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Avoid allocating when parsing \u{...} literals.

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`char_lit` uses an allocation in order to ignore '_' chars in \u{...}
literals. This patch changes it to not do that by processing the chars
more directly.

This improves various rustc-perf benchmark measurements by up to 6%,
particularly regex, futures, clap, coercions, hyper, and encoding.
This commit is contained in:
Nicholas Nethercote 2018-04-18 14:09:27 +10:00
parent 23561c6747
commit 9f145022ef
1 changed files with 10 additions and 2 deletions
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12
src/libsyntax/parse/mod.rs
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@ -271,8 +271,16 @@ pub fn char_lit(lit: &str, diag: Option<(Span, &Handler)>) -> (char, isize) {
'u' => {
assert_eq!(lit.as_bytes()[2], b'{');
let idx = lit.find('}').unwrap();
let s = &lit[3..idx].chars().filter(|&c| c != '_').collect::<String>();
let v = u32::from_str_radix(&s, 16).unwrap();
// All digits and '_' are ascii, so treat each byte as a char.
let mut v: u32 = 0;
for c in lit[3..idx].bytes() {
let c = char::from(c);
if c != '_' {
let x = c.to_digit(16).unwrap();
v = v.checked_mul(16).unwrap().checked_add(x).unwrap();
}
}
let c = char::from_u32(v).unwrap_or_else(|| {
if let Some((span, diag)) = diag {
let mut diag = diag.struct_span_err(span, "invalid unicode character escape");