Update the-parser.md

src/the-parser.md

@@ -1,68 +1,72 @@
 # Lexing and Parsing
 
-The very first thing the compiler does is take the program (in Unicode
-characters) and turn it into something the compiler can work with more
-conveniently than strings. This happens in two stages: Lexing and Parsing.
+The very first thing the compiler does is take the program (in UTF-8 Unicode text)
+and turn it into a data format the compiler can work with more conveniently than strings.
+This happens in two stages: Lexing and Parsing.
 
-Lexing takes strings and turns them into streams of [tokens]. For example,
-`foo.bar + buz` would be turned into the tokens `foo`, `.`,
-`bar`, `+`, and `buz`.  The lexer lives in [`rustc_lexer`][lexer].
+  1. _Lexing_ takes strings and turns them into streams of [tokens]. For
+  example, `foo.bar + buz` would be turned into the tokens `foo`, `.`, `bar`,
+  `+`, and `buz`.
 
 [tokens]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/token/index.html
 [lexer]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html
 
-Parsing then takes streams of tokens and turns them into a structured
-form which is easier for the compiler to work with, usually called an [*Abstract
-Syntax Tree*][ast] (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 AST is defined in [`rustc_ast`][rustc_ast], along with some definitions for
-tokens and token streams, data structures/traits for mutating ASTs, and shared
-definitions for other AST-related parts of the compiler (like the lexer and
+  2. _Parsing_ takes streams of tokens and turns them into a structured form
+  which is easier for the compiler to work with, usually called an [*Abstract
+  Syntax Tree* (AST)][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 AST is defined in
+[`rustc_ast`][rustc_ast], along with some definitions for tokens and token
+streams, data structures/traits for mutating ASTs, and shared definitions for
+other AST-related parts of the compiler (like the lexer and
 macro-expansion).
 
+The lexer is developed in [`rustc_lexer`][lexer].
+
 The parser is defined in [`rustc_parse`][rustc_parse], along with a
 high-level interface to the lexer and some validation routines that run after
 macro expansion. In particular, the [`rustc_parse::parser`][parser] contains
 the parser implementation.
 
-The main entrypoint to the parser is via the various `parse_*` functions and others in the
-[parser crate][parser_lib]. They let you do things like turn a [`SourceFile`][sourcefile]
+The main entrypoint to the parser is via the various `parse_*` functions and others in
+[rustc_parse][rustc_parse]. They let you do things like turn a [`SourceFile`][sourcefile]
 (e.g. the source in a single file) into a token stream, create a parser from
-the token stream, and then execute the parser to get a `Crate` (the root AST
+the token stream, and then execute the parser to get a [`Crate`] (the root AST
 node).
 
 To minimize the amount of copying that is done,
-both [`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.
+both [`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.
 
-Note that while parsing, we may encounter macro definitions or invocations. We
-set these aside to be expanded (see [this chapter](./macro-expansion.md)).
-Expansion may itself require parsing the output of the macro, which may reveal
-more macros to be expanded, and so on.
+Note that while parsing, we may encounter macro definitions or invocations.
+We set these aside to be expanded (see [Macro Expansion](./macro-expansion.md)).
+Expansion itself may require parsing the output of a macro, which may reveal more macros to be expanded, and so on.
 
 ## More on Lexical Analysis
 
 Code for lexical analysis is split between two crates:
 
-- `rustc_lexer` crate is responsible for breaking a `&str` into chunks
+- [`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.
+  finite state machines, the lexer in [`rustc_lexer`] is hand-written.
 
-- [`StringReader`] integrates `rustc_lexer` with data structures specific to `rustc`.
-  Specifically,
-  it adds `Span` information to tokens returned by `rustc_lexer` and interns identifiers.
+- [`StringReader`] integrates [`rustc_lexer`] with data structures specific to
+  `rustc`. Specifically, it adds `Span` information to tokens returned by
+  [`rustc_lexer`] and interns identifiers.
 
-[rustc_ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/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
+[`Crate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/struct.Crate.html
+[`Parser`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html
+[`ParseSess`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/parse/struct.ParseSess.html
+[`rustc_lexer`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html
 [`SourceMap`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/source_map/struct.SourceMap.html
+[`StringReader`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.StringReader.html
 [ast module]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html
-[rustc_parse]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html
-[parser_lib]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html
+[ast]: ./ast-validation.md
 [parser]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/index.html
-[`Parser`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html
-[`StringReader`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.StringReader.html
-[visit module]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/index.html
+[rustc_ast]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/index.html
+[rustc_errors]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/index.html
+[rustc_parse]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html
 [sourcefile]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.SourceFile.html
+[visit module]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/visit/index.html