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                         <h1 id="lexing-and-parsing"><a class="header" href="#lexing-and-parsing">Lexing and parsing</a></h1>
 <p>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.</p>
 <ol>
 <li><em>Lexing</em> takes strings and turns them into streams of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/token/index.html">tokens</a>. For
 example, <code>foo.bar + buz</code> would be turned into the tokens <code>foo</code>, <code>.</code>, <code>bar</code>,
 <code>+</code>, and <code>buz</code>. This is implemented in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a>.</li>
 </ol>
 <ol start="2">
 <li><em>Parsing</em> takes streams of tokens and turns them into a structured form
 which is easier for the compiler to work with, usually called an <a href="./ast-validation.html"><em>Abstract
 Syntax Tree</em> (AST)</a> .</li>
 </ol>
 <h2 id="the-ast"><a class="header" href="#the-ast">The AST</a></h2>
 <p>The AST mirrors the structure of a Rust program in memory, using a <code>Span</code> to
 link a particular AST node back to its source text. The AST is defined in
 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/index.html"><code>rustc_ast</code></a>, 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).</p>
 <p>Every node in the AST has its own <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>, including top-level items
 such as structs, but also individual statements and expressions. A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>
 is an identifier number that uniquely identifies an AST node within a crate.</p>
 <p>However, because they are absolute within a crate, adding or removing a single
 node in the AST causes all the subsequent <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>s to change. This renders
 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>s pretty much useless for incremental compilation, where you want as
 few things as possible to change.</p>
 <p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>s are used in all the <code>rustc</code> bits that operate directly on the AST,
 like macro expansion and name resolution (more on these over the next couple chapters).</p>
 <h2 id="parsing"><a class="header" href="#parsing">Parsing</a></h2>
 <p>The parser is defined in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html"><code>rustc_parse</code></a>, along with a
 high-level interface to the lexer and some validation routines that run after
 macro expansion. In particular, the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/index.html"><code>rustc_parse::parser</code></a> contains
 the parser implementation.</p>
 <p>The main entrypoint to the parser is via the various <code>parse_*</code> functions and others in
 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html">rustc_parse</a>. They let you do things like turn a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.SourceFile.html"><code>SourceFile</code></a>
 (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 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/struct.Crate.html"><code>Crate</code></a> (the root AST
 node).</p>
 <p>To minimize the amount of copying that is done,
 both <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.Lexer.html"><code>Lexer</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html"><code>Parser</code></a> have lifetimes which bind them to the parent <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/parse/struct.ParseSess.html"><code>ParseSess</code></a>.
 This contains all the information needed while parsing, as well as the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/source_map/struct.SourceMap.html"><code>SourceMap</code></a> itself.</p>
 <p>Note that while parsing, we may encounter macro definitions or invocations.
 We set these aside to be expanded (see <a href="./macro-expansion.html">Macro Expansion</a>).
 Expansion itself may require parsing the output of a macro, which may reveal more macros to be expanded, and so on.</p>
 <h2 id="more-on-lexical-analysis"><a class="header" href="#more-on-lexical-analysis">More on lexical analysis</a></h2>
 <p>Code for lexical analysis is split between two crates:</p>
 <ul>
 <li>
 <p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> crate is responsible for breaking a <code>&amp;str</code> into chunks
 constituting tokens. Although it is popular to implement lexers as generated
 finite state machines, the lexer in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> is hand-written.</p>
 </li>
 <li>
 <p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.Lexer.html"><code>Lexer</code></a> integrates <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> with data structures specific to
 <code>rustc</code>. Specifically, it adds <code>Span</code> information to tokens returned by
 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> and interns identifiers.</p>
 </li>
 </ul>

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	<h1 id="lexing-and-parsing"><a class="header" href="#lexing-and-parsing">Lexing and parsing</a></h1>
	<p>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.</p>
	<ol>
	<li><em>Lexing</em> takes strings and turns them into streams of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/token/index.html">tokens</a>. For
	example, <code>foo.bar + buz</code> would be turned into the tokens <code>foo</code>, <code>.</code>, <code>bar</code>,
	<code>+</code>, and <code>buz</code>. This is implemented in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a>.</li>
	</ol>
	<ol start="2">
	<li><em>Parsing</em> takes streams of tokens and turns them into a structured form
	which is easier for the compiler to work with, usually called an <a href="./ast-validation.html"><em>Abstract
	Syntax Tree</em> (AST)</a> .</li>
	</ol>
	<h2 id="the-ast"><a class="header" href="#the-ast">The AST</a></h2>
	<p>The AST mirrors the structure of a Rust program in memory, using a <code>Span</code> to
	link a particular AST node back to its source text. The AST is defined in
	<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/index.html"><code>rustc_ast</code></a>, 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).</p>
	<p>Every node in the AST has its own <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>, including top-level items
	such as structs, but also individual statements and expressions. A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>
	is an identifier number that uniquely identifies an AST node within a crate.</p>
	<p>However, because they are absolute within a crate, adding or removing a single
	node in the AST causes all the subsequent <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>s to change. This renders
	<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>s pretty much useless for incremental compilation, where you want as
	few things as possible to change.</p>
	<p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/node_id/struct.NodeId.html"><code>NodeId</code></a>s are used in all the <code>rustc</code> bits that operate directly on the AST,
	like macro expansion and name resolution (more on these over the next couple chapters).</p>
	<h2 id="parsing"><a class="header" href="#parsing">Parsing</a></h2>
	<p>The parser is defined in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html"><code>rustc_parse</code></a>, along with a
	high-level interface to the lexer and some validation routines that run after
	macro expansion. In particular, the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/index.html"><code>rustc_parse::parser</code></a> contains
	the parser implementation.</p>
	<p>The main entrypoint to the parser is via the various <code>parse_*</code> functions and others in
	<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/index.html">rustc_parse</a>. They let you do things like turn a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.SourceFile.html"><code>SourceFile</code></a>
	(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 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/struct.Crate.html"><code>Crate</code></a> (the root AST
	node).</p>
	<p>To minimize the amount of copying that is done,
	both <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.Lexer.html"><code>Lexer</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html"><code>Parser</code></a> have lifetimes which bind them to the parent <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/parse/struct.ParseSess.html"><code>ParseSess</code></a>.
	This contains all the information needed while parsing, as well as the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/source_map/struct.SourceMap.html"><code>SourceMap</code></a> itself.</p>
	<p>Note that while parsing, we may encounter macro definitions or invocations.
	We set these aside to be expanded (see <a href="./macro-expansion.html">Macro Expansion</a>).
	Expansion itself may require parsing the output of a macro, which may reveal more macros to be expanded, and so on.</p>
	<h2 id="more-on-lexical-analysis"><a class="header" href="#more-on-lexical-analysis">More on lexical analysis</a></h2>
	<p>Code for lexical analysis is split between two crates:</p>
	<ul>
	<li>
	<p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> crate is responsible for breaking a <code>&str</code> into chunks
	constituting tokens. Although it is popular to implement lexers as generated
	finite state machines, the lexer in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> is hand-written.</p>
	</li>
	<li>
	<p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/lexer/struct.Lexer.html"><code>Lexer</code></a> integrates <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> with data structures specific to
	<code>rustc</code>. Specifically, it adds <code>Span</code> information to tokens returned by
	<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html"><code>rustc_lexer</code></a> and interns identifiers.</p>
	</li>
	</ul>

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