Rollup merge of #86455 - tlyu:check-where-before-suggesting-unsized, r=estebank

check where-clause for explicit `Sized` before suggesting `?Sized`

Fixes #85945.

Based on #86454.

``@rustbot`` label +A-diagnostics +A-traits +A-typesystem +D-papercut +T-compiler

src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.rs

@@ -0,0 +1,8 @@
+// Regression test for #85945: Don't suggest `?Sized` bound if an explicit
+// `Sized` bound is already in a `where` clause.
+fn foo<T>(_: &T) where T: Sized {}
+fn bar() { foo(""); }
+//~^ERROR the size for values of type
+
+pub fn main() {
+}

src/test/ui/suggestions/issue-85945-check-where-clause-before-suggesting-unsized.stderr

@@ -0,0 +1,18 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/issue-85945-check-where-clause-before-suggesting-unsized.rs:4:16
+   |
+LL | fn bar() { foo(""); }
+   |            --- ^^ doesn't have a size known at compile-time
+   |            |
+   |            required by a bound introduced by this call
+   |
+   = help: the trait `Sized` is not implemented for `str`
+note: required by a bound in `foo`
+  --> $DIR/issue-85945-check-where-clause-before-suggesting-unsized.rs:3:8
+   |
+LL | fn foo<T>(_: &T) where T: Sized {}
+   |        ^ required by this bound in `foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.