`GenericArgs::types` triage + possible fixes by LorrensP-2158466 · Pull Request #159899 · rust-lang/rust

LorrensP-2158466 · 2026-07-25T13:02:17Z

cc #t-types/call-for-participation > `args.types()` triage and fixes

LorrensP-2158466 · 2026-07-25T13:02:49Z

Don't know what tests can/will fail, so i'll see what a CI run says and go from there.

LorrensP-2158466 · 2026-07-27T10:42:02Z

This is all of the list posted in #t-types/call-for-participation > `args.types()` triage and fixes expect 2, both seem to be a bit harder to fix (to me at least:).

~~Currently separate commits because wanted to check CI after each "major enough" change~~

ping @lcnr

rustbot · 2026-07-27T10:42:07Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

Some changes occurred in need_type_info.rs

cc @lcnr

rustbot · 2026-07-27T10:42:10Z

r? @chenyukang

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LorrensP-2158466 · 2026-07-27T12:10:37Z

                && matches!(cx.tcx.get_diagnostic_name(adt_def.did()), Some(sym::Rc | sym::Arc))
            {
-                args.types().next() == Some(parent_ty)
+                args.iter().filter_map(ty::GenericArg::as_type).nth(0) == Some(parent_ty)


From @lcnr in the zulip discussion:

yeah, the fact that accessing just types is often vaguely wrong and doesn't allow for random access, so doing iter().filter_map(GenericArg::as_type).nth(1) is beter for that

View changes since the review

that can just be next().as_type() 🤔

chenyukang · 2026-07-30T01:43:53Z

is it this PR still in WIP?
r? @lcnr

lcnr · 2026-08-06T13:28:53Z

 fn get_ty_param(ty: Ty<'_>) -> Option<Ty<'_>> {
    if let ty::Adt(_, subs) = ty.kind() {
-        subs.types().next()
+        subs.iter().filter_map(ty::GenericArg::as_type).nth(0)


subs.iter().filter_map(ty::GenericArg::as_type).next() .nth(0) is always worse than next 🤔

View changes since the review

so iter().next().as_type(), gotcha.

khyperia

nice, thanks! lcnr asked me to take a peek.

I have a vague preference for .iter().nth(N).and_then(ty::GenericArg::as_type) over .iter().filter_map(ty::GenericArg::as_type).nth(N) when fetching the Nth type arg of a known type (the latter gives me spidey sense tingles around weird indexing, even though they're both equivalent since there's only type args for these known types), but that's an opinion that you're free to disregard~

View changes since this review

khyperia · 2026-08-07T04:37:42Z

+            },
+            // FIXME: check const parameters better as well. Currently this will consider `Array<5>` the same as
+            // `Array<6>`
+            (ty::TermKind::Const(c1), ty::TermKind::Const(c2)) if c1 == c2 => todo!(),


there's a todo!() here - probably should either keep the old behavior of considering all consts to be equal, or properly implement this, instead of panicing~

(today I learned that tidy doesn't run on src/tools/clippy, todo!() is banned by tidy)

Yeah, I meant to ask a follow up question on this.

Consts are interned as well, right? But are there cases where they are not the same "const" but are still the same? Otherwise this can just return true indeed.

(today I learned that tidy doesn't run on src/tools/clippy, todo!() is banned by tidy)

Yeah me to :D.

I changed it to => true for now.

For purposes of this real wonky function, yeah, consts can contain params. IMO, drop the guard, just have (ty::TermKind::Const(c1), ty::TermKind::Const(c2)) => (), to retain the old behavior, with the old comment. This function is only called in clippy::transmute_undefined_repr, which "has had multiple problems in the past and was moved to nursery" rust-lang/rust-clippy#8496 so preserving whatever wonky behavior it has for now seems best. (Hypothesizing a future fix, this should probably be a type relation or somesuch rather than doing a wonky custom relation on just ADTs? I mean, properly, I think maybe it should be doing a trait solver .eq() and checking if there's a solution. Idk.)

(Also, shouldn't return true but rather continue to the next term)

LorrensP-2158466 · 2026-08-08T08:23:21Z

I have a vague preference for .iter().nth(N).and_then(ty::GenericArg::as_type) over .iter().filter_map(ty::GenericArg::as_type).nth(N) when fetching the Nth type arg of a known type

I can get behind why this is better in the case of a known type. I'll change them.

LorrensP-2158466 · 2026-08-08T08:55:46Z

So i went with the recommendation of lcnr, doing next().and_then(as_type) instead of nth(0).and_then(as_type).

@rustbot ready.

LorrensP-2158466 · 2026-08-08T10:16:23Z

@rustbot author

rustbot · 2026-08-08T10:16:27Z

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

khyperia · 2026-08-10T05:21:13Z

 fn get_ty_param(ty: Ty<'_>) -> Option<Ty<'_>> {
    if let ty::Adt(_, subs) = ty.kind() {
-        subs.types().next()
+        subs.iter().next().and_then(ty::GenericArg::as_type)


could do subs.iter().next()?.as_type() in a few of these places, but whatever, that's a style choice, I don't care :P

View changes since the review

I mean, why not :D? It does look cleaner.

Oh, I just now noticed that using types() is actually correct here.

it is also used on the Cow<'a, T> type, and the first parameter is the lifetime. That's why the filter_map is correct here and that this new change cause a ci job to fail.

LorrensP-2158466 · 2026-08-10T10:59:47Z

@rustbot ready

- be explicit when only needing types of `GenericArgs`

LorrensP-2158466 · 2026-08-18T07:28:36Z

rebased to fix merge conflict and squash the commits that fix the CI failures.
@rustbot ready.

rustbot · 2026-08-18T07:30:20Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot added A-CI Area: Our Github Actions CI A-testsuite Area: The testsuite used to check the correctness of rustc T-infra Relevant to the infrastructure team, which will review and decide on the PR/issue. labels Jul 25, 2026

LorrensP-2158466 force-pushed the generic_args_terms branch 2 times, most recently from a16db39 to ea9f31c Compare July 25, 2026 16:36

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LorrensP-2158466 force-pushed the generic_args_terms branch from 26018f7 to 043bfcc Compare July 27, 2026 12:08

LorrensP-2158466 commented Jul 27, 2026

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rustbot assigned lcnr and unassigned chenyukang Jul 30, 2026

LorrensP-2158466 changed the title ~~[WIP]: GenericArgs::types triage + possible fixes~~ GenericArgs::types triage + possible fixes Jul 31, 2026

lcnr reviewed Aug 6, 2026

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LorrensP-2158466 added 2 commits August 18, 2026 09:27

- implement GenericArgs::terms and use where needed.

558dbb6

- be explicit when only needing types of `GenericArgs`

use next().and_then(as_type) instead of filter_map(as_type).nth(0)

715bd31

LorrensP-2158466 force-pushed the generic_args_terms branch from 1fc152f to 715bd31 Compare August 18, 2026 07:30

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