Auto merge of rust-lang#55052 - newpavlov:patch-2, r=alexcrichton

bors · bors · commit c4371c854f5c · 2018-11-13T01:08:13.000Z
Use read_unaligned instead of read in transmute_copy Closes: rust-lang#55044 This change could result in performance regression on non-x86 platforms. (but it also can fix some of UB which lurks in existing programs) An alternative would be to update `transmute_copy` documentation with alignment requirements.
diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
@@ -809,7 +809,7 @@ pub fn drop<T>(_x: T) { }
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
-    ptr::read(src as *const T as *const U)
+    ptr::read_unaligned(src as *const T as *const U)
 }
 
 /// Opaque type representing the discriminant of an enum.

Original file line number	Diff line number	Diff line change
`@@ -809,7 +809,7 @@ pub fn drop<T>(_x: T) { }`
`809`	`809`	`#[inline]`
`810`	`810`	`#[stable(feature = "rust1", since = "1.0.0")]`
`811`	`811`	`pub unsafe fn transmute_copy<T, U>(src: &T) -> U {`
`812`		`- ptr::read(src as const T as const U)`
	`812`	`+ ptr::read_unaligned(src as const T as const U)`
`813`	`813`	`}`
`814`	`814`
`815`	`815`	`/// Opaque type representing the discriminant of an enum.`