`create_matches` creates a `Vec<Rc<Vec<NamedMatch>>>`. Even though all the
inner `Vec`s are empty, each one is created separately.
This commit changes `create_matches` so it instead creates one empty inner
`Vec`, and shares it.
The commit also changes `MatcherPos::matches` to a boxed slice, because its
length doesn't change.
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%
```
Started rebasing @sgrif's PR #33135 off of current master. (Well, actually merging it into a new branch based off current master.)
The following files still need to be fixed or at least reviewed:
- `src/libsyntax/ext/tt/macro_parser.rs`: calls `Parser::parse_lifetime`, which doesn't exist anymore
- `src/libsyntax/parse/parser.rs`: @sgrif added an error message to `Parser::parse_lifetime`. Code has since been refactored, so I just took it out for now.
- `src/libsyntax/ext/tt/transcribe.rs`: This code has been refactored bigtime. Not sure whether @sgrif's changes here are still necessary. Took it out for this commit.
syntax: Relax path grammar
TLDR: Accept the disambiguator `::` in "type" paths (`Type::<Args>`), accept the disambiguator `::` before parenthesized generic arguments (`Fn::(Args)`).
The "turbofish" disambiguator `::<>` in expression paths is a necessary evil required for path parsing to be both simple and to give reasonable results.
Since paths in expressions usually refer to values (but not necessarily, e.g. `Struct::<u8> { field: 0 }` is disambiguated, but refers to a type), people often consider `::<>` to be inherent to *values*, and not *expressions* and want to write disambiguated paths for values even in contexts where disambiguation is not strictly necessary, for example when a path is passed to a macro `m!(Vec::<i32>::new)`.
The problem is that currently, if the disambiguator is not *required*, then it's *prohibited*. This results in confusion - see https://github.com/rust-lang/rust/issues/41740, https://internals.rust-lang.org/t/macro-path-uses-novel-syntax/5561.
This PR makes the disambiguator *optional* instead of prohibited in contexts where it's not strictly required, so people can pass paths to macros in whatever form they consider natural (e.g. disambiguated form for value paths).
This PR also accepts the disambiguator in paths with parenthesized arguments (`Fn::(Args)`) for consistency and to simplify testing of stuff like https://github.com/rust-lang/rust/pull/41856#issuecomment-301219194.
Closes https://github.com/rust-lang/rust/issues/41740
cc @rust-lang/lang
r? @nikomatsakis
This is useful if parsing from stdin or a String and don't want to try and read in a module from another file. Instead we just leave a stub in the AST.
