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write a short paragraph about the new lexer #394

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Jul 22, 2019
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write a short paragraph about the new lexer #394

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13 changes: 13 additions & 0 deletions src/the-parser.md
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Expand Up @@ -32,6 +32,19 @@ To minimise the amount of copying that is done, both the `StringReader` and
`Parser` have lifetimes which bind them to the parent `ParseSess`. This contains
all the information needed while parsing, as well as the `SourceMap` itself.

## More on Lexical Analysis

Code for lexical analysis is split between two crates:

- `rustc_lexer` crate is responsible for breaking a `&str` into chunks
constituting tokens. Although it is popular to implement lexers as generated
finite state machines, the lexer in `rustc_lexer` is hand-written.

- [`StringReader`] from [libsyntax] integrates `rustc_lexer` with `rustc`
specific data structures. Specifically, it adds `Span` information to tokens
returned by `rustc_lexer` and interns identifiers.


[libsyntax]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/index.html
[rustc_errors]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/index.html
[ast]: https://en.wikipedia.org/wiki/Abstract_syntax_tree
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