From 924374c75b6ec99c5d95847f6c54554d8d590716 Mon Sep 17 00:00:00 2001
From: Redddy <rust@redddy.com>
Date: Tue, 17 Mar 2026 06:30:04 +0000
Subject: [PATCH] mGCA: Lower const generic args to infer when needed

---
 compiler/rustc_ast_lowering/src/lib.rs        | 10 +++--
 .../const-generics/mgca/braced-const-infer.rs |  9 ++++
 .../mgca/braced-const-infer.stderr            |  9 ++++
 .../mgca/macro-const-arg-infer.rs             | 20 +++++++++
 .../mgca/macro-const-arg-infer.stderr         | 44 +++++++++++++++++++
 5 files changed, 89 insertions(+), 3 deletions(-)
 create mode 100644 tests/ui/const-generics/mgca/braced-const-infer.rs
 create mode 100644 tests/ui/const-generics/mgca/braced-const-infer.stderr
 create mode 100644 tests/ui/const-generics/mgca/macro-const-arg-infer.rs
 create mode 100644 tests/ui/const-generics/mgca/macro-const-arg-infer.stderr

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index e8092b540ec87..7a0537b0e7c6e 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1269,9 +1269,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
                 GenericArg::Type(self.lower_ty_alloc(ty, itctx).try_as_ambig_ty().unwrap())
             }
-            ast::GenericArg::Const(ct) => GenericArg::Const(
-                self.lower_anon_const_to_const_arg_and_alloc(ct).try_as_ambig_ct().unwrap(),
-            ),
+            ast::GenericArg::Const(ct) => {
+                let ct = self.lower_anon_const_to_const_arg_and_alloc(ct);
+                match ct.try_as_ambig_ct() {
+                    Some(ct) => GenericArg::Const(ct),
+                    None => GenericArg::Infer(hir::InferArg { hir_id: ct.hir_id, span: ct.span }),
+                }
+            }
         }
     }
 
diff --git a/tests/ui/const-generics/mgca/braced-const-infer.rs b/tests/ui/const-generics/mgca/braced-const-infer.rs
new file mode 100644
index 0000000000000..9b007d338d1da
--- /dev/null
+++ b/tests/ui/const-generics/mgca/braced-const-infer.rs
@@ -0,0 +1,9 @@
+//! Regression test for https://github.com/rust-lang/rust/issues/153198
+#![feature(min_generic_const_args)]
+#![allow(incomplete_features, rust_2021_compatibility)]
+
+trait Trait<T> {}
+
+impl dyn Trait<{_}> {} //~ ERROR: the placeholder `_` is not allowed within types on item signatures
+
+fn main() {}
diff --git a/tests/ui/const-generics/mgca/braced-const-infer.stderr b/tests/ui/const-generics/mgca/braced-const-infer.stderr
new file mode 100644
index 0000000000000..d8ffee4cd0e8f
--- /dev/null
+++ b/tests/ui/const-generics/mgca/braced-const-infer.stderr
@@ -0,0 +1,9 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for implementations
+  --> $DIR/braced-const-infer.rs:7:17
+   |
+LL | impl dyn Trait<{_}> {}
+   |                 ^ not allowed in type signatures
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0121`.
diff --git a/tests/ui/const-generics/mgca/macro-const-arg-infer.rs b/tests/ui/const-generics/mgca/macro-const-arg-infer.rs
new file mode 100644
index 0000000000000..017c63d8b0db5
--- /dev/null
+++ b/tests/ui/const-generics/mgca/macro-const-arg-infer.rs
@@ -0,0 +1,20 @@
+//! Regression test for https://github.com/rust-lang/rust/issues/153198
+#![feature(min_generic_const_args)]
+#![allow(incomplete_features)]
+macro_rules! y {
+    ( $($matcher:tt)*) => {
+        _ //~ ERROR: the placeholder `_` is not allowed within types on item signatures
+    };
+}
+
+struct A<T>; //~ ERROR: type parameter `T` is never used
+
+const y: A<
+    {
+        y! {
+            x
+        }
+    },
+> = 1; //~ ERROR: mismatched types
+
+fn main() {}
diff --git a/tests/ui/const-generics/mgca/macro-const-arg-infer.stderr b/tests/ui/const-generics/mgca/macro-const-arg-infer.stderr
new file mode 100644
index 0000000000000..406206c59e18f
--- /dev/null
+++ b/tests/ui/const-generics/mgca/macro-const-arg-infer.stderr
@@ -0,0 +1,44 @@
+error[E0392]: type parameter `T` is never used
+  --> $DIR/macro-const-arg-infer.rs:10:10
+   |
+LL | struct A<T>;
+   |          ^ unused type parameter
+   |
+   = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
+
+error[E0308]: mismatched types
+  --> $DIR/macro-const-arg-infer.rs:18:5
+   |
+LL |   const y: A<
+   |  __________-
+LL | |     {
+LL | |         y! {
+LL | |             x
+LL | |         }
+LL | |     },
+LL | | > = 1;
+   | | -   ^ expected `A<_>`, found integer
+   | |_|
+   |   expected because of the type of the constant
+   |
+   = note: expected struct `A<_>`
+                found type `{integer}`
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
+  --> $DIR/macro-const-arg-infer.rs:6:9
+   |
+LL |           _
+   |           ^ not allowed in type signatures
+...
+LL | /         y! {
+LL | |             x
+LL | |         }
+   | |_________- in this macro invocation
+   |
+   = note: this error originates in the macro `y` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0121, E0308, E0392.
+For more information about an error, try `rustc --explain E0121`.