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Fix macro parser quadratic complexity in small repeating groups
Observed in #51754, and more easily demonstrated with the following:
```rust
macro_rules! stress {
($($t:tt)+) => { };
}
fn main() {
stress!{
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
// ... 65536 copies of "a" total ...
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
}
}
```
which takes 50 seconds to compile prior to the fix and <1s after.
I hope this has a visible impact on the compile times for real code. (I think it is most likely to affect incremental TT munchers that deal with large inputs, though it depends on how they are written)
For a fuller description of the performance issue: https://github.com/rust-lang/rust/issues/51754#issuecomment-403242159
---
There is no test (yet) because I'm not sure how easily to measure this for regressions.
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For a fuller description of the performance issue fixed by this:
https://github.com/rust-lang/rust/issues/51754#issuecomment-403242159
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This reverts commit d6ba1b9b021c408fcad60ee52acf8af5e1b2eb00, reversing
changes made to 8de5353f75dcde04abe947e0560dc5edd861cf3a.
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Speed up the macro parser
These three commits reduce the number of allocations done by the macro parser, in some cases dramatically. For example, for a clean check builds of html5ever, the number of allocations is reduced by 40%.
Here are the rustc-benchmarks that are sped up by at least 1%.
```
html5ever-check
avg: -6.6% min: -10.3% max: -4.1%
html5ever
avg: -5.2% min: -9.5% max: -2.8%
html5ever-opt
avg: -4.3% min: -9.3% max: -1.6%
crates.io-check
avg: -1.8% min: -2.9% max: -0.6%
crates.io-opt
avg: -1.0% min: -2.2% max: -0.1%
crates.io
avg: -1.1% min: -2.2% max: -0.2%
```
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Because we create a lot of these in the macro parser, but only very
rarely modify them.
This speeds up some html5ever runs by 2--3%.
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This lets us store most `MatcherPos` instances on the stack. This speeds
up various runs of html5ever, the best by 3%.
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This avoids a `to_owned` call that can be hot, speeding up the various
runs of html5ever by 1--5%, and some runs of crates.io by 2--3%.
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Implements RFC 1576.
See: https://github.com/rust-lang/rfcs/blob/master/text/1576-macros-literal-matcher.md
Changes are mostly in libsyntax, docs, and tests. Feature gate is
enabled for 1.27.0.
Many thanks to Vadim Petrochenkov for following through code reviews
and suggestions.
Example:
````rust
macro_rules! test_literal {
($l:literal) => {
println!("literal: {}", $l);
};
($e:expr) => {
println!("expr: {}", $e);
};
}
fn main() {
let a = 1;
test_literal!(a);
test_literal!(2);
test_literal!(-3);
}
```
Output:
```
expr: 1
literal: 2
literal: -3
```
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Update `?` repetition disambiguation.
**Do not merge** (yet)
This is a test implementation of some ideas from discussion in https://github.com/rust-lang/rust/issues/48075 . This PR
- disallows `?` repetition from taking a separator, since the separator is never used.
- disallows the use of `?` as a separator. This allows patterns like `$(a)?+` to match `+` and `a+` rather than `a?a?a`. This is a _breaking change_, but maybe that's ok? Perhaps a crater run is the right approach?
cc @durka @alexreg @nikomatsakis
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check stability of macro invocations
I haven't implemented tests yet but this should be a pretty solid prototype. I think as-implemented it will also stability-check macro invocations in the same crate, dunno if we want that or not.
I don't know if we want this to go through `rustc::middle::stability` or not, considering the information there wouldn't be available at the time of macro expansion (even for external crates, right?).
r? @nrc
closes #34079
cc @petrochenkov @durka @jseyfried #38356
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