From 7403ee9d07a1b096e9628871bd97e39f464c3aa5 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Micha=C5=82=20Krasnoborski?= Date: Wed, 1 Feb 2017 23:47:03 +0100 Subject: [PATCH] Adjust heuristics to better handle "{}..." format strings. --- src/libcore/fmt/mod.rs | 21 ++++++++++++--------- src/libcoretest/fmt/mod.rs | 4 +++- 2 files changed, 15 insertions(+), 10 deletions(-) diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs index ed9c5d3337f0..a870b6f88fb8 100644 --- a/src/libcore/fmt/mod.rs +++ b/src/libcore/fmt/mod.rs @@ -281,17 +281,20 @@ impl<'a> Arguments<'a> { let pieces_length: W = 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 } + } } diff --git a/src/libcoretest/fmt/mod.rs b/src/libcoretest/fmt/mod.rs index 71b3a440f7be..5d204c7d523d 100644 --- a/src/libcoretest/fmt/mod.rs +++ b/src/libcoretest/fmt/mod.rs @@ -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); }