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Adjust heuristics to better handle "{}..." format strings.

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This commit is contained in:
Michał Krasnoborski 2017-02-01 23:47:03 +01:00
parent e74b55b9da
commit 7403ee9d07
2 changed files with 15 additions and 10 deletions
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21
src/libcore/fmt/mod.rs
View file

@ -281,17 +281,20 @@ impl<'a> Arguments<'a> {
let pieces_length: W<usize> = self.pieces.iter()
.map(|x| W(x.len())).sum();
// If they are any arguments to format, the string will most likely
// double in size. So we're pre-doubling it here.
let multiplier = if self.args.is_empty() { W(1) } else { W(2) };
let capacity = multiplier * pieces_length;
if multiplier == W(2) && (W(1)..W(8)).contains(capacity) {
// Allocations smaller than 8 don't really make sense for String.
8
if self.args.is_empty() {
pieces_length.0
} else if self.pieces[0] == "" && pieces_length < W(16) {
// If the format string starts with an argument,
// don't preallocate anything, unless length
// of pieces is significant.
0
} else {
capacity.0
// There are some arguments, so any additional push
// will reallocate the string. To avoid that,
// we're "pre-doubling" the capacity here.
(pieces_length * W(2)).0
}
}
}

4
src/libcoretest/fmt/mod.rs
View file

@ -31,8 +31,10 @@ fn test_pointer_formats_data_pointer() {
#[test]
fn test_estimated_capacity() {
assert_eq!(format_args!("").estimated_capacity(), 0);
assert_eq!(format_args!("{}", "").estimated_capacity(), 0);
assert_eq!(format_args!("Hello").estimated_capacity(), 5);
assert_eq!(format_args!("Hello, {}!", "").estimated_capacity(), 16);
assert_eq!(format_args!("{}, hello!", "World").estimated_capacity(), 16);
assert_eq!(format_args!("{}, hello!", "World").estimated_capacity(), 0);
assert_eq!(format_args!("{}. 16-bytes piece", "World").estimated_capacity(), 32);
}