Suppress lint type_alias_bounds for ty aliases containing const projections under GCE

Author: fmease

compiler/rustc_lint/src/builtin.rs

@@ -1437,6 +1437,16 @@ impl<'tcx> LateLintPass<'tcx> for TypeAliasBounds {
             return;
         }
 
+
+        // FIXME(generic_const_exprs): Revisit this before stabilization.
+        // See also `tests/ui/const-generics/generic_const_exprs/type-alias-bounds.rs`.
+        let ty = cx.tcx.type_of(item.owner_id).instantiate_identity();
+        if ty.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION)
+            && cx.tcx.features().generic_const_exprs
+        {
+            return;
+        }
+
         // NOTE(inherent_associated_types): While we currently do take some bounds in type
         // aliases into consideration during IAT *selection*, we don't perform full use+def
         // site wfchecking for such type aliases. Therefore TAB should still trigger.

tests/ui/const-generics/generic_const_exprs/type-alias-bounds.neg.stderr

@@ -0,0 +1,63 @@
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/type-alias-bounds.rs:23:12
+   |
+LL |     let _: AliasConstUnused<String>;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+note: required by a bound in `ct_unused_0::AliasConstUnused`
+  --> $DIR/type-alias-bounds.rs:20:30
+   |
+LL |     type AliasConstUnused<T: Copy> = (T, I32<{ DATA }>);
+   |                              ^^^^ required by this bound in `AliasConstUnused`
+
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/type-alias-bounds.rs:31:12
+   |
+LL |     let _: AliasConstUnused;
+   |            ^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+note: required by a bound in `ct_unused_1::AliasConstUnused`
+  --> $DIR/type-alias-bounds.rs:29:41
+   |
+LL |     type AliasConstUnused where String: Copy = I32<{ 0; 0 }>;
+   |                                         ^^^^ required by this bound in `AliasConstUnused`
+
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/type-alias-bounds.rs:39:12
+   |
+LL |     let _: AliasFnUnused<String>;
+   |            ^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+note: required by a bound in `AliasFnUnused`
+  --> $DIR/type-alias-bounds.rs:36:27
+   |
+LL |     type AliasFnUnused<T: Copy> = (T, I32<{ code() }>);
+   |                           ^^^^ required by this bound in `AliasFnUnused`
+
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/type-alias-bounds.rs:57:12
+   |
+LL |     let _: AliasAssocConstUsed<String>;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+note: required by a bound in `AliasAssocConstUsed`
+  --> $DIR/type-alias-bounds.rs:55:41
+   |
+LL |     type AliasAssocConstUsed<T: Trait + Copy> = I32<{ T::DATA }>;
+   |                                         ^^^^ required by this bound in `AliasAssocConstUsed`
+
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/type-alias-bounds.rs:65:12
+   |
+LL |     let _: AliasFnUsed<String>;
+   |            ^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+note: required by a bound in `AliasFnUsed`
+  --> $DIR/type-alias-bounds.rs:62:33
+   |
+LL |     type AliasFnUsed<T: Trait + Copy> = I32<{ code::<T>() }>;
+   |                                 ^^^^ required by this bound in `AliasFnUsed`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/const-generics/generic_const_exprs/type-alias-bounds.rs

@@ -0,0 +1,71 @@
+//@ revisions: pos neg
+//@[pos] check-pass
+
+#![feature(generic_const_exprs)]
+#![feature(trivial_bounds)] // only used in test case `ct_unused_1`
+#![allow(incomplete_features)]
+
+// FIXME(generic_const_exprs): Revisit this before stabilization.
+// Check that we don't emit the lint `type_alias_bounds` for (eager) type aliases
+// whose RHS contains a const projection (aka uneval'ed const).
+// Since anon consts inherit the parent generics and predicates and we effectively
+// check them before and after instantiaton for well-formedness, the type alias
+// bounds are in every sense "enforced".
+// Note that the test cases whose name ends in "unused" just demonstrate that this
+// holds even if the const projections don't "visibly" capture any generics and/or
+// predicates.
+#![deny(type_alias_bounds)]
+
+fn ct_unused_0() {
+    type AliasConstUnused<T: Copy> = (T, I32<{ DATA }>);
+    const DATA: i32 = 0;
+    #[cfg(neg)]
+    let _: AliasConstUnused<String>;
+    //[neg]~^ ERROR the trait bound `String: Copy` is not satisfied
+}
+
+fn ct_unused_1() {
+    #[allow(trivial_bounds)]
+    type AliasConstUnused where String: Copy = I32<{ 0; 0 }>;
+    #[cfg(neg)]
+    let _: AliasConstUnused;
+    //[neg]~^ ERROR the trait bound `String: Copy` is not satisfied
+}
+
+fn fn_unused() {
+    type AliasFnUnused<T: Copy> = (T, I32<{ code() }>);
+    const fn code() -> i32 { 0 }
+    #[cfg(neg)]
+    let _: AliasFnUnused<String>;
+    //[neg]~^ ERROR the trait bound `String: Copy` is not satisfied
+}
+
+trait Trait {
+    type Proj;
+    const DATA: i32;
+}
+
+impl Trait for String {
+    type Proj = i32;
+    const DATA: i32 = 0;
+}
+
+// Regression test for issue #94398.
+fn assoc_ct_used() {
+    type AliasAssocConstUsed<T: Trait + Copy> = I32<{ T::DATA }>;
+    #[cfg(neg)]
+    let _: AliasAssocConstUsed<String>;
+    //[neg]~^ ERROR the trait bound `String: Copy` is not satisfied
+}
+
+fn fn_used() {
+    type AliasFnUsed<T: Trait + Copy> = I32<{ code::<T>() }>;
+    const fn code<T: Trait>() -> i32 { T::DATA }
+    #[cfg(neg)]
+    let _: AliasFnUsed<String>;
+    //[neg]~^ ERROR the trait bound `String: Copy` is not satisfied
+}
+
+struct I32<const N: i32>;
+
+fn main() {}