WIP: Started working on the Parser

src/the-parser.md

@@ -1 +1,42 @@
-# The parser
+# The Parser
+
+The parser is responsible for converting raw Rust source code into a structured
+form which is easier for the compiler to work with, usually called an [*Abstract
+Syntax Tree*][ast]. An AST mirrors the structure of a Rust program in memory, 
+using a `Span` to link a particular AST node back to its source text.
+
+The bulk of the parser lives in the [libsyntax] crate.
+
+Like most parsers, the parsing process is composed of two main steps,
+
+- lexical analysis - turn a stream of characters into a stream of token trees
+- parsing - turn the token trees into an AST
+
+The `syntax` crate contains several main players,
+
+- a [`CodeMap`] for mapping AST nodes to their source code
+- the [ast module] contains types corresponding to each AST node
+- a [`StringReader`] for lexing source code into tokens
+- the [parser module] and [`Parser`] struct are in charge of actually parsing
+  tokens into AST nodes,
+- and a [visit module] for walking the AST and inspecting or mutating the AST
+  nodes.
+
+The main entrypoint to the parser is via the various `parse_*` functions
+in the [parser module]. They let you do things like turn a filemap into a
+token stream, create a parser from the token stream, and then execute the
+parser to get a `Crate` (the root AST node).
+
+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 `CodeMap` itself.
+
+[libsyntax]: https://github.com/rust-lang/rust/tree/master/src/libsyntax
+[rustc_errors]: https://github.com/rust-lang/rust/tree/master/src/librustc_errors
+[ast]: https://en.wikipedia.org/wiki/Abstract_syntax_tree
+[`CodeMap`]: https://github.com/rust-lang/rust/blob/master/src/libsyntax/codemap.rs
+[ast module]: https://github.com/rust-lang/rust/blob/master/src/libsyntax/ast.rs
+[parser module]: https://github.com/rust-lang/rust/tree/master/src/libsyntax/parse
+[`Parser`]: https://github.com/rust-lang/rust/blob/master/src/libsyntax/parse/parser.rs
+[`StringReader`]: https://github.com/rust-lang/rust/blob/master/src/libsyntax/parse/lexer/mod.rs
+[visit module]: https://github.com/rust-lang/rust/blob/master/src/libsyntax/visit.rs