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Use String::with_capacity in format! #39356

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Feb 3, 2017
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3 changes: 2 additions & 1 deletion src/libcollections/fmt.rs
Original file line number Diff line number Diff line change
Expand Up @@ -539,7 +539,8 @@ use string;
/// [format!]: ../macro.format.html
#[stable(feature = "rust1", since = "1.0.0")]
pub fn format(args: Arguments) -> string::String {
let mut output = string::String::new();
let capacity = args.estimated_capacity();
let mut output = string::String::with_capacity(capacity);
let _ = output.write_fmt(args);
output
}
26 changes: 26 additions & 0 deletions src/libcore/fmt/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -265,6 +265,32 @@ impl<'a> Arguments<'a> {
args: args
}
}

/// Estimates the length of the formatted text.
///
/// This is intended to be used for setting initial `String` capacity
/// when using `format!`. Note: this is neither the lower nor upper bound.
#[doc(hidden)] #[inline]
#[unstable(feature = "fmt_internals", reason = "internal to format_args!",
issue = "0")]
pub fn estimated_capacity(&self) -> usize {
let pieces_length: usize = self.pieces.iter()
.map(|x| x.len()).sum();

if self.args.is_empty() {
pieces_length
} else if self.pieces[0] == "" && pieces_length < 16 {
// If the format string starts with an argument,
// don't preallocate anything, unless length
// of pieces is significant.
0
} else {
// There are some arguments, so any additional push
// will reallocate the string. To avoid that,
// we're "pre-doubling" the capacity here.
pieces_length.checked_mul(2).unwrap_or(0)
}
}
}

/// This structure represents a safely precompiled version of a format string
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10 changes: 10 additions & 0 deletions src/libcoretest/fmt/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -28,3 +28,13 @@ fn test_pointer_formats_data_pointer() {
assert_eq!(format!("{:p}", s), format!("{:p}", s.as_ptr()));
assert_eq!(format!("{:p}", b), format!("{:p}", b.as_ptr()));
}

#[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(), 0);
assert_eq!(format_args!("{}. 16-bytes piece", "World").estimated_capacity(), 32);
}
1 change: 1 addition & 0 deletions src/libcoretest/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,7 @@
#![feature(ordering_chaining)]
#![feature(ptr_unaligned)]
#![feature(move_cell)]
#![feature(fmt_internals)]

extern crate core;
extern crate test;
Expand Down