delegation: refactor AST -> HIR lowering by aerooneqq · Pull Request #158434 · rust-lang/rust

aerooneqq · 2026-06-26T08:41:06Z

After a lot of new features were added to delegation the code became a bit worse, as resolution and lowering logic is mixed in delegation.rs. In this file the output of resolution step of delegation is transferred as function params (in some functions up to 7 or 8 params), which is not good looking for me.

As I mentioned in the previous paragraph delegation AST -> HIR lowering can be split into two phases:

Resolution - we collect all information about signature, signature parent context, delegation parent, delegation generics. Then this information is used in AST -> HIR lowering,
AST -> HIR lowering - we generate everything based on the output of resolution.

This PR explicitly splits those two phases: as a first step we collect all needed information into DelegationResolution struct and then use it during AST -> HIR lowering. The delegation.rs file is moved into delegation folder and renamed to mod.rs, the final file structure is as follows:

mod.rs - contains lowering logic for everything but generics,
generics.rs - contains logic for resolution and uplifting of generic params, placed in a separate file as it is a significant and independent part of lowering,
attributes.rs - contains relatively small piece of logic which connects to attributes inheritance,
resolution.rs - contains logic for resolution of everything but generics.

As resolution of delegation and generics logically does not depend on LoweringContext, as from its name LoweringContext contains many additional information which is used for lowering, I've created LoweringContextForResolution trait which abstracts operations which are used during delegation resolution.

Benefits of this PR:

Resolution information is grouped into single struct, is extracted from AST -> HIR lowering which leads to better transfer of resolution across lowering (through a single struct, not through many arguments) and enhanced understanding of which information we use for delegation lowering,
There is now a single path for generating error delegation - if an error was returned from delegation resolution,
Refactorings of uplift_delegation_generics which was creating generic params for delegation made the code more understandable.

Part of #118212.
r? @petrochenkov

aerooneqq · 2026-06-26T08:42:08Z

Now this PR is based on #158397, so it is draft until #158397 is merged, but feedback is anyway welcomed.

petrochenkov · 2026-07-02T10:58:02Z

Could you split this into 2 commits - one moving code around (to different files, or within files), and the second one doing the code changes/refactorings?
@rustbot author

rustbot · 2026-07-02T10:58:06Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

aerooneqq · 2026-07-02T11:12:11Z

@rustbot ready

aerooneqq · 2026-07-02T12:59:35Z

@rustbot ready

petrochenkov · 2026-07-02T13:12:11Z

+    /// Abstracts operations that are needed for delegation's resolution, so resolution
+    /// is independent of `LoweringContext`. Placed in a separate module so `LoweringContext`
+    /// can not be accessed directly.
+    pub(crate) struct DelegationResolverWrapper<'a, 'hir>(&'a LoweringContext<'a, 'hir>);


Suggested change

pub(crate) struct DelegationResolverWrapper<'a, 'hir>(&'a LoweringContext<'a, 'hir>);

pub(crate) struct DelegationResolver<'a, 'hir>(&'a LoweringContext<'a, 'hir>);

There's no naming ambiguity with the trait anymore.

View changes since the review

petrochenkov · 2026-07-02T13:18:46Z

+}

-    pub(super) fn check_for_cycles(
+impl<'tcx> wrapper::DelegationResolverWrapper<'_, 'tcx> {


Suggested change

impl<'tcx> wrapper::DelegationResolverWrapper<'_, 'tcx> {

impl<'tcx> DelegationResolver<'_, 'tcx> {

Import, here and in one more place.

View changes since the review

petrochenkov · 2026-07-02T13:21:06Z

+        (!args.args.is_empty()).then(|| args)
+    }
+
+    pub(super) fn resolve_generics(


Suggested change

pub(super) fn resolve_generics(

pub(super) fn resolve_and_generate_generics(

Because we produce GenericsGenerationResults here and not GenericsResolution.
create_generics_resolution can then be renamed to resolve_generics.

View changes since the review

petrochenkov · 2026-07-02T13:24:16Z

r=me after addressing the remaining comments and squashing the refactoring commits.
@rustbot author
@bors delegate+

rust-bors · 2026-07-02T13:24:20Z

✌️ @aerooneqq, you can now approve this pull request!

If @petrochenkov told you to "r=me" after making some further change, then please make that change and post @bors r=petrochenkov.

View changes since this delegation.

rust-log-analyzer · 2026-07-02T14:10:25Z

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...............................F.................. (50/144)
.................................................. (100/144)
............................................       (144/144)

======== tests/rustdoc-gui/go-to-collapsed-elem.goml ========

[ERROR] line 40
    at `tests/rustdoc-gui/go-to-collapsed-elem.goml` line 21: Error: Node is detached from document: for command `click: "//*[@id='search']//a[@href='../test_docs/struct.Foo.html#method.must_use']"`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/struct.Foo.html?search=t_use>


<= doc-ui tests done: 143 succeeded, 1 failed, 0 filtered out

Error: ()

rustbot assigned petrochenkov Jun 26, 2026

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aerooneqq force-pushed the delegation-refactoring branch from d6dd2be to c2d7c9c Compare June 29, 2026 12:03

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aerooneqq force-pushed the delegation-refactoring branch from ae9c536 to faa250b Compare July 1, 2026 06:56

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aerooneqq force-pushed the delegation-refactoring branch from faa250b to 002fc94 Compare July 2, 2026 05:42

aerooneqq marked this pull request as ready for review July 2, 2026 05:45

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 2, 2026

petrochenkov added the F-fn_delegation `#![feature(fn_delegation)]` label Jul 2, 2026

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 2, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 2, 2026

aerooneqq added 4 commits July 2, 2026 14:01

delegation.rs -> mod.rs

090b0f2

Split attributes

94c1760

Split resolution

9b715ee

Refactorings

2196005

aerooneqq force-pushed the delegation-refactoring branch from 67f05b8 to 2196005 Compare July 2, 2026 11:11

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 2, 2026

aerooneqq mentioned this pull request Jul 2, 2026

delegation: support mapping of all arguments with Self type #158690

Draft

petrochenkov reviewed Jul 2, 2026

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petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 2, 2026

aerooneqq added 10 commits July 2, 2026 15:34

Review: remove DelegationResolver trait, inline get_resolution_id

474ac51

Review: attributes -> attrs

9a91051

Review: pub(super) -> pub

316ba07

Review: GenericsResolutionDto -> GenericsResolution

8f2baa0

Review: typo

14d0ba2

Review: dto -> res

9510356

Review: separate generics and resolution

5a30ddb

Review: remove res. access

9cb31b2

Review: fix other bad accessed to res.

8bb9b11

Review: add comment for DelegationResolverWrapper

1429873

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 2, 2026

create_resolution_dto -> create_generics_resolution

aec2af7

aerooneqq commented Jul 2, 2026

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Comment thread compiler/rustc_ast_lowering/src/delegation/resolution.rs

petrochenkov reviewed Jul 2, 2026

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rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 2, 2026

	pub(crate) struct DelegationResolverWrapper<'a, 'hir>(&'a LoweringContext<'a, 'hir>);
	pub(crate) struct DelegationResolver<'a, 'hir>(&'a LoweringContext<'a, 'hir>);

	impl<'tcx> wrapper::DelegationResolverWrapper<'_, 'tcx> {
	impl<'tcx> DelegationResolver<'_, 'tcx> {

	pub(super) fn resolve_generics(
	pub(super) fn resolve_and_generate_generics(

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