, label: impl Into<Estring>, url: impl Into<Estring>) -> Response {
         Hyperlink::new(url).text(label).ui(self)
     }
 
@@ -1031,7 +1031,7 @@ impl Ui {
     /// See also [`Button`].
     #[must_use = "You should check if the user clicked this with `if ui.button(…).clicked() { … } "]
     #[inline(always)]
-    pub fn button(&mut self, text: impl ToString) -> Response {
+    pub fn button(&mut self, text: impl Into<Estring>) -> Response {
         Button::new(text).ui(self)
     }
 
@@ -1041,19 +1041,19 @@ impl Ui {
     ///
     /// Shortcut for `add(Button::new(text).small())`
     #[must_use = "You should check if the user clicked this with `if ui.small_button(…).clicked() { … } "]
-    pub fn small_button(&mut self, text: impl ToString) -> Response {
+    pub fn small_button(&mut self, text: impl Into<Estring>) -> Response {
         Button::new(text).small().ui(self)
     }
 
     /// Show a checkbox.
-    pub fn checkbox(&mut self, checked: &mut bool, text: impl ToString) -> Response {
+    pub fn checkbox(&mut self, checked: &mut bool, text: impl Into<Estring>) -> Response {
         Checkbox::new(checked, text).ui(self)
     }
 
     /// Show a [`RadioButton`].
     /// Often you want to use [`Self::radio_value`] instead.
     #[must_use = "You should check if the user clicked this with `if ui.radio(…).clicked() { … } "]
-    pub fn radio(&mut self, selected: bool, text: impl ToString) -> Response {
+    pub fn radio(&mut self, selected: bool, text: impl Into<Estring>) -> Response {
         RadioButton::new(selected, text).ui(self)
     }
 
@@ -1078,7 +1078,7 @@ impl Ui {
         &mut self,
         current_value: &mut Value,
         selected_value: Value,
-        text: impl ToString,
+        text: impl Into<Estring>,
     ) -> Response {
         let mut response = self.radio(*current_value == selected_value, text);
         if response.clicked() {
@@ -1092,7 +1092,7 @@ impl Ui {
     ///
     /// See also [`SelectableLabel`].
     #[must_use = "You should check if the user clicked this with `if ui.selectable_label(…).clicked() { … } "]
-    pub fn selectable_label(&mut self, checked: bool, text: impl ToString) -> Response {
+    pub fn selectable_label(&mut self, checked: bool, text: impl Into<Estring>) -> Response {
         SelectableLabel::new(checked, text).ui(self)
     }
 
@@ -1106,7 +1106,7 @@ impl Ui {
         &mut self,
         current_value: &mut Value,
         selected_value: Value,
-        text: impl ToString,
+        text: impl Into<Estring>,
     ) -> Response {
         let mut response = self.selectable_label(*current_value == selected_value, text);
         if response.clicked() {
@@ -1303,7 +1303,7 @@ impl Ui {
     /// A [`CollapsingHeader`] that starts out collapsed.
     pub fn collapsing<R>(
         &mut self,
-        heading: impl ToString,
+        heading: impl Into<Estring>,
         add_contents: impl FnOnce(&mut Ui) -> R,
     ) -> CollapsingResponse<R> {
         CollapsingHeader::new(heading).show(self, add_contents)
@@ -1644,7 +1644,7 @@ impl Ui {
 
     /// Shows the given text where the next widget is to be placed
     /// if when [`Context::set_debug_on_hover`] has been turned on and the mouse is hovering the Ui.
-    pub fn trace_location(&self, text: impl ToString) {
+    pub fn trace_location(&self, text: impl Into<Estring>) {
         let rect = self.max_rect();
         if self.style().debug.debug_on_hover && self.rect_contains_pointer(rect) {
             self.placer

egui/src/widgets/button.rs

@@ -22,7 +22,7 @@ fn select<T>(b: bool, if_true: T, if_false: T) -> T {
 /// ```
 #[must_use = "You should put this widget in an ui with `ui.add(widget);`"]
 pub struct Button {
-    text: String,
+    text: Estring,
     text_color: Option<Color32>,
     text_style: Option<TextStyle>,
     /// None means default for interact
@@ -36,10 +36,9 @@ pub struct Button {
 }
 
 impl Button {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(text: impl Into<Estring>) -> Self {
-    pub fn new(text: impl ToString) -> Self {
         Self {
-            text: text.to_string(),
+            text: text.into(),
             text_color: None,
             text_style: None,
             fill: None,
@@ -235,17 +234,16 @@ impl Widget for Button {
 #[derive(Debug)]
 pub struct Checkbox<'a> {
     checked: &'a mut bool,
-    text: String,
+    text: Estring,
     text_color: Option<Color32>,
     text_style: Option<TextStyle>,
 }
 
 impl<'a> Checkbox<'a> {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(checked: &'a mut bool, text: impl Into<Estring>) -> Self {
-    pub fn new(checked: &'a mut bool, text: impl ToString) -> Self {
         Checkbox {
             checked,
-            text: text.to_string(),
+            text: text.into(),
             text_color: None,
             text_style: None,
         }
@@ -360,17 +358,16 @@ impl<'a> Widget for Checkbox<'a> {
 #[derive(Debug)]
 pub struct RadioButton {
     checked: bool,
-    text: String,
+    text: Estring,
     text_color: Option<Color32>,
     text_style: Option<TextStyle>,
 }
 
 impl RadioButton {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(checked: bool, text: impl Into<Estring>) -> Self {
-    pub fn new(checked: bool, text: impl ToString) -> Self {
         Self {
             checked,
-            text: text.to_string(),
+            text: text.into(),
             text_color: None,
             text_style: None,
         }

egui/src/widgets/drag_value.rs

@@ -1,5 +1,3 @@
-#![allow(clippy::needless_pass_by_value)] // False positives with `impl ToString`
-
 use std::ops::RangeInclusive;
 
 use crate::*;
@@ -50,8 +48,8 @@ fn set(get_set_value: &mut GetSetValue<'_>, value: f64) {
 pub struct DragValue<'a> {
     get_set_value: GetSetValue<'a>,
     speed: f64,
-    prefix: String,
+    prefix: Estring,
-    suffix: String,
+    suffix: Estring,
     clamp_range: RangeInclusive<f64>,
     min_decimals: usize,
     max_decimals: Option<usize>,
@@ -100,14 +98,14 @@ impl<'a> DragValue<'a> {
     }
 
     /// Show a prefix before the number, e.g. "x: "
-    pub fn prefix(mut self, prefix: impl ToString) -> Self {
+    pub fn prefix(mut self, prefix: impl Into<Estring>) -> Self {
-        self.prefix = prefix.to_string();
+        self.prefix = prefix.into();
         self
     }
 
     /// Add a suffix to the number, this can be e.g. a unit ("°" or " m")
-    pub fn suffix(mut self, suffix: impl ToString) -> Self {
+    pub fn suffix(mut self, suffix: impl Into<Estring>) -> Self {
-        self.suffix = suffix.to_string();
+        self.suffix = suffix.into();
         self
     }
 

egui/src/widgets/hyperlink.rs

@@ -11,32 +11,29 @@ use crate::*;
 /// ```
 #[must_use = "You should put this widget in an ui with `ui.add(widget);`"]
 pub struct Hyperlink {
-    url: String,
+    url: Estring,
     label: Label,
 }
 
 impl Hyperlink {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(url: impl Into<Estring>) -> Self {
-    pub fn new(url: impl ToString) -> Self {
+        let url = url.into();
-        let url = url.to_string();
         Self {
             url: url.clone(),
             label: Label::new(url).sense(Sense::click()),
         }
     }
 
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn from_label_and_url(label: impl Into<Label>, url: impl Into<Estring>) -> Self {
-    pub fn from_label_and_url(label: impl Into<Label>, url: impl ToString) -> Self {
         Self {
-            url: url.to_string(),
+            url: url.into(),
             label: label.into(),
         }
     }
 
     /// Show some other text than the url
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn text(mut self, text: impl Into<Estring>) -> Self {
-    pub fn text(mut self, text: impl ToString) -> Self {
+        self.label.text = text.into();
-        self.label.text = text.to_string();
         self
     }
 
@@ -63,13 +60,13 @@ impl Widget for Hyperlink {
         if response.clicked() {
             let modifiers = ui.ctx().input().modifiers;
             ui.ctx().output().open_url = Some(crate::output::OpenUrl {
-                url: url.clone(),
+                url: url.to_string(),
                 new_tab: modifiers.any(),
             });
         }
         if response.middle_clicked() {
             ui.ctx().output().open_url = Some(crate::output::OpenUrl {
-                url: url.clone(),
+                url: url.to_string(),
                 new_tab: true,
             });
         }

egui/src/widgets/label.rs

@@ -16,7 +16,7 @@ use std::sync::Arc;
 #[must_use = "You should put this widget in an ui with `ui.add(widget);`"]
 pub struct Label {
     // TODO: not pub
-    pub(crate) text: String,
+    pub(crate) text: Estring,
     pub(crate) wrap: Option<bool>,
     pub(crate) text_style: Option<TextStyle>,
     pub(crate) background_color: Color32,
@@ -32,10 +32,9 @@ pub struct Label {
 }
 
 impl Label {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(text: impl Into<Estring>) -> Self {
-    pub fn new(text: impl ToString) -> Self {
         Self {
-            text: text.to_string(),
+            text: text.into(),
             wrap: None,
             text_style: None,
             background_color: Color32::TRANSPARENT,
@@ -51,7 +50,7 @@ impl Label {
         }
     }
 
-    pub fn text(&self) -> &str {
+    pub(crate) fn text(&self) -> &Estring {
         &self.text
     }
 
@@ -211,7 +210,7 @@ impl Label {
         };
 
         let job = LayoutJob {
-            text: self.text.clone(), // TODO: avoid clone
+            text: self.text.clone(),
             sections: vec![LayoutSection {
                 leading_space,
                 byte_range: 0..self.text.len(),
@@ -358,8 +357,20 @@ impl Widget for Label {
     }
 }
 
-impl From<&str> for Label {
+impl From<&'static str> for Label {
-    fn from(s: &str) -> Label {
+    fn from(s: &'static str) -> Label {
+        Label::new(s)
+    }
+}
+
+impl From<&Estring> for Label {
+    fn from(s: &Estring) -> Label {
+        Label::new(s)
+    }
+}
+
+impl From<Estring> for Label {
+    fn from(s: Estring) -> Label {
         Label::new(s)
     }
 }

egui/src/widgets/mod.rs

@@ -92,7 +92,7 @@ pub fn reset_button<T: Default + PartialEq>(ui: &mut Ui, value: &mut T) {
 
 // ----------------------------------------------------------------------------
 
-pub fn stroke_ui(ui: &mut crate::Ui, stroke: &mut epaint::Stroke, text: &str) {
+pub fn stroke_ui(ui: &mut crate::Ui, stroke: &mut epaint::Stroke, text: &'static str) {
     let epaint::Stroke { width, color } = stroke;
     ui.horizontal(|ui| {
         ui.add(DragValue::new(width).speed(0.1).clamp_range(0.0..=5.0))
@@ -108,7 +108,7 @@ pub fn stroke_ui(ui: &mut crate::Ui, stroke: &mut epaint::Stroke, text: &str) {
     });
 }
 
-pub(crate) fn shadow_ui(ui: &mut Ui, shadow: &mut epaint::Shadow, text: &str) {
+pub(crate) fn shadow_ui(ui: &mut Ui, shadow: &mut epaint::Shadow, text: &'static str) {
     let epaint::Shadow { extrusion, color } = shadow;
     ui.horizontal(|ui| {
         ui.label(text);

egui/src/widgets/plot/items.rs

@@ -126,7 +126,7 @@ impl ToString for LineStyle {
 pub struct HLine {
     pub(super) y: f64,
     pub(super) stroke: Stroke,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
     pub(super) style: LineStyle,
 }
@@ -136,7 +136,7 @@ impl HLine {
         Self {
             y: y.into(),
             stroke: Stroke::new(1.0, Color32::TRANSPARENT),
-            name: String::default(),
+            name: Estring::default(),
             highlight: false,
             style: LineStyle::Solid,
         }
@@ -178,9 +178,8 @@ impl HLine {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -204,7 +203,7 @@ impl PlotItem for HLine {
     fn initialize(&mut self, _x_range: RangeInclusive<f64>) {}
 
     fn name(&self) -> &str {
-        &self.name
+        self.name.as_str()
     }
 
     fn color(&self) -> Color32 {
@@ -236,7 +235,7 @@ impl PlotItem for HLine {
 pub struct VLine {
     pub(super) x: f64,
     pub(super) stroke: Stroke,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
     pub(super) style: LineStyle,
 }
@@ -246,7 +245,7 @@ impl VLine {
         Self {
             x: x.into(),
             stroke: Stroke::new(1.0, Color32::TRANSPARENT),
-            name: String::default(),
+            name: Estring::default(),
             highlight: false,
             style: LineStyle::Solid,
         }
@@ -288,9 +287,8 @@ impl VLine {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -314,7 +312,7 @@ impl PlotItem for VLine {
     fn initialize(&mut self, _x_range: RangeInclusive<f64>) {}
 
     fn name(&self) -> &str {
-        &self.name
+        self.name.as_str()
     }
 
     fn color(&self) -> Color32 {
@@ -542,7 +540,7 @@ impl MarkerShape {
 pub struct Line {
     pub(super) series: Values,
     pub(super) stroke: Stroke,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
     pub(super) fill: Option<f32>,
     pub(super) style: LineStyle,
@@ -602,9 +600,8 @@ impl Line {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -709,7 +706,7 @@ impl PlotItem for Line {
 pub struct Polygon {
     pub(super) series: Values,
     pub(super) stroke: Stroke,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
     pub(super) fill_alpha: f32,
     pub(super) style: LineStyle,
@@ -770,9 +767,8 @@ impl Polygon {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -842,20 +838,19 @@ impl PlotItem for Polygon {
 
 /// Text inside the plot.
 pub struct Text {
-    pub(super) text: String,
+    pub(super) text: Estring,
     pub(super) style: TextStyle,
     pub(super) position: Value,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
     pub(super) color: Color32,
     pub(super) anchor: Align2,
 }
 
 impl Text {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(position: Value, text: impl Into<Estring>) -> Self {
-    pub fn new(position: Value, text: impl ToString) -> Self {
         Self {
-            text: text.to_string(),
+            text: text.into(),
             style: TextStyle::Small,
             position,
             name: Default::default(),
@@ -895,9 +890,8 @@ impl Text {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -965,7 +959,7 @@ pub struct Points {
     pub(super) filled: bool,
     /// The maximum extent of the marker from its center.
     pub(super) radius: f32,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
     pub(super) stems: Option<f32>,
 }
@@ -1026,9 +1020,8 @@ impl Points {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -1221,7 +1214,7 @@ pub struct Arrows {
     pub(super) origins: Values,
     pub(super) tips: Values,
     pub(super) color: Color32,
-    pub(super) name: String,
+    pub(super) name: Estring,
     pub(super) highlight: bool,
 }
 
@@ -1254,9 +1247,8 @@ impl Arrows {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }
@@ -1340,7 +1332,7 @@ pub struct PlotImage {
     pub(super) bg_fill: Color32,
     pub(super) tint: Color32,
     pub(super) highlight: bool,
-    pub(super) name: String,
+    pub(super) name: Estring,
 }
 
 impl PlotImage {
@@ -1388,9 +1380,8 @@ impl PlotImage {
     ///
     /// Multiple plot items may share the same name, in which case they will also share an entry in
     /// the legend.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn name(mut self, name: impl Into<Estring>) -> Self {
-    pub fn name(mut self, name: impl ToString) -> Self {
+        self.name = name.into();
-        self.name = name.to_string();
         self
     }
 }

egui/src/widgets/progress_bar.rs

@@ -1,7 +1,7 @@
 use crate::*;
 
 enum ProgressBarText {
-    Custom(String),
+    Custom(Estring),
     Percentage,
 }
 
@@ -31,9 +31,8 @@ impl ProgressBar {
     }
 
     /// A custom text to display on the progress bar.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn text(mut self, text: impl Into<Estring>) -> Self {
-    pub fn text(mut self, text: impl ToString) -> Self {
+        self.text = Some(ProgressBarText::Custom(text.into()));
-        self.text = Some(ProgressBarText::Custom(text.to_string()));
         self
     }
 
@@ -125,9 +124,9 @@ impl Widget for ProgressBar {
         }
 
         if let Some(text_kind) = text {
-            let text = match text_kind {
+            let text: Estring = match text_kind {
                 ProgressBarText::Custom(string) => string,
-                ProgressBarText::Percentage => format!("{}%", (progress * 100.0) as usize),
+                ProgressBarText::Percentage => format!("{}%", (progress * 100.0) as usize).into(),
             };
             ui.painter().sub_region(outer_rect).text(
                 outer_rect.left_center() + vec2(ui.spacing().item_spacing.x, 0.0),

egui/src/widgets/selected_label.rs

@@ -24,16 +24,15 @@ use crate::*;
 #[derive(Debug)]
 pub struct SelectableLabel {
     selected: bool,
-    text: String,
+    text: Estring,
     text_style: Option<TextStyle>,
 }
 
 impl SelectableLabel {
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn new(selected: bool, text: impl Into<Estring>) -> Self {
-    pub fn new(selected: bool, text: impl ToString) -> Self {
         Self {
             selected,
-            text: text.to_string(),
+            text: text.into(),
             text_style: None,
         }
     }

egui/src/widgets/slider.rs

@@ -1,5 +1,3 @@
-#![allow(clippy::needless_pass_by_value)] // False positives with `impl ToString`
-
 use crate::{widgets::Label, *};
 use std::ops::RangeInclusive;
 
@@ -58,9 +56,9 @@ pub struct Slider<'a> {
     clamp_to_range: bool,
     smart_aim: bool,
     show_value: bool,
-    prefix: String,
+    prefix: Estring,
-    suffix: String,
+    suffix: Estring,
-    text: String,
+    text: Estring,
     text_color: Option<Color32>,
     min_decimals: usize,
     max_decimals: Option<usize>,
@@ -115,20 +113,20 @@ impl<'a> Slider<'a> {
     }
 
     /// Show a prefix before the number, e.g. "x: "
-    pub fn prefix(mut self, prefix: impl ToString) -> Self {
+    pub fn prefix(mut self, prefix: impl Into<Estring>) -> Self {
-        self.prefix = prefix.to_string();
+        self.prefix = prefix.into();
         self
     }
 
     /// Add a suffix to the number, this can be e.g. a unit ("°" or " m")
-    pub fn suffix(mut self, suffix: impl ToString) -> Self {
+    pub fn suffix(mut self, suffix: impl Into<Estring>) -> Self {
-        self.suffix = suffix.to_string();
+        self.suffix = suffix.into();
         self
     }
 
     /// Show a text next to the slider (e.g. explaining what the slider controls).
-    pub fn text(mut self, text: impl ToString) -> Self {
+    pub fn text(mut self, text: impl Into<Estring>) -> Self {
-        self.text = text.to_string();
+        self.text = text.into();
         self
     }
 

egui/src/widgets/text_edit.rs

@@ -225,7 +225,7 @@ impl TextBuffer for String {
 #[must_use = "You should put this widget in an ui with `ui.add(widget);`"]
 pub struct TextEdit<'t, S: TextBuffer = String> {
     text: &'t mut S,
-    hint_text: String,
+    hint_text: Estring,
     id: Option<Id>,
     id_source: Option<Id>,
     text_style: Option<TextStyle>,
@@ -302,9 +302,8 @@ impl<'t, S: TextBuffer> TextEdit<'t, S> {
     }
 
     /// Show a faint hint text when the text field is empty.
-    #[allow(clippy::needless_pass_by_value)]
+    pub fn hint_text(mut self, hint_text: impl Into<Estring>) -> Self {
-    pub fn hint_text(mut self, hint_text: impl ToString) -> Self {
+        self.hint_text = hint_text.into();
-        self.hint_text = hint_text.to_string();
         self
     }
 

egui_demo_lib/benches/benchmark.rs

@@ -57,7 +57,7 @@ pub fn criterion_benchmark(c: &mut Criterion) {
                 ui.label("the quick brown fox jumps over the lazy dog");
             })
         });
-        c.bench_function("label format!", |b| {
+        c.bench_function("label String", |b| {
             b.iter(|| {
                 ui.label("the quick brown fox jumps over the lazy dog".to_owned());
             })
@@ -77,20 +77,16 @@ pub fn criterion_benchmark(c: &mut Criterion) {
             b.iter(|| {
                 use egui::epaint::text::{layout, LayoutJob};
 
-                let job = LayoutJob::simple(
+                let job =
-                    LOREM_IPSUM_LONG.to_owned(),
+                    LayoutJob::simple(LOREM_IPSUM_LONG, egui::TextStyle::Body, color, wrap_width);
-                    egui::TextStyle::Body,
-                    color,
-                    wrap_width,
-                );
                 layout(&fonts, job.into())
             })
         });
         c.bench_function("text_layout_cached", |b| {
-            b.iter(|| fonts.layout(LOREM_IPSUM_LONG.to_owned(), text_style, color, wrap_width))
+            b.iter(|| fonts.layout(LOREM_IPSUM_LONG, text_style, color, wrap_width))
         });
 
-        let galley = fonts.layout(LOREM_IPSUM_LONG.to_owned(), text_style, color, wrap_width);
+        let galley = fonts.layout(LOREM_IPSUM_LONG, text_style, color, wrap_width);
         let mut tessellator = egui::epaint::Tessellator::from_options(Default::default());
         let mut mesh = egui::epaint::Mesh::default();
         c.bench_function("tessellate_text", |b| {

egui_demo_lib/src/apps/color_test.rs

@@ -262,7 +262,7 @@ impl ColorTest {
         &mut self,
         ui: &mut Ui,
         tex_allocator: &mut Option<&mut dyn epi::TextureAllocator>,
-        label: &str,
+        label: &'static str,
         bg_fill: Color32,
         gradient: &Gradient,
     ) {
@@ -284,7 +284,13 @@ impl ColorTest {
         }
     }
 
-    fn vertex_gradient(&mut self, ui: &mut Ui, label: &str, bg_fill: Color32, gradient: &Gradient) {
+    fn vertex_gradient(
+        &mut self,
+        ui: &mut Ui,
+        label: &'static str,
+        bg_fill: Color32,
+        gradient: &Gradient,
+    ) {
         if !self.vertex_gradients {
             return;
         }

egui_demo_lib/src/apps/demo/code_editor.rs

@@ -117,7 +117,7 @@ impl MemoizedSyntaxHighlighter {
                 .highlight(is_dark_mode, code, language)
                 .unwrap_or_else(|| {
                     LayoutJob::simple(
-                        code.into(),
+                        code.to_owned(),
                         egui::TextStyle::Monospace,
                         if is_dark_mode {
                             egui::Color32::LIGHT_GRAY
@@ -172,7 +172,7 @@ impl Highligher {
         use egui::text::{LayoutSection, TextFormat};
 
         let mut job = LayoutJob {
-            text: text.into(),
+            text: text.to_owned().into(),
             ..Default::default()
         };
 
@@ -226,7 +226,7 @@ impl Highligher {
     fn highlight(&self, is_dark_mode: bool, mut text: &str, _language: &str) -> Option<LayoutJob> {
         // Extremely simple syntax highlighter for when we compile without syntect
 
-        use egui::text::TextFormat;
+        use egui::text::{LayoutJobBuilder, TextFormat};
         use egui::Color32;
         let monospace = egui::TextStyle::Monospace;
 
@@ -265,7 +265,7 @@ impl Highligher {
             },
         );
 
-        let mut job = LayoutJob::default();
+        let mut job = LayoutJobBuilder::default();
 
         while !text.is_empty() {
             if text.starts_with("//") {
@@ -308,7 +308,7 @@ impl Highligher {
             }
         }
 
-        Some(job)
+        Some(job.build())
     }
 }
 

egui_demo_lib/src/apps/demo/font_book.rs

@@ -90,10 +90,12 @@ impl super::View for FontBook {
 
                 for (&chr, name) in named_chars {
                     if filter.is_empty() || name.contains(filter) || *filter == chr.to_string() {
-                        let button = egui::Button::new(chr).text_style(text_style).frame(false);
+                        let button = egui::Button::new(chr.to_string())
+                            .text_style(text_style)
+                            .frame(false);
 
                         let tooltip_ui = |ui: &mut egui::Ui| {
-                            ui.add(egui::Label::new(chr).text_style(text_style));
+                            ui.add(egui::Label::new(chr.to_string()).text_style(text_style));
                             ui.label(format!("{}\nU+{:X}\n\nClick to copy", name, chr as u32));
                         };
 

egui_demo_lib/src/apps/demo/misc_demo_window.rs

@@ -329,7 +329,7 @@ impl Tree {
         )
     }
     pub fn ui(&mut self, ui: &mut Ui) -> Action {
-        self.1.ui(ui, 0, "root", &mut self.0)
+        self.1.ui(ui, 0, "root".to_owned(), &mut self.0)
     }
 }
 
@@ -342,16 +342,16 @@ impl SubTree {
         &mut self,
         ui: &mut Ui,
         depth: usize,
-        name: &str,
+        name: String,
         selected_name: &mut String,
     ) -> Action {
-        let response = CollapsingHeader::new(name)
+        let response = CollapsingHeader::new(name.clone())
             .default_open(depth < 1)
             .selectable(true)
             .selected(selected_name.as_str() == name)
-            .show(ui, |ui| self.children_ui(ui, name, depth, selected_name));
+            .show(ui, |ui| self.children_ui(ui, &name, depth, selected_name));
         if response.header_response.clicked() {
-            *selected_name = name.to_string();
+            *selected_name = name;
         }
         response.body_returned.unwrap_or(Action::Keep)
     }
@@ -379,7 +379,7 @@ impl SubTree {
                 if tree.ui(
                     ui,
                     depth + 1,
-                    &format!("{}/{}", parent_name, i),
+                    format!("{}/{}", parent_name, i),
                     selected_name,
                 ) == Action::Keep
                 {
@@ -401,9 +401,9 @@ impl SubTree {
 // ----------------------------------------------------------------------------
 
 fn text_layout_ui(ui: &mut egui::Ui) {
-    use egui::epaint::text::{LayoutJob, TextFormat};
+    use egui::epaint::text::{LayoutJobBuilder, TextFormat};
 
-    let mut job = LayoutJob::default();
+    let mut job = LayoutJobBuilder::default();
 
     let first_row_indentation = 10.0;
 
@@ -565,6 +565,7 @@ fn text_layout_ui(ui: &mut egui::Ui) {
         },
     );
 
+    let mut job = job.build();
     job.wrap_width = ui.available_width();
 
     let galley = ui.fonts().layout_job(job);

egui_demo_lib/src/apps/demo/widget_gallery.rs

@@ -241,7 +241,7 @@ fn example_plot() -> egui::plot::Plot {
         .data_aspect(1.0)
 }
 
-fn doc_link_label<'a>(title: &'a str, search_term: &'a str) -> impl egui::Widget + 'a {
+fn doc_link_label(title: &'static str, search_term: &'static str) -> impl egui::Widget {
     let label = format!("{}:", title);
     let url = format!("https://docs.rs/egui?search={}", search_term);
     move |ui: &mut egui::Ui| {

egui_demo_lib/src/apps/http_app.rs

@@ -132,10 +132,12 @@ impl epi::App for HttpApp {
                     }
                     Err(error) => {
                         // This should only happen if the fetch API isn't available or something similar.
-                        ui.add(
+                        let error = if error.is_empty() {
-                            egui::Label::new(if error.is_empty() { "Error" } else { error })
+                            "Error".to_owned()
-                                .text_color(egui::Color32::RED),
+                        } else {
-                        );
+                            error.clone()
+                        };
+                        ui.add(egui::Label::new(error).text_color(egui::Color32::RED));
                     }
                 }
             }
@@ -325,7 +327,7 @@ impl ColoredText {
         use egui::text::{LayoutJob, LayoutSection, TextFormat};
 
         let mut job = LayoutJob {
-            text: text.into(),
+            text: text.to_owned().into(),
             ..Default::default()
         };
 

egui_demo_lib/src/easy_mark/easy_mark_viewer.rs

@@ -43,7 +43,7 @@ pub fn item_ui(ui: &mut Ui, item: easy_mark::Item<'_>) {
         }
         easy_mark::Item::Hyperlink(style, text, url) => {
             let label = label_from_style(text, &style);
-            ui.add(Hyperlink::from_label_and_url(label, url));
+            ui.add(Hyperlink::from_label_and_url(label, url.to_owned()));
         }
 
         easy_mark::Item::Separator => {
@@ -75,7 +75,7 @@ pub fn item_ui(ui: &mut Ui, item: easy_mark::Item<'_>) {
         }
         easy_mark::Item::CodeBlock(_language, code) => {
             let where_to_put_background = ui.painter().add(Shape::Noop);
-            let mut rect = ui.monospace(code).rect;
+            let mut rect = ui.monospace(code.to_owned()).rect;
             rect = rect.expand(1.0); // looks better
             rect.max.x = ui.max_rect().max.x;
             let code_bg_color = ui.visuals().code_bg_color;
@@ -102,7 +102,7 @@ fn label_from_style(text: &str, style: &easy_mark::Style) -> Label {
 
     let small = small || raised; // Raised text is also smaller
 
-    let mut label = Label::new(text);
+    let mut label = Label::new(text.to_owned());
     if heading && !small {
         label = label.heading().strong();
     }

egui_demo_lib/src/wrap_app.rs

@@ -121,7 +121,7 @@ impl WrapApp {
 
             for (anchor, app) in self.apps.iter_mut() {
                 if ui
-                    .selectable_label(self.selected_anchor == anchor, app.name())
+                    .selectable_label(self.selected_anchor == anchor, app.name().to_owned())
                     .clicked()
                 {
                     self.selected_anchor = anchor.to_owned();

epaint/src/shape.rs

@@ -1,5 +1,5 @@
 use crate::{
-    text::{Fonts, Galley, TextStyle},
+    text::{Estring, Fonts, Galley, TextStyle},
     Color32, Mesh, Stroke,
 };
 use emath::*;
@@ -166,16 +166,15 @@ impl Shape {
         }
     }
 
-    #[allow(clippy::needless_pass_by_value)]
     pub fn text(
         fonts: &Fonts,
         pos: Pos2,
         anchor: Align2,
-        text: impl ToString,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         color: Color32,
     ) -> Self {
-        let galley = fonts.layout_no_wrap(text.to_string(), text_style, color);
+        let galley = fonts.layout_no_wrap(text.into(), text_style, color);
         let rect = anchor.anchor_rect(Rect::from_min_size(pos, galley.size));
         Self::galley(rect.min, galley)
     }

epaint/src/text/estring.rs

@@ -0,0 +1,173 @@
+use std::sync::Arc;
+
+/// An immutable string, backed by either `&'static str` or `Arc<String>`.
+///
+/// Wherever you see `impl Into<Estring>` pass either a `String` or
+/// a `&'static str` (a `"string literal"`).
+///
+/// Estring provides fast `Clone`.
+#[derive(Clone)]
+pub enum Estring {
+    Static(&'static str),
+    Owned(Arc<str>),
+}
+
+impl Estring {
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        match self {
+            Self::Static(s) => s,
+            Self::Owned(s) => s,
+        }
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.as_str().is_empty()
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.as_str().len()
+    }
+}
+
+impl Default for Estring {
+    fn default() -> Self {
+        Self::Static("")
+    }
+}
+
+impl std::convert::AsRef<str> for Estring {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl std::borrow::Borrow<str> for Estring {
+    fn borrow(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl std::hash::Hash for Estring {
+    #[inline]
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.as_str().hash(state)
+    }
+}
+
+impl PartialEq for Estring {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl std::cmp::Eq for Estring {}
+
+impl std::cmp::PartialOrd for Estring {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl std::cmp::Ord for Estring {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.as_str().cmp(other.as_str())
+    }
+}
+
+impl std::fmt::Display for Estring {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.as_str().fmt(f)
+    }
+}
+
+impl std::fmt::Debug for Estring {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.as_str().fmt(f)
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+impl std::convert::From<&'static str> for Estring {
+    fn from(s: &'static str) -> Self {
+        Self::Static(s)
+    }
+}
+
+impl std::convert::From<String> for Estring {
+    fn from(s: String) -> Self {
+        Self::Owned(s.into())
+    }
+}
+
+impl std::convert::From<&String> for Estring {
+    fn from(s: &String) -> Self {
+        Self::Owned(s.clone().into())
+    }
+}
+
+impl std::convert::From<&Estring> for Estring {
+    fn from(s: &Estring) -> Self {
+        s.clone()
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+impl std::ops::Index<std::ops::Range<usize>> for Estring {
+    type Output = str;
+
+    #[inline]
+    fn index(&self, index: std::ops::Range<usize>) -> &str {
+        self.as_str().index(index)
+    }
+}
+
+impl std::ops::Index<std::ops::RangeTo<usize>> for Estring {
+    type Output = str;
+
+    #[inline]
+    fn index(&self, index: std::ops::RangeTo<usize>) -> &str {
+        self.as_str().index(index)
+    }
+}
+
+impl std::ops::Index<std::ops::RangeFrom<usize>> for Estring {
+    type Output = str;
+
+    #[inline]
+    fn index(&self, index: std::ops::RangeFrom<usize>) -> &str {
+        self.as_str().index(index)
+    }
+}
+
+impl std::ops::Index<std::ops::RangeFull> for Estring {
+    type Output = str;
+
+    #[inline]
+    fn index(&self, index: std::ops::RangeFull) -> &str {
+        self.as_str().index(index)
+    }
+}
+
+impl std::ops::Index<std::ops::RangeInclusive<usize>> for Estring {
+    type Output = str;
+
+    #[inline]
+    fn index(&self, index: std::ops::RangeInclusive<usize>) -> &str {
+        self.as_str().index(index)
+    }
+}
+
+impl std::ops::Index<std::ops::RangeToInclusive<usize>> for Estring {
+    type Output = str;
+
+    #[inline]
+    fn index(&self, index: std::ops::RangeToInclusive<usize>) -> &str {
+        self.as_str().index(index)
+    }
+}

epaint/src/text/fonts.rs

@@ -10,7 +10,7 @@ use crate::{
     mutex::Mutex,
     text::{
         font::{Font, FontImpl},
-        Galley, LayoutJob,
+        Estring, Galley, LayoutJob,
     },
     Texture, TextureAtlas,
 };
@@ -330,7 +330,7 @@ impl Fonts {
     /// The implementation uses memoization so repeated calls are cheap.
     pub fn layout(
         &self,
-        text: String,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         color: crate::Color32,
         wrap_width: f32,
@@ -344,7 +344,7 @@ impl Fonts {
     /// The implementation uses memoization so repeated calls are cheap.
     pub fn layout_no_wrap(
         &self,
-        text: String,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         color: crate::Color32,
     ) -> Arc<Galley> {
@@ -357,7 +357,7 @@ impl Fonts {
     /// The implementation uses memoization so repeated calls are cheap.
     pub fn layout_delayed_color(
         &self,
-        text: String,
+        text: impl Into<Estring>,
         text_style: TextStyle,
         wrap_width: f32,
     ) -> Arc<Galley> {

epaint/src/text/mod.rs

@@ -1,6 +1,7 @@
 //! Everything related to text, fonts, text layout, cursors etc.
 
 pub mod cursor;
+mod estring;
 mod font;
 mod fonts;
 mod text_layout;
@@ -10,6 +11,7 @@ mod text_layout_types;
 pub const TAB_SIZE: usize = 4;
 
 pub use {
+    estring::Estring,
     fonts::{FontDefinitions, FontFamily, Fonts, TextStyle},
     text_layout::layout,
     text_layout_types::*,

epaint/src/text/text_layout_types.rs

@@ -1,7 +1,7 @@
 use std::ops::Range;
 use std::sync::Arc;
 
-use super::{cursor::*, font::UvRect};
+use super::{cursor::*, font::UvRect, Estring};
 use crate::{Color32, Mesh, Stroke, TextStyle};
 use emath::*;
 
@@ -13,7 +13,7 @@ use emath::*;
 #[derive(Clone, Debug)]
 pub struct LayoutJob {
     /// The complete text of this job, referenced by `LayoutSection`.
-    pub text: String, // TODO: Cow<'static, str>
+    pub text: Estring,
 
     /// The different section, which can have different fonts, colors, etc.
     pub sections: Vec<LayoutSection>,
@@ -53,7 +53,13 @@ impl Default for LayoutJob {
 impl LayoutJob {
     /// Break on `\n` and at the given wrap width.
     #[inline]
-    pub fn simple(text: String, text_style: TextStyle, color: Color32, wrap_width: f32) -> Self {
+    pub fn simple(
+        text: impl Into<Estring>,
+        text_style: TextStyle,
+        color: Color32,
+        wrap_width: f32,
+    ) -> Self {
+        let text = text.into();
         Self {
             sections: vec![LayoutSection {
                 leading_space: 0.0,
@@ -69,7 +75,12 @@ impl LayoutJob {
 
     /// Does not break on `\n`, but shows the replacement character instead.
     #[inline]
-    pub fn simple_singleline(text: String, text_style: TextStyle, color: Color32) -> Self {
+    pub fn simple_singleline(
+        text: impl Into<Estring>,
+        text_style: TextStyle,
+        color: Color32,
+    ) -> Self {
+        let text = text.into();
         Self {
             sections: vec![LayoutSection {
                 leading_space: 0.0,
@@ -87,18 +98,6 @@ impl LayoutJob {
     pub fn is_empty(&self) -> bool {
         self.sections.is_empty()
     }
-
-    /// Helper for adding a new section when building a `LayoutJob`.
-    pub fn append(&mut self, text: &str, leading_space: f32, format: TextFormat) {
-        let start = self.text.len();
-        self.text += text;
-        let byte_range = start..self.text.len();
-        self.sections.push(LayoutSection {
-            leading_space,
-            byte_range,
-            format,
-        });
-    }
 }
 
 impl std::hash::Hash for LayoutJob {
@@ -135,6 +134,37 @@ impl std::cmp::Eq for LayoutJob {}
 
 // ----------------------------------------------------------------------------
 
+/// Build a [`LayoutJob`] from many small pieces.
+#[derive(Clone, Debug, Default)]
+pub struct LayoutJobBuilder {
+    text: String,
+    sections: Vec<LayoutSection>,
+}
+
+impl LayoutJobBuilder {
+    /// Helper for adding a new section when building a `LayoutJob`.
+    pub fn append(&mut self, text: &str, leading_space: f32, format: TextFormat) {
+        let start = self.text.len();
+        self.text += text;
+        let byte_range = start..self.text.len();
+        self.sections.push(LayoutSection {
+            leading_space,
+            byte_range,
+            format,
+        });
+    }
+
+    pub fn build(self) -> LayoutJob {
+        LayoutJob {
+            text: self.text.into(),
+            sections: self.sections,
+            ..Default::default()
+        }
+    }
+}
+
+// ----------------------------------------------------------------------------
+
 #[derive(Clone, Debug)]
 pub struct LayoutSection {
     /// Can be used for first row indentation.
@@ -366,7 +396,7 @@ impl Galley {
 
     #[inline(always)]
     pub fn text(&self) -> &str {
-        &self.job.text
+        self.job.text.as_str()
     }
 }