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/egui_demo_app/src/accessibility_inspector.rs

@@ -1,4 +1,4 @@
-use std::mem;
+use core::mem;
 
 use accesskit::{Action, ActionRequest};
 use accesskit_consumer::{FilterResult, Node, NodeId, Tree, TreeChangeHandler};
@@ -168,7 +168,7 @@ impl AccessibilityInspectorPlugin {
             ui.horizontal_wrapped(|ui| {
                 // Iterate through all possible actions via the `Action::n` helper.
                 let mut current_action = 0;
-                let all_actions = std::iter::from_fn(|| {
+                let all_actions = core::iter::from_fn(|| {
                     let action = Action::n(current_action);
                     current_action += 1;
                     action

crates/egui_demo_app/src/apps/custom3d_wgpu.rs

@@ -1,6 +1,6 @@
 #![expect(clippy::unwrap_used)] // TODO(emilk): avoid unwraps
 
-use std::num::NonZeroU64;
+use core::num::NonZeroU64;
 
 use eframe::{
     egui_wgpu::wgpu::util::DeviceExt as _,

crates/egui_demo_app/src/apps/fractal_clock.rs

@@ -1,10 +1,10 @@
+use core::f32::consts::TAU;
 use egui::{
     Color32, Painter, Pos2, Rect, Shape, Stroke, Ui, Vec2,
     containers::{CollapsingHeader, Frame},
     emath, pos2,
     widgets::Slider,
 };
-use std::f32::consts::TAU;
 
 #[derive(PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
@@ -202,7 +202,7 @@ impl FractalClock {
                 }
             }
 
-            std::mem::swap(&mut nodes, &mut new_nodes);
+            core::mem::swap(&mut nodes, &mut new_nodes);
         }
         self.line_count = shapes.len();
         painter.extend(shapes);

crates/egui_demo_app/src/backend_panel.rs

@@ -160,9 +160,9 @@ impl BackendPanel {
                     {
                         log::info!("Waiting 2s before requesting repaint…");
                         let ctx = ui.ctx().clone();
-                        call_after_delay(std::time::Duration::from_secs(2), move || {
+                        call_after_delay(core::time::Duration::from_secs(2), move || {
                             log::info!("Request a repaint in 3s…");
-                            ctx.request_repaint_after(std::time::Duration::from_secs(3));
+                            ctx.request_repaint_after(core::time::Duration::from_secs(3));
                         });
                     }
 
@@ -525,7 +525,7 @@ impl EguiWindows {
 // ----------------------------------------------------------------------------
 
 #[cfg(not(target_arch = "wasm32"))]
-fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + Send + 'static) {
+fn call_after_delay(delay: core::time::Duration, f: impl FnOnce() + Send + 'static) {
     std::thread::Builder::new()
         .name("call_after_delay".to_owned())
         .spawn(move || {
@@ -536,7 +536,7 @@ fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + Send + 'stati
 }
 
 #[cfg(target_arch = "wasm32")]
-fn call_after_delay(delay: std::time::Duration, f: impl FnOnce() + Send + 'static) {
+fn call_after_delay(delay: core::time::Duration, f: impl FnOnce() + Send + 'static) {
     #![expect(clippy::unwrap_used)]
 
     use wasm_bindgen::prelude::*;

crates/egui_demo_app/src/main.rs

@@ -38,7 +38,7 @@ fn main() {
         });
         for loud_crate in ["naga", "wgpu_core", "wgpu_hal"] {
             if !rust_log.contains(&format!("{loud_crate}=")) {
-                use std::fmt::Write as _;
+                use core::fmt::Write as _;
                 write!(rust_log, ",{loud_crate}=warn").ok();
             }
         }
@@ -103,7 +103,7 @@ fn start_puffin_server() {
             // We can store the server if we want, but in this case we just want
             // it to keep running. Dropping it closes the server, so let's not drop it!
             #[expect(clippy::mem_forget)]
-            std::mem::forget(puffin_server);
+            core::mem::forget(puffin_server);
         }
         Err(err) => {
             log::error!("Failed to start puffin server: {err}");

crates/egui_demo_app/src/wrap_app.rs

@@ -127,8 +127,8 @@ impl Anchor {
     }
 }
 
-impl std::fmt::Display for Anchor {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Display for Anchor {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         let mut name = format!("{self:?}");
         name.make_ascii_lowercase();
         f.write_str(&name)
@@ -473,8 +473,8 @@ impl WrapApp {
     }
 
     fn ui_file_drag_and_drop(&mut self, ctx: &egui::Context) {
+        use core::fmt::Write as _;
         use egui::{Align2, Color32, Id, LayerId, Order, TextStyle};
-        use std::fmt::Write as _;
 
         // Preview hovering files:
         if !ctx.input(|i| i.raw.hovered_files.is_empty()) {