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| <h1 id="name-resolution"><a class="header" href="#name-resolution">Name resolution</a></h1> |
| <p>In the previous chapters, we saw how the <a href="./ast-validation.html"><em>Abstract Syntax Tree</em> (<code>AST</code>)</a> |
| is built with all macros expanded. We saw how doing that requires doing some |
| name resolution to resolve imports and macro names. In this chapter, we show |
| how this is actually done and more.</p> |
| <p>In fact, we don’t do full name resolution during macro expansion – we only |
| resolve imports and macros at that time. This is required to know what to even |
| expand. Later, after we have the whole AST, we do full name resolution to |
| resolve all names in the crate. This happens in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/index.html"><code>rustc_resolve::late</code></a>. |
| Unlike during macro expansion, in this late expansion, we only need to try to |
| resolve a name once, since no new names can be added. If we fail to resolve a |
| name, then it is a compiler error.</p> |
| <p>Name resolution is complex. There are different namespaces (e.g. |
| macros, values, types, lifetimes), and names may be valid at different (nested) |
| scopes. Also, different types of names can fail resolution differently, and |
| failures can happen differently at different scopes. For example, in a module |
| scope, failure means no unexpanded macros and no unresolved glob imports in |
| that module. On the other hand, in a function body scope, failure requires that a |
| name be absent from the block we are in, all outer scopes, and the global |
| scope.</p> |
| <h2 id="basics"><a class="header" href="#basics">Basics</a></h2> |
| <p>In our programs we refer to variables, types, functions, etc, by giving them |
| a name. These names are not always unique. For example, take this valid Rust |
| program:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>type x = u32; |
| let x: x = 1; |
| let y: x = 2; |
| <span class="boring">}</span></code></pre> |
| <p>How do we know on line 3 whether <code>x</code> is a type (<code>u32</code>) or a value (1)? These |
| conflicts are resolved during name resolution. In this specific case, name |
| resolution defines that type names and variable names live in separate |
| namespaces and therefore can co-exist.</p> |
| <p>The name resolution in Rust is a two-phase process. In the first phase, which runs |
| during <code>macro</code> expansion, we build a tree of modules and resolve imports. Macro |
| expansion and name resolution communicate with each other via the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/trait.ResolverAstLoweringExt.html"><code>ResolverAstLoweringExt</code></a> trait.</p> |
| <p>The input to the second phase is the syntax tree, produced by parsing input |
| files and expanding <code>macros</code>. This phase produces links from all the names in the |
| source to relevant places where the name was introduced. It also generates |
| helpful error messages, like typo suggestions, traits to import or lints about |
| unused items.</p> |
| <p>A successful run of the second phase (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/struct.Resolver.html#method.resolve_crate"><code>Resolver::resolve_crate</code></a>) creates kind |
| of an index the rest of the compilation may use to ask about the present names |
| (through the <code>hir::lowering::Resolver</code> interface).</p> |
| <p>The name resolution lives in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/index.html"><code>rustc_resolve</code></a> crate, with the bulk in |
| <code>lib.rs</code> and some helpers or symbol-type specific logic in the other modules.</p> |
| <h2 id="namespaces"><a class="header" href="#namespaces">Namespaces</a></h2> |
| <p>Different kind of symbols live in different namespaces ‒ e.g. types don’t |
| clash with variables. This usually doesn’t happen, because variables start with |
| lower-case letter while types with upper-case one, but this is only a |
| convention. This is legal Rust code that will compile (with warnings):</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>type x = u32; |
| let x: x = 1; |
| let y: x = 2; // See? x is still a type here. |
| <span class="boring">}</span></code></pre> |
| <p>To cope with this, and with slightly different scoping rules for these |
| namespaces, the resolver keeps them separated and builds separate structures for |
| them.</p> |
| <p>In other words, when the code talks about namespaces, it doesn’t mean the module |
| hierarchy, it’s types vs. values vs. macros.</p> |
| <h2 id="scopes-and-ribs"><a class="header" href="#scopes-and-ribs">Scopes and ribs</a></h2> |
| <p>A name is visible only in certain area in the source code. This forms a |
| hierarchical structure, but not necessarily a simple one ‒ if one scope is |
| part of another, it doesn’t mean a name visible in the outer scope is also |
| visible in the inner scope, or that it refers to the same thing.</p> |
| <p>To cope with that, the compiler introduces the concept of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a>s. This is |
| an abstraction of a scope. Every time the set of visible names potentially changes, |
| a new <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a> is pushed onto a stack. The places where this can happen include for |
| example:</p> |
| <ul> |
| <li>The obvious places ‒ curly braces enclosing a block, function boundaries, |
| modules.</li> |
| <li>Introducing a <code>let</code> binding ‒ this can shadow another binding with the same |
| name.</li> |
| <li>Macro expansion border ‒ to cope with macro hygiene.</li> |
| </ul> |
| <p>When searching for a name, the stack of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.LateResolutionVisitor.html#structfield.ribs"><code>ribs</code></a> is traversed from the innermost |
| outwards. This helps to find the closest meaning of the name (the one not |
| shadowed by anything else). The transition to outer <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a> may also affect |
| what names are usable ‒ if there are nested functions (not closures), |
| the inner one can’t access parameters and local bindings of the outer one, |
| even though they should be visible by ordinary scoping rules. An example:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>fn do_something<T: Default>(val: T) { // <- New rib in both types and values (1) |
| // `val` is accessible, as is the helper function |
| // `T` is accessible |
| let helper = || { // New rib on the block (2) |
| // `val` is accessible here |
| }; // End of (2), new rib on `helper` (3) |
| // `val` is accessible, `helper` variable shadows `helper` function |
| fn helper() { // <- New rib in both types and values (4) |
| // `val` is not accessible here, (4) is not transparent for locals |
| // `T` is not accessible here |
| } // End of (4) |
| let val = T::default(); // New rib (5) |
| // `val` is the variable, not the parameter here |
| } // End of (5), (3) and (1) |
| <span class="boring">}</span></code></pre> |
| <p>Because the rules for different namespaces are a bit different, each namespace |
| has its own independent <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a> stack that is constructed in parallel to the others. |
| In addition, there’s also a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a> stack for local labels (e.g. names of loops or |
| blocks), which isn’t a full namespace in its own right.</p> |
| <h2 id="overall-strategy"><a class="header" href="#overall-strategy">Overall strategy</a></h2> |
| <p>To perform the name resolution of the whole crate, the syntax tree is traversed |
| top-down and every encountered name is resolved. This works for most kinds of |
| names, because at the point of use of a name it is already introduced in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a> |
| hierarchy.</p> |
| <p>There are some exceptions to this. Items are bit tricky, because they can be |
| used even before encountered ‒ therefore every block needs to be first scanned |
| for items to fill in its <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.Rib.html"><code>Rib</code></a>.</p> |
| <p>Other, even more problematic ones, are imports which need recursive fixed-point |
| resolution and macros, that need to be resolved and expanded before the rest of |
| the code can be processed.</p> |
| <p>Therefore, the resolution is performed in multiple stages.</p> |
| <h2 id="speculative-crate-loading"><a class="header" href="#speculative-crate-loading">Speculative crate loading</a></h2> |
| <p>To give useful errors, rustc suggests importing paths into scope if they’re |
| not found. How does it do this? It looks through every module of every crate |
| and looks for possible matches. This even includes crates that haven’t yet |
| been loaded!</p> |
| <p>Eagerly loading crates to include import suggestions that haven’t yet been |
| loaded is called <em>speculative crate loading</em>, because any errors it encounters |
| shouldn’t be reported: <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/index.html"><code>rustc_resolve</code></a> decided to load them, not the user. The function |
| that does this is <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/struct.Resolver.html#method.lookup_import_candidates"><code>lookup_import_candidates</code></a> and lives in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/diagnostics/index.html"><code>rustc_resolve::diagnostics</code></a>.</p> |
| <p>To tell the difference between speculative loads and loads initiated by the |
| user, <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/index.html"><code>rustc_resolve</code></a> passes around a <code>record_used</code> parameter, which is <code>false</code> when |
| the load is speculative.</p> |
| <h2 id="todo-16"><a class="header" href="#todo-16">TODO: <a href="https://github.com/rust-lang/rustc-dev-guide/issues/16">#16</a></a></h2> |
| <p>This is a result of the first pass of learning the code. It is definitely |
| incomplete and not detailed enough. It also might be inaccurate in places. |
| Still, it probably provides useful first guidepost to what happens in there.</p> |
| <ul> |
| <li>What exactly does it link to and how is that published and consumed by |
| following stages of compilation?</li> |
| <li>Who calls it and how it is actually used.</li> |
| <li>Is it a pass and then the result is only used, or can it be computed |
| incrementally?</li> |
| <li>The overall strategy description is a bit vague.</li> |
| <li>Where does the name <code>Rib</code> come from?</li> |
| <li>Does this thing have its own tests, or is it tested only as part of some e2e |
| testing?</li> |
| </ul> |
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