Rename the Sized widget to SetSize

It shadowed the marker trait, so a `?Sized` bound in any crate that imports
the prelude failed to resolve -- a compile error in someone else's code that
nothing here would have caught. Three files inside iris already imported
`std::marker::Sized` to get out from under it; they no longer need to.

`SetSize` rather than `FixedSize` because the size it sets need not be fixed:
`width(rest(2))` and `width(rel(0.5))` build the same widget. It pairs with
the `MaxSize` beside it -- one sets a length, the other caps it.

`tests/prelude_bounds.rs` is a compile-level guard: it fails to build if the
prelude shadows `Sized` again. The pad tab of the tabs example, which is what
uses `sized` and the flexible widths, renders pixel-identical.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/default/mod.rs

@@ -1,10 +1,6 @@
 use crate::prelude::*;
 use arboard::Clipboard;
-use std::{
-    marker::{PhantomData, Sized},
-    sync::Arc,
-    time::Instant,
-};
+use std::{marker::PhantomData, sync::Arc, time::Instant};
 use winit::{
     event::{Ime, WindowEvent},
     event_loop::{ActiveEventLoop, EventLoopProxy},

src/widget/position/mod.rs

@@ -4,7 +4,7 @@ mod max_size;
 mod offset;
 mod pad;
 mod scroll;
-mod sized;
+mod set_size;
 mod span;
 mod stack;
 
@@ -14,6 +14,6 @@ pub use max_size::*;
 pub use offset::*;
 pub use pad::*;
 pub use scroll::*;
-pub use sized::*;
+pub use set_size::*;
 pub use span::*;
 pub use stack::*;

src/widget/position/set_size.rs

@@ -1,12 +1,12 @@
 use crate::prelude::*;
 
-pub struct Sized {
+pub struct SetSize {
     pub inner: StrongWidget,
     pub x: Option<Len>,
     pub y: Option<Len>,
 }
 
-impl Sized {
+impl SetSize {
     fn apply_to_outer(&self, ctx: &mut SizeCtx) {
         if let Some(x) = self.x {
             ctx.outer.x.select_len(x.apply_rest());
@@ -17,7 +17,7 @@ impl Sized {
     }
 }
 
-impl Widget for Sized {
+impl Widget for SetSize {
     fn draw(&mut self, painter: &mut Painter) {
         painter.widget(&self.inner);
     }

src/widget/ptr.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use std::marker::{Sized, Unsize};
+use std::marker::Unsize;
 
 pub struct WidgetPtr {
     pub inner: Option<StrongWidget>,

src/widget/text/build.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use std::marker::{PhantomData, Sized};
+use std::marker::PhantomData;
 
 pub struct TextBuilder<State, O = TextOutput, H: WidgetOption<State> = ()> {
     pub content: String,

src/widget/trait_fns.rs

@@ -31,9 +31,9 @@ widget_trait! {
         }
     }
 
-    fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, Sized> {
+    fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, SetSize> {
         let size = size.into();
-        move |state| Sized {
+        move |state| SetSize {
             inner: self.add_strong(state),
             x: Some(size.x),
             y: Some(size.y),
@@ -58,18 +58,18 @@ widget_trait! {
         }
     }
 
-    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
+    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
         let len = len.into();
-        move |state| Sized {
+        move |state| SetSize {
             inner: self.add_strong(state),
             x: Some(len),
             y: None,
         }
     }
 
-    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
+    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
         let len = len.into();
-        move |state| Sized {
+        move |state| SetSize {
             inner: self.add_strong(state),
             x: None,
             y: Some(len),

tests/prelude_bounds.rs

@@ -0,0 +1,13 @@
+//! The prelude must not shadow names from the standard prelude. A widget
+//! called `Sized` meant every `?Sized` bound in a crate using iris resolved
+//! to the widget and failed to compile, which is an error in someone else's
+//! code that nothing here would have noticed.
+
+use iris::prelude::*;
+
+fn takes_unsized<T: ?Sized>(_: &T) {}
+
+#[test]
+fn the_prelude_leaves_sized_alone() {
+    takes_unsized("a `?Sized` bound resolves to the marker trait");
+}