don't suggest unsized indirection in where-clauses

Skip where-clauses when suggesting using indirection in combination with
`?Sized` bounds on type parameters.

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1878,6 +1878,10 @@ impl<'v> Visitor<'v> for FindTypeParam {
         hir::intravisit::NestedVisitorMap::None
     }
 
+    fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
+        // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
+    }
+
     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be

src/test/ui/suggestions/issue-85943-no-suggest-unsized-indirection-in-where-clause.rs

@@ -0,0 +1,9 @@
+// Regression test for #85943: should not emit suggestions for adding
+// indirection to type parameters in where-clauses when suggesting
+// adding `?Sized`.
+struct A<T>(T) where T: Send;
+struct B(A<[u8]>);
+//~^ ERROR the size for values of type
+
+pub fn main() {
+}

src/test/ui/suggestions/issue-85943-no-suggest-unsized-indirection-in-where-clause.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/issue-85943-no-suggest-unsized-indirection-in-where-clause.rs:5:10
+   |
+LL | struct A<T>(T) where T: Send;
+   |          - required by this bound in `A`
+LL | struct B(A<[u8]>);
+   |          ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+help: you could relax the implicit `Sized` bound on `T` if it were used through indirection like `&T` or `Box<T>`
+  --> $DIR/issue-85943-no-suggest-unsized-indirection-in-where-clause.rs:4:10
+   |
+LL | struct A<T>(T) where T: Send;
+   |          ^  - ...if indirection were used here: `Box<T>`
+   |          |
+   |          this could be changed to `T: ?Sized`...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.