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

examples/external_eventloop/src/main.rs

@@ -1,8 +1,9 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![expect(rustdoc::missing_crate_level_docs, clippy::unwrap_used)] // it's an example
 
+use core::cell::Cell;
 use eframe::{UserEvent, egui};
-use std::{cell::Cell, rc::Rc};
+use std::rc::Rc;
 use winit::event_loop::{ControlFlow, EventLoop};
 
 fn main() -> eframe::Result {

examples/external_eventloop_async/src/app.rs

@@ -1,6 +1,7 @@
 #![expect(clippy::unwrap_used)] // It's an example
 
-use std::{cell::Cell, io, os::fd::AsRawFd as _, rc::Rc, time::Duration};
+use core::{cell::Cell, time::Duration};
+use std::{io, os::fd::AsRawFd as _, rc::Rc};
 
 use tokio::task::LocalSet;
 use winit::event_loop::{ControlFlow, EventLoop};

examples/file_dialog/src/main.rs

@@ -83,8 +83,8 @@ impl eframe::App for MyApp {
 
 /// Preview hovering files:
 fn preview_files_being_dropped(ctx: &egui::Context) {
+    use core::fmt::Write as _;
     use egui::{Align2, Color32, Id, LayerId, Order, TextStyle};
-    use std::fmt::Write as _;
 
     if !ctx.input(|i| i.raw.hovered_files.is_empty()) {
         let text = ctx.input(|i| {

examples/hello_world_par/src/main.rs

@@ -106,10 +106,10 @@ impl MyApp {
     }
 }
 
-impl std::ops::Drop for MyApp {
+impl core::ops::Drop for MyApp {
     fn drop(&mut self) {
         for (handle, show_tx) in self.threads.drain(..) {
-            std::mem::drop(show_tx);
+            core::mem::drop(show_tx);
             handle.join().unwrap();
         }
     }

examples/multiple_viewports/src/main.rs

@@ -1,10 +1,8 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![expect(rustdoc::missing_crate_level_docs)] // it's an example
 
-use std::sync::{
-    Arc,
-    atomic::{AtomicBool, Ordering},
-};
+use core::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
 
 use eframe::egui;
 

examples/puffin_profiler/src/main.rs

@@ -1,10 +1,8 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![expect(rustdoc::missing_crate_level_docs)] // it's an example
 
-use std::sync::{
-    Arc,
-    atomic::{AtomicBool, Ordering},
-};
+use core::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
 
 use eframe::egui;
 
@@ -88,7 +86,7 @@ impl eframe::App for MyApp {
                 .clicked()
             {
                 puffin::profile_scope!("long_sleep");
-                std::thread::sleep(std::time::Duration::from_millis(50));
+                std::thread::sleep(core::time::Duration::from_millis(50));
             }
 
             ui.checkbox(
@@ -172,7 +170,7 @@ 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}");

examples/serial_windows/src/main.rs

@@ -19,7 +19,7 @@ fn main() -> eframe::Result {
         Box::new(|_cc| Ok(Box::new(MyApp { has_next: true }))),
     )?;
 
-    std::thread::sleep(std::time::Duration::from_secs(2));
+    std::thread::sleep(core::time::Duration::from_secs(2));
 
     log::info!("Starting second window…");
     eframe::run_native(
@@ -28,7 +28,7 @@ fn main() -> eframe::Result {
         Box::new(|_cc| Ok(Box::new(MyApp { has_next: true }))),
     )?;
 
-    std::thread::sleep(std::time::Duration::from_secs(2));
+    std::thread::sleep(core::time::Duration::from_secs(2));
 
     log::info!("Starting third window…");
     eframe::run_native(

examples/user_attention/src/main.rs

@@ -4,7 +4,8 @@
 use eframe::{CreationContext, NativeOptions, egui};
 use egui::{Button, CentralPanel, UserAttentionType};
 
-use std::time::{Duration, SystemTime};
+use core::time::Duration;
+use std::time::SystemTime;
 
 fn main() -> eframe::Result {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).