Enable the clippy::std_instead_of_core lint (#8394)

Prefer `core::` over `std::` where either work

* Part of https://github.com/emilk/egui/issues/5735

crates/epaint/src/shapes/bezier_shape.rs

@@ -1,6 +1,6 @@
 #![expect(clippy::many_single_char_names)]
 
-use std::ops::Range;
+use core::ops::Range;
 
 use crate::{Color32, PathShape, PathStroke, Shape};
 use emath::{Pos2, Rect, RectTransform, fast_midpoint};
@@ -256,8 +256,8 @@ impl CubicBezierShape {
         let theta = (-q / (2.0 * r)).acos() / 3.0;
 
         let t1 = 2.0 * r.cbrt() * theta.cos() + h;
-        let t2 = 2.0 * r.cbrt() * (theta + 120.0 * std::f32::consts::PI / 180.0).cos() + h;
-        let t3 = 2.0 * r.cbrt() * (theta + 240.0 * std::f32::consts::PI / 180.0).cos() + h;
+        let t2 = 2.0 * r.cbrt() * (theta + 120.0 * core::f32::consts::PI / 180.0).cos() + h;
+        let t3 = 2.0 * r.cbrt() * (theta + 240.0 * core::f32::consts::PI / 180.0).cos() + h;
 
         if t1 > epsilon && t1 < 1.0 - epsilon {
             return Some(t1);

crates/epaint/src/shapes/paint_callback.rs

@@ -1,4 +1,5 @@
-use std::{any::Any, sync::Arc};
+use core::any::Any;
+use std::sync::Arc;
 
 use crate::*;
 
@@ -32,7 +33,7 @@ fn test_viewport_rounding() {
         let left = Rect::from_min_max(pos2(0.0, 0.0), pos2(100.0, 100.0)).with_max_x(x);
         let right = Rect::from_min_max(pos2(0.0, 0.0), pos2(100.0, 100.0)).with_min_x(x);
 
-        for pixels_per_point in [0.618, 1.0, std::f32::consts::PI] {
+        for pixels_per_point in [0.618, 1.0, core::f32::consts::PI] {
             let left = ViewportInPixels::from_points(&left, pixels_per_point, [100, 100]);
             let right = ViewportInPixels::from_points(&right, pixels_per_point, [100, 100]);
             assert_eq!(left.left_px + left.width_px, right.left_px);
@@ -81,15 +82,15 @@ pub struct PaintCallback {
     pub callback: Arc<dyn Any + Send + Sync>,
 }
 
-impl std::fmt::Debug for PaintCallback {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for PaintCallback {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         f.debug_struct("CustomShape")
             .field("rect", &self.rect)
             .finish_non_exhaustive()
     }
 }
 
-impl std::cmp::PartialEq for PaintCallback {
+impl core::cmp::PartialEq for PaintCallback {
     fn eq(&self, other: &Self) -> bool {
         self.rect.eq(&other.rect) && Arc::ptr_eq(&self.callback, &other.callback)
     }

crates/epaint/src/shapes/rect_shape.rs

@@ -62,12 +62,12 @@ pub struct RectShape {
 #[test]
 fn rect_shape_size() {
     assert_eq!(
-        std::mem::size_of::<RectShape>(),
+        core::mem::size_of::<RectShape>(),
         56,
         "RectShape changed size! If it shrank - good! Update this test. If it grew - bad! Try to find a way to avoid it."
     );
     assert!(
-        std::mem::size_of::<RectShape>() <= 64,
+        core::mem::size_of::<RectShape>() <= 64,
         "RectShape is getting way too big!"
     );
 }

crates/epaint/src/shapes/shape.rs

@@ -73,12 +73,12 @@ pub enum Shape {
 #[test]
 fn shape_size() {
     assert_eq!(
-        std::mem::size_of::<Shape>(),
+        core::mem::size_of::<Shape>(),
         64,
         "Shape changed size! If it shrank - good! Update this test. If it grew - bad! Try to find a way to avoid it."
     );
     assert!(
-        std::mem::size_of::<Shape>() <= 64,
+        core::mem::size_of::<Shape>() <= 64,
         "Shape is getting way too big!"
     );
 }

crates/epaint/src/shapes/text_shape.rs

@@ -201,7 +201,7 @@ mod tests {
 
         // 90 degree rotation
         if let Shape::Text(ts) = &mut t {
-            ts.angle = std::f32::consts::PI / 2.0;
+            ts.angle = core::f32::consts::PI / 2.0;
         }
 
         let size_rot = t.visual_bounding_rect().size();