`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.
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the
rustc guide:
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