GCI: Check where-clauses at the def site for well-formedness

Author: fmease

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -307,7 +307,11 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
             check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
         }
         hir::ItemKind::Const(ty, ..) => {
-            check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
+            let res = check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid);
+            res.and(enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
+                check_where_clauses(wfcx, item.span, def_id);
+                Ok(())
+            }))
         }
         hir::ItemKind::Struct(_, hir_generics) => {
             let res = check_type_defn(tcx, item, false);

tests/ui/generic-const-items/def-site-predicates-wf.current.stderr

@@ -0,0 +1,22 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:11:5
+   |
+LL |     <Vec<str> as Discard>::Output:;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/def-site-predicates-wf.rs:9:1
+   |
+LL | / const _: () = ()
+LL | | where
+LL | |     <Vec<str> as Discard>::Output:;
+   | |___________________________________^ entering unreachable code
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0080, E0277.
+For more information about an error, try `rustc --explain E0080`.

tests/ui/generic-const-items/def-site-predicates-wf.next.stderr

@@ -0,0 +1,32 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:11:5
+   |
+LL |     <Vec<str> as Discard>::Output:;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/def-site-predicates-wf.rs:17:36
+   |
+LL |     <Vec<str> as Discard>::Output: Sized;
+   |                                    ^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by an implicit `Sized` bound in `Vec`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/def-site-predicates-wf.rs:9:1
+   |
+LL | / const _: () = ()
+LL | | where
+LL | |     <Vec<str> as Discard>::Output:;
+   | |___________________________________^ entering unreachable code
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0080, E0277.
+For more information about an error, try `rustc --explain E0080`.

tests/ui/generic-const-items/def-site-predicates-wf.rs

@@ -0,0 +1,22 @@
+//! Ensure that we check the predicates at the definition site for well-formedness.
+#![feature(generic_const_items)]
+#![allow(incomplete_features)]
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+const _: () = ()
+where
+    <Vec<str> as Discard>::Output:; //~ ERROR the size for values of type `str` cannot be known at compilation time
+//~^^^ ERROR evaluation of constant value failed
+
+// FIXME(#100041): This should error in the current solver, too.
+const _: () = ()
+where
+    <Vec<str> as Discard>::Output: Sized; //[next]~ ERROR the size for values of type `str` cannot be known at compilation time
+
+trait Discard { type Output; }
+impl<T> Discard for T { type Output = (); }
+
+fn main() {}