Auto merge of #64456 - Centril:rollup-ytqdwaq, r=Centril

Rollup of 17 pull requests

Successful merges:

 - #63846 (Added table containing the system calls used by Instant and SystemTime.)
 - #64116 (Fix minor typo in docs.)
 - #64203 (A few cosmetic improvements to code & comments in liballoc and libcore)
 - #64302 (Shrink `ObligationCauseCode`)
 - #64372 (use randSecure and randABytes)
 - #64374 (Box `DiagnosticBuilder`.)
 - #64375 (Fast path for vec.clear/truncate )
 - #64378 (Fix inconsistent link formatting.)
 - #64384 (Trim rustc-workspace-hack)
 - #64393 ( declare EnvKey before use to fix build error)
 - #64420 (Inline `mark_neighbours_as_waiting_from`.)
 - #64422 (Remove raw string literal quotes from error index descriptions)
 - #64423 (Add self to .mailmap)
 - #64425 (typo fix)
 - #64431 (fn ptr is structural match)
 - #64435 (codegen: use "_N" (like for other locals) instead of "argN", for argument names.)
 - #64439 (fix #64430, confusing `owned_box` error message in no_std build)

Failed merges:

r? @ghost

1 files changed, 17 insertions, 13 deletions

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index d5dc2d4b868..c513658c842 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -685,21 +685,25 @@ impl<T> Vec<T> {
     /// [`drain`]: #method.drain
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn truncate(&mut self, len: usize) {
-        let current_len = self.len;
-        unsafe {
-            let mut ptr = self.as_mut_ptr().add(self.len);
-            // Set the final length at the end, keeping in mind that
-            // dropping an element might panic. Works around a missed
-            // optimization, as seen in the following issue:
-            // https://github.com/rust-lang/rust/issues/51802
-            let mut local_len = SetLenOnDrop::new(&mut self.len);
+        if mem::needs_drop::<T>() {
+            let current_len = self.len;
+            unsafe {
+                let mut ptr = self.as_mut_ptr().add(self.len);
+                // Set the final length at the end, keeping in mind that
+                // dropping an element might panic. Works around a missed
+                // optimization, as seen in the following issue:
+                // https://github.com/rust-lang/rust/issues/51802
+                let mut local_len = SetLenOnDrop::new(&mut self.len);
 
-            // drop any extra elements
-            for _ in len..current_len {
-                local_len.decrement_len(1);
-                ptr = ptr.offset(-1);
-                ptr::drop_in_place(ptr);
+                // drop any extra elements
+                for _ in len..current_len {
+                    local_len.decrement_len(1);
+                    ptr = ptr.offset(-1);
+                    ptr::drop_in_place(ptr);
+                }
             }
+        } else if len <= self.len {
+            self.len = len;
         }
     }