mGCA: error on region vars lowering - #157188

Human9000-bit · 2026-05-31T10:44:14Z

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This also needs to error on min_generic_const_args feature

cc @BoxyUwU (didn't r her as she is unavailable rn)

Fixes #157147

rustbot · 2026-05-31T10:44:17Z

HIR ty lowering was modified

cc @fmease

rustbot · 2026-05-31T10:44:18Z

Failed to set assignee to her: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

Human9000-bit · 2026-05-31T10:44:51Z

@rustbot reroll

reddevilmidzy

And if you wrap it in backticks, it probably won't be recognized as a command.😄

View changes since this review

BoxyUwU · 2026-06-01T14:33:37Z

r? BoxyUwU

BoxyUwU · 2026-06-01T14:34:19Z

I expect that a similar case should be possible without min_generic_const_args, instead using generic_const_parameter_types e.g. something like Foo<'a, const N: &'a ()> and then Foo<'_, { &() }> . In general I don't see a problem extending this fix to happen even on stable as it should never be meaningfully reachable (though it would be good to have a test case where it actually matters)

Human9000-bit · 2026-06-02T13:50:44Z

Is that what you meant?

I found a case with generic_const_parameter_types, but it seems that there is no obvious case to trigger the ICE without const generics features
(lowk used ai to find gcpt case)

Human9000-bit · 2026-06-03T13:22:10Z

it seems that ~~that clanker~~ ai hasn't checked that it triggers ICE even on non-incremental. Is there a problem with that?

EDIT: ofc there is. Removed the gcpt case

BoxyUwU · 2026-06-07T20:39:08Z

I actually am somewhat confused here, I thought that the has_free_regions check would trigger if there are region variables in the expected type. For example the following does error:

#![feature(generic_const_parameter_types, adt_const_params, const_param_ty_unchecked)]

fn evil<T, const R: T>() {}

fn foo<const N: usize>() {
    evil::<&u32, { &1_u32 }>();
}

what's the difference in expected types of the { &1_u32 } anon const in this example, and the const { &0_u8 } anon const in the mgca example? I would expect both of these to ICE not just the mGCA one

BoxyUwU · 2026-06-07T21:28:26Z

@rustbot author

Human9000-bit · 2026-06-14T15:02:50Z

I would expect both of these to ICE not just the mGCA one

Your example is correct in terms of it containing region var, but it being caught by existing check:

rust/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

Lines 2313 to 2314 in 4f8ea98

    
           if tcx.features().generic_const_parameter_types() 
        
               && (ty.has_free_regions() || ty.has_erased_regions())

, because gcpt enabled and const having free regions. Does that make sense?
@rustbot ready

BoxyUwU · 2026-06-15T15:40:59Z

hmm, what I meant is why does the ICEing example not get caught by that existing check. it too should have free regions in the type. i got confused before though and I realise now it's jsut because the check for this stuff only happens under gcpt but mgca also allows for gcpts implicitly.

I also realise now there's operator precedence jank at play in this diff 😅 :

if tcx.features().generic_const_parameter_types()
    && (ty.has_free_regions() || ty.has_erased_regions())
    || ty.has_infer_regions()
{

is actually

if (tcx.features().generic_const_parameter_types()
    && (ty.has_free_regions() || ty.has_erased_regions()))
    || ty.has_infer_regions()
{

I think your original diff was fine- making it be ``if (mgca || gcpt) && (has_free_regions || has_erased_regions)` seems correct enough to me 🤔

Human9000-bit · 2026-06-16T10:34:28Z

making it be if (mgca || gcpt) && (has_free_regions || has_erased_regions)

But this one gives some false negatives 🤔

BoxyUwU · 2026-06-19T13:00:56Z

do you have an example of these false negatives?

rust-log-analyzer · 2026-06-20T09:39:03Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

---- [ui] tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs#nogate stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/const-generics/mgca/type_const-generic-param-in-type.nogate/type_const-generic-param-in-type.nogate.stderr`
diff of stderr:

52 LL |     type const ASSOC_LT<'a>: [&'a (); 0] = const { [] };
53    |                                ^^ the type must not depend on the parameter `'a`
54 
- error: aborting due to 9 previous errors
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/type_const-generic-param-in-type.rs:16:35
+    |
+ LL | type const BAZ<'a>: [&'a (); 0] = const { [] };
+    |                                   ^^^^^^^^^^^^
+ 
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/type_const-generic-param-in-type.rs:40:44
+    |
+ LL |     type const ASSOC_LT<'a>: [&'a (); 0] = const { [] };
+    |                                            ^^^^^^^^^^^^
+ 
+ error: aborting due to 11 previous errors
56 
57 For more information about this error, try `rustc --explain E0770`.
---
-   --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:40:44
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/type_const-generic-param-in-type.rs:16:35
+    |
+ LL | type const BAZ<'a>: [&'a (); 0] = const { [] };
+    |                                   ^^^^^^^^^^^^
+ 
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/type_const-generic-param-in-type.rs:40:44
+    |
+ LL |     type const ASSOC_LT<'a>: [&'a (); 0] = const { [] };
+    |                                            ^^^^^^^^^^^^
+ 
+ error: aborting due to 11 previous errors


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args const-generics/mgca/type_const-generic-param-in-type.rs`

error in revision `nogate`: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--cfg" "nogate" "--check-cfg" "cfg(test,FALSE,nogate,gate)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/const-generics/mgca/type_const-generic-param-in-type.nogate" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:8:50
   |
LL | type const FOO<T: core::marker::ConstParamTy_>: [T; 0] = const { [] };
   |                                                  ^ the type must not depend on the parameter `T`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:12:38
   |
LL | type const BAR<const N: usize>: [(); N] = const { [] };
   |                                      ^ the type must not depend on the parameter `N`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:16:23
   |
LL | type const BAZ<'a>: [&'a (); 0] = const { [] };
   |                       ^^ the type must not depend on the parameter `'a`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:21:56
   |
LL |     type const ASSOC<T: core::marker::ConstParamTy_>: [T; 0];
   |                                                        ^ the type must not depend on the parameter `T`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:24:50
   |
LL |     type const ASSOC_CONST<const N: usize>: [(); N];
   |                                                  ^ the type must not depend on the parameter `N`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:27:32
   |
LL |     type const ASSOC_LT<'a>: [&'a (); 0];
   |                                ^^ the type must not depend on the parameter `'a`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:32:56
   |
LL |     type const ASSOC<T: core::marker::ConstParamTy_>: [T; 0] = const { [] };
   |                                                        ^ the type must not depend on the parameter `T`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:36:50
   |
LL |     type const ASSOC_CONST<const N: usize>: [(); N] = const { [] };
   |                                                  ^ the type must not depend on the parameter `N`

error[E0770]: the type of const parameters must not depend on other generic parameters
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:40:32
   |
LL |     type const ASSOC_LT<'a>: [&'a (); 0] = const { [] };
   |                                ^^ the type must not depend on the parameter `'a`

error: anonymous constants with lifetimes in their type are not yet supported
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:16:35
   |
LL | type const BAZ<'a>: [&'a (); 0] = const { [] };
   |                                   ^^^^^^^^^^^^

error: anonymous constants with lifetimes in their type are not yet supported
##[error]  --> /checkout/tests/ui/const-generics/mgca/type_const-generic-param-in-type.rs:40:44
   |
LL |     type const ASSOC_LT<'a>: [&'a (); 0] = const { [] };
   |                                            ^^^^^^^^^^^^

error: aborting due to 11 previous errors

For more information about this error, try `rustc --explain E0770`.
---

+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/assoc-const-no-infer-ice-115806.rs:17:59
+    |
+ LL | impl<USART, T> Pins<USART> for T where T: PinA<USART, A = const { &() }> {}
+    |                                                           ^^^^^^^^^^^^^
+ 
1 error[E0119]: conflicting implementations of trait `Pins<_>` for type `NoPin`
2   --> $DIR/assoc-const-no-infer-ice-115806.rs:17:1
3    |

9    |
10    = note: downstream crates may implement trait `PinA<_>` for type `NoPin`
11 
- error: aborting due to 1 previous error
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/assoc-const-no-infer-ice-115806.rs:12:33
+    |
+ LL |     type const A: &'static () = const { &() };
+    |                                 ^^^^^^^^^^^^^
+ 
+ error: aborting due to 3 previous errors
13 
14 For more information about this error, try `rustc --explain E0119`.
---
-   --> /checkout/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.rs:12:33
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/assoc-const-no-infer-ice-115806.rs:17:59
+    |
+ LL | impl<USART, T> Pins<USART> for T where T: PinA<USART, A = const { &() }> {}
+    |                                                           ^^^^^^^^^^^^^
+ 
+ error: anonymous constants with lifetimes in their type are not yet supported
+   --> $DIR/assoc-const-no-infer-ice-115806.rs:12:33
+    |
+ LL |     type const A: &'static () = const { &() };
+    |                                 ^^^^^^^^^^^^^
+ 
+ error: aborting due to 3 previous errors


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args generic-const-items/assoc-const-no-infer-ice-115806.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/generic-const-items/assoc-const-no-infer-ice-115806" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: anonymous constants with lifetimes in their type are not yet supported
##[error]  --> /checkout/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.rs:17:59
   |
LL | impl<USART, T> Pins<USART> for T where T: PinA<USART, A = const { &() }> {}
   |                                                           ^^^^^^^^^^^^^

error[E0119]: conflicting implementations of trait `Pins<_>` for type `NoPin`
##[error]  --> /checkout/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.rs:17:1
   |
LL | impl<TA> Pins<TA> for NoPin {}
   | --------------------------- first implementation here
...
LL | impl<USART, T> Pins<USART> for T where T: PinA<USART, A = const { &() }> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `NoPin`
   |
   = note: downstream crates may implement trait `PinA<_>` for type `NoPin`

error: anonymous constants with lifetimes in their type are not yet supported
##[error]  --> /checkout/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.rs:12:33
   |
LL |     type const A: &'static () = const { &() };
   |                                 ^^^^^^^^^^^^^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0119`.

Human9000-bit · 2026-06-20T11:16:10Z

does this count?

BoxyUwU · 2026-06-23T09:52:30Z

those errors seem expected/okay to me

BoxyUwU · 2026-07-08T14:13:09Z

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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 May 31, 2026

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Comment thread tests/ui/const-generics/mgca/region_vars_hashed.rs Outdated

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Avoid regions vars in const args lowering

615a10c

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