mGCA: Validate const literal against expected type #152001

reddevilmidzy · 2026-02-02T16:10:33Z

also fix: #133966 (moved crashes test)

tests/ui/const-generics/mgca/literal-type-mismatch.rs

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

BoxyUwU · 2026-02-04T12:44:13Z

very cool :) thanks for working on this

…=oli-obk Dont strip const blocks in array lengths r? oli-obk mGCA now handles const blocks by *always* handling them during `lower_expr_to_const_arg_direct` instead of *sometimes* stripping them out at parse time. This is just generally a lot clearer/nicer but also means parsing isn't lossy which is just straight up wrong. We now use `MgcaDisambiguation::Direct` for const blocks because we "directly" represent a const block as `hir::ConstArgKind::Anon` :> The only time that an anon const for const generics uses `MgcaDisambiguation::AnonConst` is for unbraced literals. Once we properly support literals in `hir::ConstArgKind` (see rust-lang#152139 rust-lang#152001) then `MgcaDisambiguation` can be renamed to `AnonConstKind` with `TypeSystem` and `NonTypeSystem` variants. We can also get rid of `mgca_direct_lit_hack`. I expect this to be a very nice cleanup :) Fixes rust-lang/rustfmt#6788 The diff relating to passing spans around is to avoid a bunch of mGCA diagnostics changing from `const {}` to `{}`. I'm not entirely sure why this was happening. cc @rust-lang/rustfmt How do I run the tests in the rustfmt repo from here? `x test rustfmt` only seems to run like 100 tests and doesn't result in a `target/issue-6788.rs` getting created. I've verified locally that this formats correctly though

Rollup merge of #152234 - BoxyUwU:dont_strip_const_blocks, r=oli-obk Dont strip const blocks in array lengths r? oli-obk mGCA now handles const blocks by *always* handling them during `lower_expr_to_const_arg_direct` instead of *sometimes* stripping them out at parse time. This is just generally a lot clearer/nicer but also means parsing isn't lossy which is just straight up wrong. We now use `MgcaDisambiguation::Direct` for const blocks because we "directly" represent a const block as `hir::ConstArgKind::Anon` :> The only time that an anon const for const generics uses `MgcaDisambiguation::AnonConst` is for unbraced literals. Once we properly support literals in `hir::ConstArgKind` (see #152139 #152001) then `MgcaDisambiguation` can be renamed to `AnonConstKind` with `TypeSystem` and `NonTypeSystem` variants. We can also get rid of `mgca_direct_lit_hack`. I expect this to be a very nice cleanup :) Fixes rust-lang/rustfmt#6788 The diff relating to passing spans around is to avoid a bunch of mGCA diagnostics changing from `const {}` to `{}`. I'm not entirely sure why this was happening. cc @rust-lang/rustfmt How do I run the tests in the rustfmt repo from here? `x test rustfmt` only seems to run like 100 tests and doesn't result in a `target/issue-6788.rs` getting created. I've verified locally that this formats correctly though

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

compiler/rustc_mir_build/src/thir/constant.rs

reddevilmidzy · 2026-02-07T12:19:39Z

Haha, ICE occurs in other tests...

reddevilmidzy

Thanks for your quick review. 😊

View changes since this review

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

tests/ui/const-generics/mgca/type_const-mismatched-types.rs

reddevilmidzy

In ten crash tests, the ICE disappeared; however, in ‎tests/crashes/117460.rs, tests/crashes/114212-2.rs and tests/crashes/114212.rs, adding the main function caused the ICE to occur again.

View changes since this review

reddevilmidzy · 2026-02-08T09:12:14Z

tests/ui/const-generics/generic_const_exprs/enum-const-generic-default-param.rs

@@ -0,0 +1,23 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/116554>


This issue was closed; is there a need to add further testing?

unsure, I will look over all the GCE tests (outside of this PR) at some point soon and see which ones are worth keeping

tests/crashes/101036.rs

reddevilmidzy · 2026-02-08T09:35:23Z

Could the following issues also be closed? 🤔

#119824
#133966
#136416

already closed
#94846
#106473
#116554

compiler/rustc_mir_build/src/thir/constant.rs

compiler/rustc_trait_selection/src/traits/wf.rs

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

compiler/rustc_mir_build/src/thir/pattern/mod.rs

compiler/rustc_mir_build/src/thir/constant.rs

BoxyUwU · 2026-02-08T17:41:24Z

cool! sorry for a lot of comments but this is looking really good :)

reddevilmidzy · 2026-02-09T16:13:51Z

In my current code, an ICE will occur in the tests/ui/const-generics/generic_const_exprs/lit_type_mismatch.rs test.

Looking at the stacktrace and log, it appears that after returning None due to a type mismatch, the path continues without stopping at hir_ty_lowering, which then instantiates the default value. This causes an ICE error as instantiate is called with an empty preceding_args.

How would you recommend handling this situation?

reddevilmidzy · 2026-02-09T16:16:34Z

cool! sorry for a lot of comments but this is looking really good :)

No worries at all — I really appreciate the detailed feedback. Thanks for taking the time to review (_ _)

BoxyUwU · 2026-02-09T17:04:59Z

it appears that after returning None due to a type mismatch, the path continues without stopping at hir_ty_lowering, which then instantiates the default value

does it still ICE after removing some of the checks you had for types being equal? If so I'd just remove that test

BoxyUwU · 2026-02-10T12:34:59Z

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

-            .map(|l| tcx.at(expr.span).lit_to_const(l))
+        lit_input.and_then(|l| {
+            tcx.at(expr.span)
+                .lit_to_const(l)


so we now get a bunch of extra errors I think because this succeeds in more cases and results in duplication between the type checking errors of the anon const this was derived from, and type checking errors for the const arg matching the thing its a parameter to.

I think it would probably make sense (for diagnostics reasons) to do like .filter(|| const_lit_matches_ty()) so that we only do this fast path if the types are ok 🤔

Oh, that was the problem!! Added filtering

and const_lit_matches_ty() is used in two places: compiler/rustc_mir_build/src/thir/pattern/mod.rs and compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs. I was going to create a function for each, but I ended up creating a function in compiler/rustc_middle/src/mir/interpret/mod.rs.

Co-authored-by: Boxy <rust@boxyuwu.dev>

…eneric_const_exprs/lit_type_mismatch.rs

reddevilmidzy · 2026-02-11T05:18:48Z

tests/ui/const-generics/generic_const_exprs/lit_type_mismatch.rs

Removed tests related to generic_const_args that were causing crashes.

reddevilmidzy · 2026-02-11T05:20:20Z

tests/ui/const-generics/mgca/nonsensical-negated-literal.rs

+    foo::<{ Foo { field: -1_usize } }>();
+    //~^ ERROR: type annotations needed for the literal
+    foo::<{ Foo { field: { -1_usize } } }>();
+    //~^ ERROR: complex const arguments must be placed inside of a `const` block
+    foo::<{ Foo { field: -true } }>();
+    //~^ ERROR: the constant `true` is not of type `isize`
+    foo::<{ Foo { field: { -true } } }>();
+    //~^ ERROR: complex const arguments must be placed inside of a `const` block
+    foo::<{ Foo { field: -"<3" } }>();
+    //~^ ERROR: the constant `"<3"` is not of type `isize`
+    foo::<{ Foo { field: { -"<3" } } }>();
+    //~^ ERROR: complex const arguments must be placed inside of a `const` block


Added the test mentioned in #152139.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 2, 2026

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mGCA: Support directly represented negated literals #152123

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khyperia mentioned this pull request Feb 6, 2026

mGCA: Support directly represented negated literals #152139

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This was referenced Feb 6, 2026

Dont strip const blocks in array lengths #152234

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mGCA: remove parser special case for literals #152246

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reddevilmidzy and others added 2 commits February 11, 2026 05:01

mGCA: Validate const literal against expected type

5898b00

Co-authored-by: Boxy <rust@boxyuwu.dev>

modify error comment and bless test, delete tests/ui/const-generics/g…

177b1b0

…eneric_const_exprs/lit_type_mismatch.rs

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		@@ -0,0 +1,23 @@
		//! Regression test for <https://github.com/rust-lang/rust/issues/116554>

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