Lower multi-segment const paths as `ConstArgKind::Path` #135186

camelid · 2025-01-07T05:21:38Z

This PR:

Lower all const paths in arg position to hir::ConstArgKind::Path
Then lower assoc const paths to ty::ConstKind::Unevaluated
(incomplete) Introduce #[type_const] attribute for trait assoc consts that are allowed as const args

Next steps:

Implement code to check that assoc const definitions satisfy #[type_const] if present (basically is it a const path or monomorphic anon const)

r? @BoxyUwU

rustbot · 2025-01-11T02:51:48Z

Some changes occurred in const_evaluatable.rs

cc @BoxyUwU

HIR ty lowering was modified

cc @fmease

BoxyUwU

thx for working on this :3

BoxyUwU · 2025-01-11T03:15:01Z

compiler/rustc_ast_lowering/src/lib.rs

@@ -1100,7 +1100,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                            .and_then(|partial_res| partial_res.full_res())
                        {
                            if !res.matches_ns(Namespace::TypeNS)
-                                && path.is_potential_trivial_const_arg()
+                            // FIXME: should this only allow single-segment paths?


Suggested change

// FIXME: should this only allow single-segment paths?

// FIXME(mgca): should this only allow single-segment paths?

unannotated FIXME will just get lost tbh, no way of tracking them down lol

Ah yeah, I meant it as a FIXME to deal with before merging this PR -- which is why I didn't label it. But since all of this is experimental anyway, we could deal with it later.

Oh in that case using // TODO will fail tidy and ensure the PR can't be merged until its resolved which ensures noone can forget about it before r+

compiler/rustc_ast/src/ast.rs

BoxyUwU · 2025-01-16T05:32:03Z

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

-    /// Literals and const generic parameters are eagerly converted to a constant, everything else
-    /// becomes `Unevaluated`.
+    #[instrument(level = "debug", skip(self), ret)]
+    pub fn lower_const_assoc_path(


We should definitely consolidate this and lower_assoc_path into one method that handles 99% of the logic since the only real behavioural differences are the AssocKind arguments we hard code and that we construct a Ty or Const at the very end depending on which method it is

BoxyUwU · 2025-01-16T05:32:27Z

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

+            match &candidates[..] {
+                [] => {}
+                &[(impl_, assoc)] => {
+                    // FIXME(mgca): adapted from temporary inherent assoc ty code that may be incorrect


same here we should just make probe_inherent_assoc_ty be able to be reused for consts somehow

BoxyUwU · 2025-01-16T05:33:38Z

compiler/rustc_hir_analysis/src/collect.rs

@@ -549,6 +549,95 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
        }
    }

+    fn lower_assoc_const(


this would probably also be good to unify with lower_assoc_ty somehow, at the very least it would be good to not duplicate almost 100 lines of diagnostics logic :3

Done. These two could now be on the dyn HirTyLowerer impl instead, with just the new lower_assoc_shared left as a trait method. I didn't do that though in case future impl-specific behavior was needed. Thoughts?

bors · 2025-01-23T01:51:57Z

☔ The latest upstream changes (presumably #135896) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned BoxyUwU Jan 7, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 7, 2025

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camelid changed the title ~~Start lowering multi-segment const paths as ConstArgKind::Path~~ Lower multi-segment const paths as ConstArgKind::Path Jan 7, 2025

camelid force-pushed the const-path-multi branch from c501669 to 4efbed3 Compare January 7, 2025 18:52

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camelid force-pushed the const-path-multi branch from 4efbed3 to 0036c2c Compare January 7, 2025 19:13

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camelid marked this pull request as ready for review January 11, 2025 02:51

camelid changed the title ~~Lower multi-segment const paths as ConstArgKind::Path~~ Lower multi-segment const paths as ConstArgKind::Path Jan 11, 2025

BoxyUwU reviewed Jan 16, 2025

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camelid force-pushed the const-path-multi branch from 263637d to a296ff8 Compare January 20, 2025 04:47

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camelid added 10 commits January 22, 2025 22:06

Start lowering multi-segment const paths as ConstArgKind::Path

39b402e

Partially fix parent_args handling for assoc const

ef6597f

Avoid "const uses param" error

afda88d

Replace TODOs with FIXMEs to allow CI to pass for now

53457f5

Properly lower assoc consts from traits

ea0691b

Avoid "const depends on generics" error for bare paths under mgca

fafacd7

Standardize mgca FIXMEs to "mgca"

2675d49

Start implementing #[type_const]

b422934

Allow const paths with generics/qself under mgca

fa9e8bf

Fix handling of const qpath args

ece9e77

camelid force-pushed the const-path-multi branch from a5a9370 to ece9e77 Compare January 23, 2025 06:13

BoxyUwU removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 23, 2025

BoxyUwU added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 23, 2025

Factor out shared code for lowering assoc item paths

030354f

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Lower multi-segment const paths as `ConstArgKind::Path` #135186

Lower multi-segment const paths as `ConstArgKind::Path` #135186

camelid commented Jan 7, 2025 •

edited

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bors commented Jan 23, 2025

	// FIXME: should this only allow single-segment paths?
	// FIXME(mgca): should this only allow single-segment paths?

Lower multi-segment const paths as ConstArgKind::Path #135186

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Lower multi-segment const paths as ConstArgKind::Path #135186

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bors commented Jan 23, 2025

Lower multi-segment const paths as `ConstArgKind::Path` #135186

Lower multi-segment const paths as `ConstArgKind::Path` #135186

camelid commented Jan 7, 2025 •

edited

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