From 223600ac2c23715f350ceae7f315f5056aae6fc0 Mon Sep 17 00:00:00 2001 From: Kornel Date: Wed, 11 Sep 2019 11:46:53 +0100 Subject: [PATCH] Guarantee vec.clear/truncate is O(1) for trivial types --- src/liballoc/vec.rs | 30 +++++++++++++++++------------- 1 file changed, 17 insertions(+), 13 deletions(-) diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs index d5dc2d4b8688d..c513658c842e5 100644 --- a/src/liballoc/vec.rs +++ b/src/liballoc/vec.rs @@ -685,21 +685,25 @@ impl Vec { /// [`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::() { + 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; } }