Reduce verbosity when calling for-loop on non-Iterator expression

compiler/rustc_middle/src/traits/mod.rs

@@ -350,6 +350,8 @@ pub enum ObligationCauseCode<'tcx> {
 
     AwaitableExpr,
 
+    ForLoopIterator,
+
     /// Well-formed checking. If a `WellFormedLoc` is provided,
     /// then it will be used to eprform HIR-based wf checking
     /// after an error occurs, in order to generate a more precise error span.

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

@@ -887,8 +887,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let span = obligation.cause.span;
 
         if let ObligationCauseCode::AwaitableExpr = obligation.cause.code {
-            // FIXME: use `trait_ref.self_ty().no_bound_vars()` to typecheck if `()` and if not
-            // maybe suggest returning instead?
+            // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
+            // and if not maybe suggest doing something else? If we kept the expression around we
+            // could also check if it is an fn call (very likely) and suggest changing *that*, if
+            // it is from the local crate.
             err.span_suggestion_verbose(
                 span,
                 "do not `.await` the expression",
@@ -1961,6 +1963,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             | ObligationCauseCode::ReturnValue(_)
             | ObligationCauseCode::BlockTailExpression(_)
             | ObligationCauseCode::AwaitableExpr
+            | ObligationCauseCode::ForLoopIterator
             | ObligationCauseCode::LetElse => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -810,8 +810,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             &substs,
             match lang_item {
                 hir::LangItem::FuturePoll => ObligationCauseCode::AwaitableExpr,
-                // FIXME: see if there are other obligation specializations we could do here beyond
-                // what we do above for `.await`.
+                hir::LangItem::IntoIterIntoIter => ObligationCauseCode::ForLoopIterator,
+                // FIXME: This could also be used for `?`.  See if there are others.
                 _ => traits::ItemObligation(def_id),
             },
         );

src/test/ui/issues/issue-33941.stderr

@@ -16,10 +16,10 @@ error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _,
   --> $DIR/issue-33941.rs:4:14
    |
 LL |     for _ in HashMap::new().iter().cloned() {}
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected reference, found tuple
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected tuple, found reference
    |
-   = note: expected reference `&_`
-                  found tuple `(&_, &_)`
+   = note:  expected tuple `(&_, &_)`
+           found reference `&_`
    = note: required because of the requirements on the impl of `Iterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`
    = note: required because of the requirements on the impl of `IntoIterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`