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Rollup merge of #32456 - bluss:str-zero, r=alexcrichton

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Hardcode accepting 0 as a valid str char boundary

If we check explicitly for index == 0, that removes the need to read the
byte at index 0, so it avoids a trip to the string's memory, and it
optimizes out the slicing index' bounds check whenever it is (a constant) zero.
This commit is contained in:
Manish Goregaokar 2016-03-26 09:07:22 +05:30
commit 671027817c
1 changed files with 5 additions and 1 deletions
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6
src/libcore/str/mod.rs
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@ -1953,7 +1953,10 @@ impl StrExt for str {
#[inline] #[inline]
fn is_char_boundary(&self, index: usize) -> bool { fn is_char_boundary(&self, index: usize) -> bool {
if index == self.len() { return true; } // 0 and len are always ok.
// Test for 0 explicitly so that it can optimize out the check
// easily and skip reading string data for that case.
if index == 0 || index == self.len() { return true; }
match self.as_bytes().get(index) { match self.as_bytes().get(index) {
None => false, None => false,
Some(&b) => b < 128 || b >= 192, Some(&b) => b < 128 || b >= 192,
@ -2026,6 +2029,7 @@ impl StrExt for str {
self.find(pat) self.find(pat)
} }
#[inline]
fn split_at(&self, mid: usize) -> (&str, &str) { fn split_at(&self, mid: usize) -> (&str, &str) {
// is_char_boundary checks that the index is in [0, .len()] // is_char_boundary checks that the index is in [0, .len()]
if self.is_char_boundary(mid) { if self.is_char_boundary(mid) {