Investigate mutating the AST during late resolution

[cjgillot]

Late name resolution happens on the AST and is responsible for finding which definition each name in the AST points to, be it a type, trait expression, pattern, lifetime or label. The name "late" means that this resolution happens after all macros have been expanded, in opposition to "early" resolution which resolves macros. The corresponding code lives in rustc_resolve::late.

The results of this late resolution are stored in tables, and used by HIR lowering to directly embed the information in HIR.

Later, lowering reinterprets some of the AST where the syntax is ambiguous. For instance, lowering is responsible for making bare trait objects explicit: interpreting types of the form T with dyn T when resolution finds that T is a trait. Lowering also introduces implicit lifetime parameters for lifetime elision. The corresponding code lives in rustc_ast_lowering.

This re-interpretation of the AST needs to closely mirror what resolution is doing. Mismatches can cause bugs, like AST and HIR disagreeing on how to resolve lifetimes in the dyn T discussed above.

The objective of this project is to have this resolution-based desugaring in only one place. We would like to have late resolution modify the AST to directly perform these transformations, and remove them from lowering.

At the same time, lowering also performs its own transformations, like transforming impl Trait to generic parameters or opaque types. Those transformations should stay in lowering for the time being. Rule of thumb: if the transformation inspects resolutions, it should go in resolution, if it's purely syntactic it can stay in lowering.

Steps:

• Refactor the visitor in rustc_resolve::late by an AST MutVisitor. MutVisitor is designed for macro expansions, so it may lack some methods to make resolution practical. There can be design work here, so feel free to suggest new methods / a different MutVisitor trait / ...
• Desugar type-relative paths during resolution. The current code in lowering is in rustc_ast_lowering::path, and aims to transform a::b::c::d to <a::b::c>::d when a::b::c can be resolved to a type and ::d cannot (for instance because it is a trait method). See rustc_resolve::late::smart_resolve_path and associated methods.
• Desugar bare trait object during late resolution when the path is actually a trait. See rustc_resolve::late::visit_ty.
• Explicit elided lifetimes on the AST by inserting fresh lifetimes.

Please contact me on zulip (cjgillot) for any questions.

Note: this project is a large body of work, with possibly many changes to the resolution code. The point is to simplify the lowering code. If this simplification goal is not reached, or if the perf impact is prohibitive, we may end-up not merging the modifications, and instead document why.
To be merged, the PR will probably require a major change proposal. I will take care of this proposal once the perspectives become a bit clear.

[petrochenkov]

I'm not sure whether it's a good idea in general, but the implementation/prototyping here can be simplified by splitting this job into two steps:

• Step 1: turn the AST mutation into a separate MutVisitor pass that runs immediately after late resolution and uses the tables built by immutable late resolution. This pass becomes a some kind of "pre AST lowering" or "early AST lowering". The equivalent code can then be removed from the "late" AST lowering.
• Step 2: merge that pass into late resolution and eliminate tables using for communication between late resolution and early lowering.

[petrochenkov]

@cjgillot
How does all this map on incremental compilation changes you've been planning to do?

[vincenzopalazzo]

I'm taking a lock on it because its feet on my learning path! and see the evolution of the discussion in the meanwhile

@rustbot claim

[Noratrieb]

@vincenzopalazzo are you still working on this?

[vincenzopalazzo]

@Nilstrieb it is in my todo list my I had some other priority, so I unassign me if you had any other good target to assign this

[jon-chuang]

@rustbot claim

Hi, I'd like to try this out.