Register ConstArgHasType obligation when normalizing trait projection consts

Author: lapla-cogito

compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

@@ -388,7 +388,16 @@ where
                 kind => panic!("expected projection, found {kind:?}"),
             };
 
-            ecx.instantiate_normalizes_to_term(goal, term.instantiate(cx, target_args));
+            let instantiated_term: I::Term = term.instantiate(cx, target_args);
+            if let Some(ct) = instantiated_term.as_const() {
+                let expected_ty = cx.type_of(target_item_def_id).instantiate(cx, target_args);
+                ecx.add_goal(
+                    GoalSource::Misc,
+                    goal.with(cx, ty::ClauseKind::ConstArgHasType(ct, expected_ty)),
+                );
+            }
+
+            ecx.instantiate_normalizes_to_term(goal, instantiated_term);
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2061,7 +2061,18 @@ fn confirm_impl_candidate<'cx, 'tcx>(
         Progress { term: err, obligations: nested }
     } else {
         assoc_term_own_obligations(selcx, obligation, &mut nested);
-        Progress { term: term.instantiate(tcx, args), obligations: nested }
+        let instantiated_term: Term<'tcx> = term.instantiate(tcx, args);
+        if let Some(ct) = instantiated_term.as_const() {
+            let expected_ty = tcx.type_of(assoc_term.item.def_id).instantiate(tcx, args);
+            nested.push(Obligation::with_depth(
+                tcx,
+                obligation.cause.clone(),
+                obligation.recursion_depth + 1,
+                obligation.param_env,
+                ty::ClauseKind::ConstArgHasType(ct, expected_ty),
+            ));
+        }
+        Progress { term: instantiated_term, obligations: nested }
     };
     Ok(Projected::Progress(progress))
 }

tests/ui/const-generics/mgca/type-const-value-type-mismatch.current.stderr

@@ -0,0 +1,32 @@
+error[E0053]: method `arr` has an incompatible type for trait
+  --> $DIR/type-const-value-type-mismatch.rs:23:5
+   |
+LL |     fn arr() -> [u8; const { Self::LEN }] {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an array with a size of 0, found one with a size of const { Self::LEN }
+   |
+note: type in trait
+  --> $DIR/type-const-value-type-mismatch.rs:15:5
+   |
+LL |     fn arr() -> [u8; Self::LEN];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected signature `fn() -> [u8; 0]`
+              found signature `fn() -> [u8; const { Self::LEN }]`
+
+error: the constant `0` is not of type `usize`
+  --> $DIR/type-const-value-type-mismatch.rs:20:5
+   |
+LL |     type const LEN: usize = 0u8;
+   |     ^^^^^^^^^^^^^^^^^^^^^ expected `usize`, found `u8`
+
+error[E0308]: mismatched types
+  --> $DIR/type-const-value-type-mismatch.rs:23:17
+   |
+LL |     fn arr() -> [u8; const { Self::LEN }] {}
+   |        ---      ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `[u8; const { Self::LEN }]`, found `()`
+   |        |
+   |        implicitly returns `()` as its body has no tail or `return` expression
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0053, E0308.
+For more information about an error, try `rustc --explain E0053`.

tests/ui/const-generics/mgca/type-const-value-type-mismatch.next.stderr

@@ -0,0 +1,45 @@
+error[E0271]: type mismatch resolving `<A as Array>::LEN normalizes-to 0`
+  --> $DIR/type-const-value-type-mismatch.rs:23:5
+   |
+LL |     fn arr() -> [u8; const { Self::LEN }] {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
+   |
+note: the requirement `<A as Array>::LEN normalizes-to 0` appears on the `impl`'s associated function `arr` but not on the corresponding trait's associated function
+  --> $DIR/type-const-value-type-mismatch.rs:15:8
+   |
+LL | pub trait Array {
+   |           ----- in this trait
+LL |     type const LEN: usize;
+LL |     fn arr() -> [u8; Self::LEN];
+   |        ^^^ this trait's associated function doesn't have the requirement `<A as Array>::LEN normalizes-to 0`
+
+error: the constant `0` is not of type `usize`
+  --> $DIR/type-const-value-type-mismatch.rs:20:5
+   |
+LL |     type const LEN: usize = 0u8;
+   |     ^^^^^^^^^^^^^^^^^^^^^ expected `usize`, found `u8`
+
+error[E0284]: type annotations needed: cannot normalize `<A as Array>::arr::{constant#0}`
+  --> $DIR/type-const-value-type-mismatch.rs:23:17
+   |
+LL |     fn arr() -> [u8; const { Self::LEN }] {}
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot normalize `<A as Array>::arr::{constant#0}`
+
+error[E0284]: type annotations needed: cannot satisfy `the constant `<Self as Array>::LEN` can be evaluated`
+  --> $DIR/type-const-value-type-mismatch.rs:15:17
+   |
+LL |     fn arr() -> [u8; Self::LEN];
+   |                 ^^^^^^^^^^^^^^^ cannot satisfy `the constant `<Self as Array>::LEN` can be evaluated`
+
+error[E0308]: mismatched types
+  --> $DIR/type-const-value-type-mismatch.rs:23:17
+   |
+LL |     fn arr() -> [u8; const { Self::LEN }] {}
+   |        ---      ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `[u8; const { Self::LEN }]`, found `()`
+   |        |
+   |        implicitly returns `()` as its body has no tail or `return` expression
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0271, E0284, E0308.
+For more information about an error, try `rustc --explain E0271`.

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

@@ -0,0 +1,30 @@
+// Regression test for https://github.com/rust-lang/rust/issues/152962
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ compile-flags: -Zvalidate-mir
+
+#![feature(min_generic_const_args)]
+#![expect(incomplete_features)]
+
+pub struct A;
+
+pub trait Array {
+    type const LEN: usize;
+    fn arr() -> [u8; Self::LEN];
+    //[next]~^ ERROR type annotations needed
+}
+
+impl Array for A {
+    type const LEN: usize = 0u8;
+    //~^ ERROR the constant `0` is not of type `usize`
+
+    fn arr() -> [u8; const { Self::LEN }] {}
+    //~^ ERROR mismatched types [E0308]
+    //[current]~| ERROR method `arr` has an incompatible type for trait [E0053]
+    //[next]~| ERROR type annotations needed
+    //[next]~| ERROR type mismatch resolving `<A as Array>::LEN normalizes-to 0` [E0271]
+}
+
+fn main() {}