Auto merge of #58081 - Centril:liballoc-2018, r=oli-obk

Transition liballoc to Rust 2018

This transitions liballoc to Rust 2018 edition and applies relevant idiom lints.
I also did a small bit of drive-by cleanup along the way.

r? @oli-obk

I started with liballoc since it seemed easiest. In particular, adding `edition = "2018"` to libcore gave me way too many errors due to stdsimd. Ideally we should be able to continue this crate-by-crate until all crates use 2018.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/liballoc/tests/vec.rs b/src/liballoc/tests/vec.rs
index 0fdcf34c783..89f2e0a046d 100644
--- a/src/liballoc/tests/vec.rs
+++ b/src/liballoc/tests/vec.rs
@@ -8,7 +8,7 @@ struct DropCounter<'a> {
     count: &'a mut u32,
 }
 
-impl<'a> Drop for DropCounter<'a> {
+impl Drop for DropCounter<'_> {
     fn drop(&mut self) {
         *self.count += 1;
     }
@@ -638,7 +638,7 @@ fn test_splice_unbounded() {
 fn test_splice_forget() {
     let mut v = vec![1, 2, 3, 4, 5];
     let a = [10, 11, 12];
-    ::std::mem::forget(v.splice(2..4, a.iter().cloned()));
+    std::mem::forget(v.splice(2..4, a.iter().cloned()));
     assert_eq!(v, &[1, 2]);
 }