Merge from rustc

2 files changed, 16 insertions, 9 deletions

diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs
index 5a10121bbbe..3751f2a2454 100644
--- a/library/alloc/src/raw_vec.rs
+++ b/library/alloc/src/raw_vec.rs
@@ -241,10 +241,15 @@ impl<T, A: Allocator> RawVec<T, A> {
         if T::IS_ZST || self.cap == 0 {
             None
         } else {
-            // We have an allocated chunk of memory, so we can bypass runtime
-            // checks to get our current layout.
+            // We could use Layout::array here which ensures the absence of isize and usize overflows
+            // and could hypothetically handle differences between stride and size, but this memory
+            // has already been allocated so we know it can't overflow and currently rust does not
+            // support such types. So we can do better by skipping some checks and avoid an unwrap.
+            let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
             unsafe {
-                let layout = Layout::array::<T>(self.cap).unwrap_unchecked();
+                let align = mem::align_of::<T>();
+                let size = mem::size_of::<T>().unchecked_mul(self.cap);
+                let layout = Layout::from_size_align_unchecked(size, align);
                 Some((self.ptr.cast().into(), layout))
             }
         }
@@ -426,11 +431,13 @@ impl<T, A: Allocator> RawVec<T, A> {
         assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
 
         let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
-
+        // See current_memory() why this assert is here
+        let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
         let ptr = unsafe {
             // `Layout::array` cannot overflow here because it would have
             // overflowed earlier when capacity was larger.
-            let new_layout = Layout::array::<T>(cap).unwrap_unchecked();
+            let new_size = mem::size_of::<T>().unchecked_mul(cap);
+            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
             self.alloc
                 .shrink(ptr, layout, new_layout)
                 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 75659188515..2b843647dd5 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -42,8 +42,6 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-#[cfg(not(no_global_oom_handling))]
-use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
 use core::error::Error;
 use core::fmt;
 use core::hash;
@@ -683,7 +681,7 @@ impl String {
         // This isn't done via collect::<Result<_, _>>() for performance reasons.
         // FIXME: the function can be simplified again when #48994 is closed.
         let mut ret = String::with_capacity(v.len());
-        for c in decode_utf16(v.iter().cloned()) {
+        for c in char::decode_utf16(v.iter().cloned()) {
             if let Ok(c) = c {
                 ret.push(c);
             } else {
@@ -722,7 +720,9 @@ impl String {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16_lossy(v: &[u16]) -> String {
-        decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
+        char::decode_utf16(v.iter().cloned())
+            .map(|r| r.unwrap_or(char::REPLACEMENT_CHARACTER))
+            .collect()
     }
 
     /// Decomposes a `String` into its raw components.