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proc_macro: make TokenStream::from_streams pre-allocate its vector.

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This requires a pre-pass over the input streams. But that is cheap
compared to the quadratic blowup associated with reallocating the
accumulating vector on-the-fly.
This commit is contained in:
Felix S. Klock II 2019-01-30 15:12:41 +01:00
parent 5f60208ba1
commit 1a18336808
1 changed files with 7 additions and 1 deletions
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8
src/libsyntax/tokenstream.rs
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@ -255,7 +255,13 @@ impl TokenStream {
0 => TokenStream::empty(),
1 => streams.pop().unwrap(),
_ => {
let mut vec = vec![];
// rust-lang/rust#57735: pre-allocate vector to avoid
// quadratic blow-up due to on-the-fly reallocations.
let tree_count = streams.iter()
.map(|ts| match &ts.0 { None => 0, Some(s) => s.len() })
.sum();
let mut vec = Vec::with_capacity(tree_count);
for stream in streams {
match stream.0 {
None => {},