std: Mark Layout::repeat as #[inline]

This fixes an optimization regression by allowing LLVM to see through more
functions.

Closes #43272

src/liballoc/allocator.rs

@@ -207,6 +207,7 @@ impl Layout {
     /// of each element in the array.
     ///
     /// On arithmetic overflow, returns `None`.
+    #[inline]
     pub fn repeat(&self, n: usize) -> Option<(Self, usize)> {
         let padded_size = match self.size.checked_add(self.padding_needed_for(self.align)) {
             None => return None,

src/test/codegen/vec-optimizes-away.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+//
+// no-system-llvm
+// compile-flags: -O
+#![crate_type="lib"]
+
+#[no_mangle]
+pub fn sum_me() -> i32 {
+    // CHECK-LABEL: @sum_me
+    // CHECK-NEXT: {{^.*:$}}
+    // CHECK-NEXT: ret i32 6
+    vec![1, 2, 3].iter().sum::<i32>()
+}