Auto merge of #83245 - the8472:generalize-slice-fill, r=m-ou-se

Generalize and inline slice::fill specializations

This makes the memset specialization applicable to more types. And since the code now lives in a generic method it is also eligible for cross-crate inlining which  should fix #83235

Author: bors

library/core/benches/slice.rs

@@ -114,3 +114,16 @@ rotate!(rotate_16_usize_4, 16, |i| [i; 4]);
 rotate!(rotate_16_usize_5, 16, |i| [i; 5]);
 rotate!(rotate_64_usize_4, 64, |i| [i; 4]);
 rotate!(rotate_64_usize_5, 64, |i| [i; 5]);
+
+#[bench]
+fn fill_byte_sized(b: &mut Bencher) {
+    #[derive(Copy, Clone)]
+    struct NewType(u8);
+
+    let mut ary = [NewType(0); 1024];
+
+    b.iter(|| {
+        let slice = &mut ary[..];
+        black_box(slice.fill(black_box(NewType(42))));
+    });
+}

library/core/src/slice/specialize.rs

@@ -1,3 +1,4 @@
+use crate::mem::{size_of, transmute_copy};
 use crate::ptr::write_bytes;
 
 pub(super) trait SpecFill<T> {
@@ -17,42 +18,18 @@ impl<T: Clone> SpecFill<T> for [T] {
 }
 
 impl<T: Copy> SpecFill<T> for [T] {
-    default fn spec_fill(&mut self, value: T) {
-        for item in self.iter_mut() {
-            *item = value;
-        }
-    }
-}
-
-impl SpecFill<u8> for [u8] {
-    fn spec_fill(&mut self, value: u8) {
-        // SAFETY: this is slice of u8
-        unsafe {
-            let ptr = self.as_mut_ptr();
-            let len = self.len();
-            write_bytes(ptr, value, len);
-        }
-    }
-}
-
-impl SpecFill<i8> for [i8] {
-    fn spec_fill(&mut self, value: i8) {
-        // SAFETY: this is slice of i8
-        unsafe {
-            let ptr = self.as_mut_ptr();
-            let len = self.len();
-            write_bytes(ptr, value as u8, len);
-        }
-    }
-}
-
-impl SpecFill<bool> for [bool] {
-    fn spec_fill(&mut self, value: bool) {
-        // SAFETY: this is slice of bool
-        unsafe {
-            let ptr = self.as_mut_ptr();
-            let len = self.len();
-            write_bytes(ptr, value as u8, len);
+    fn spec_fill(&mut self, value: T) {
+        if size_of::<T>() == 1 {
+            // SAFETY: The size_of check above ensures that values are 1 byte wide, as required
+            // for the transmute and write_bytes
+            unsafe {
+                let value: u8 = transmute_copy(&value);
+                write_bytes(self.as_mut_ptr(), value, self.len());
+            }
+        } else {
+            for item in self.iter_mut() {
+                *item = value;
+            }
         }
     }
 }