Name resolution

Basics
Namespaces
Scopes and ribs
Overall strategy
Speculative crate loading
TODO: #16

In the previous chapters, we saw how the AST 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.

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 rustc_resolve::late. 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 now, then it is a compiler error.

Name resolution can be complex. There are a few different namespaces (e.g. macros, values, types, lifetimes), and names may be valid at different (nested) scopes. Also, different types of names can fail to be resolved differently, and failures can happen differently at different scopes. For example, for a module scope, failure means no unexpanded macros and no unresolved glob imports in that module. On the other hand, in a function body, failure requires that a name be absent from the block we are in, all outer scopes, and the global scope.

Basics

In our programs we can refer to variables, types, functions, etc, by giving them a name. These names are not always unique. For example, take this valid Rust program:

#![allow(unused)]
fn main() {
type x = u32;
let x: x = 1;
let y: x = 2;
}

How do we know on line 3 whether x is a type (u32) 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.

The name resolution in Rust is a two-phase process. In the first phase, which runs during macro expansion, we build a tree of modules and resolve imports. Macro expansion and name resolution communicate with each other via the ResolverAstLoweringExt trait.

The input to the second phase is the syntax tree, produced by parsing input files and expanding macros. 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.

A successful run of the second phase (Resolver::resolve_crate) creates kind of an index the rest of the compilation may use to ask about the present names (through the hir::lowering::Resolver interface).

The name resolution lives in the rustc_resolve crate, with the meat in lib.rs and some helpers or symbol-type specific logic in the other modules.

Namespaces

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'll compile (with warnings):

#![allow(unused)]
fn main() {
type x = u32;
let x: x = 1;
let y: x = 2; // See? x is still a type here.
}

To cope with this, and with slightly different scoping rules for these namespaces, the resolver keeps them separated and builds separate structures for them.

In other words, when the code talks about namespaces, it doesn't mean the module hierarchy, it's types vs. values vs. macros.

Scopes and ribs

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 the name visible in the outer one is also visible in the inner one, or that it refers to the same thing.

To cope with that, the compiler introduces the concept of Ribs. This is an abstraction of a scope. Every time the set of visible names potentially changes, a new rib is pushed onto a stack. The places where this can happen include for example:

The obvious places ‒ curly braces enclosing a block, function boundaries, modules.
Introducing a let binding ‒ this can shadow another binding with the same name.
Macro expansion border ‒ to cope with macro hygiene.

When searching for a name, the stack of ribs 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 rib 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:

#![allow(unused)]
fn main() {
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 `helper` (2) and another on the block (3)
        // `val` is accessible here
    }; // End of (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), (2) and (1)
}

Because the rules for different namespaces are a bit different, each namespace has its own independent rib stack that is constructed in parallel to the others. In addition, there's also a rib stack for local labels (e.g. names of loops or blocks), which isn't a full namespace in its own right.

Overall strategy

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 Rib hierarchy.

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 Rib.

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.

Therefore, the resolution is performed in multiple stages.

Speculative crate loading

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!

Loading crates for import suggestions that haven't yet been loaded is called speculative crate loading, because any errors it encounters shouldn't be reported: resolve decided to load them, not the user. The function that does this is lookup_import_candidates and lives in rustc_resolve/src/diagnostics.rs.

To tell the difference between speculative loads and loads initiated by the user, resolve passes around a record_used parameter, which is false when the load is speculative.

TODO: #16

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.

What exactly does it link to and how is that published and consumed by following stages of compilation?
Who calls it and how it is actually used.
Is it a pass and then the result is only used, or can it be computed incrementally?
The overall strategy description is a bit vague.
Where does the name Rib come from?
Does this thing have its own tests, or is it tested only as part of some e2e testing?

Rust Compiler Development Guide