Rollup merge of rust-lang#57589 - scottmcm:vec-set_len-debug_assert, r=alexcrichton

Add a debug_assert to Vec::set_len

Following the precedent of rust-lang#52972, which found llogiq/bytecount#42.

(This may well make a test fail; let's see what Travis says.)

Author: Centril

src/liballoc/vec.rs

@@ -819,6 +819,8 @@ impl<T> Vec<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn set_len(&mut self, new_len: usize) {
+        debug_assert!(new_len <= self.capacity());
+
         self.len = new_len;
     }