43 lines
2.2 KiB
Markdown
43 lines
2.2 KiB
Markdown
# The Parser
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The parser is responsible for converting raw Rust source code into a structured
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form which is easier for the compiler to work with, usually called an [*Abstract
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Syntax Tree*][ast]. An AST mirrors the structure of a Rust program in memory,
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using a `Span` to link a particular AST node back to its source text.
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The bulk of the parser lives in the [libsyntax] crate.
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Like most parsers, the parsing process is composed of two main steps,
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- lexical analysis – turn a stream of characters into a stream of token trees
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- parsing – turn the token trees into an AST
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The `syntax` crate contains several main players,
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- a [`CodeMap`] for mapping AST nodes to their source code
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- the [ast module] contains types corresponding to each AST node
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- a [`StringReader`] for lexing source code into tokens
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- the [parser module] and [`Parser`] struct are in charge of actually parsing
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tokens into AST nodes,
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- and a [visit module] for walking the AST and inspecting or mutating the AST
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nodes.
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The main entrypoint to the parser is via the various `parse_*` functions
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in the [parser module]. They let you do things like turn a filemap into a
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token stream, create a parser from the token stream, and then execute the
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parser to get a `Crate` (the root AST node).
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To minimise the amount of copying that is done, both the `StringReader` and
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`Parser` have lifetimes which bind them to the parent `ParseSess`. This contains
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all the information needed while parsing, as well as the `CodeMap` itself.
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[libsyntax]: https://github.com/rust-lang/rust/tree/master/src/libsyntax
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[rustc_errors]: https://github.com/rust-lang/rust/tree/master/src/librustc_errors
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[ast]: https://en.wikipedia.org/wiki/Abstract_syntax_tree
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[`CodeMap`]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/codemap/struct.CodeMap.html
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[ast module]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html
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[parser module]: https://github.com/rust-lang/rust/tree/master/src/libsyntax/parse
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[`Parser`]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/parse/parser/struct.Parser.html
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[`StringReader`]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/parse/lexer/struct.StringReader.html
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[visit module]: https://doc.rust-lang.org/nightly/nightly-rustc/syntax/visit/index.html
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