Point to closure return instead of output if defaulted

compiler/rustc_hir_typeck/src/check.rs

@@ -113,7 +113,11 @@ pub(super) fn check_fn<'a, 'tcx>(
 
     fcx.typeck_results.borrow_mut().liberated_fn_sigs_mut().insert(fn_id, fn_sig);
 
-    fcx.require_type_is_sized(declared_ret_ty, decl.output.span(), traits::SizedReturnType);
+    let return_or_body_span = match decl.output {
+        hir::FnRetTy::DefaultReturn(_) => body.value.span,
+        hir::FnRetTy::Return(ty) => ty.span,
+    };
+    fcx.require_type_is_sized(declared_ret_ty, return_or_body_span, traits::SizedReturnType);
     fcx.check_return_expr(&body.value, false);
 
     // We insert the deferred_generator_interiors entry after visiting the body.

src/tools/clippy/tests/ui/crashes/ice-6251.stderr

@@ -12,10 +12,10 @@ LL | fn bug<T>() -> impl Iterator<Item = [(); { |x: &[u8]| x }]> {
    |                                                +
 
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
-  --> $DIR/ice-6251.rs:4:53
+  --> $DIR/ice-6251.rs:4:54
    |
 LL | fn bug<T>() -> impl Iterator<Item = [(); { |x: [u8]| x }]> {
-   |                                                     ^ doesn't have a size known at compile-time
+   |                                                      ^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `[u8]`
    = note: the return type of a function must have a statically known size