Use the "nice E0277 errors"[1] for !Send impl Future from foreign crate

Partly address #78543 by making the error quieter.

We don't have access to the `typeck` tables from foreign crates, so we
used to completely skip the new code when checking foreign crates. Now,
we carry on and don't provide as nice output (we don't clarify *what* is
making the `Future: !Send`), but at least we no longer emit a sea of
derived obligations in the output.

[1]: https://blog.rust-lang.org/inside-rust/2019/10/11/AsyncAwait-Not-Send-Error-Improvements.html

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

@@ -151,7 +151,7 @@ pub trait InferCtxtExt<'tcx> {
         outer_generator: Option<DefId>,
         trait_ref: ty::TraitRef<'tcx>,
         target_ty: Ty<'tcx>,
-        typeck_results: &ty::TypeckResults<'tcx>,
+        typeck_results: Option<&ty::TypeckResults<'tcx>>,
         obligation: &PredicateObligation<'tcx>,
         next_code: Option<&ObligationCauseCode<'tcx>>,
     );
@@ -1463,11 +1463,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
 
         // Only continue if a generator was found.
-        debug!(
+        debug!(?generator, ?trait_ref, ?target_ty, "maybe_note_obligation_cause_for_async_await");
-            "maybe_note_obligation_cause_for_async_await: generator={:?} trait_ref={:?} \
-                target_ty={:?}",
-            generator, trait_ref, target_ty
-        );
         let (generator_did, trait_ref, target_ty) = match (generator, trait_ref, target_ty) {
             (Some(generator_did), Some(trait_ref), Some(target_ty)) => {
                 (generator_did, trait_ref, target_ty)
@@ -1477,14 +1473,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         let span = self.tcx.def_span(generator_did);
 
-        // Do not ICE on closure typeck (#66868).
-        if !generator_did.is_local() {
-            return false;
-        }
-
-        // Get the typeck results from the infcx if the generator is the function we are
-        // currently type-checking; otherwise, get them by performing a query.
-        // This is needed to avoid cycles.
         let in_progress_typeck_results = self.in_progress_typeck_results.map(|t| t.borrow());
         let generator_did_root = self.tcx.closure_base_def_id(generator_did);
         debug!(
@@ -1495,14 +1483,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             in_progress_typeck_results.as_ref().map(|t| t.hir_owner),
             span
         );
-        let query_typeck_results;
-        let typeck_results: &TypeckResults<'tcx> = match &in_progress_typeck_results {
-            Some(t) if t.hir_owner.to_def_id() == generator_did_root => t,
-            _ => {
-                query_typeck_results = self.tcx.typeck(generator_did.expect_local());
-                &query_typeck_results
-            }
-        };
 
         let generator_body = generator_did
             .as_local()
@@ -1545,6 +1525,20 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let mut interior_or_upvar_span = None;
         let mut interior_extra_info = None;
 
+        // Get the typeck results from the infcx if the generator is the function we are currently
+        // type-checking; otherwise, get them by performing a query.  This is needed to avoid
+        // cycles. If we can't use resolved types because the generator comes from another crate,
+        // we still provide a targeted error but without all the relevant spans.
+        let query_typeck_results;
+        let typeck_results: Option<&TypeckResults<'tcx>> = match &in_progress_typeck_results {
+            Some(t) if t.hir_owner.to_def_id() == generator_did_root => Some(&t),
+            _ if generator_did.is_local() => {
+                query_typeck_results = self.tcx.typeck(generator_did.expect_local());
+                Some(&query_typeck_results)
+            }
+            _ => None, // Do not ICE on closure typeck (#66868).
+        };
+        if let Some(typeck_results) = typeck_results {
             if let Some(upvars) = self.tcx.upvars_mentioned(generator_did) {
                 interior_or_upvar_span = upvars.iter().find_map(|(upvar_id, upvar)| {
                     let upvar_ty = typeck_results.node_type(*upvar_id);
@@ -1571,25 +1565,19 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     .into_iter()
                     .map(|id| hir.expect_expr(id))
                     .find(|await_expr| {
-                    let ty = typeck_results.expr_ty_adjusted(&await_expr);
+                        ty_matches(ty::Binder::dummy(typeck_results.expr_ty_adjusted(&await_expr)))
-                    debug!(
-                        "maybe_note_obligation_cause_for_async_await: await_expr={:?}",
-                        await_expr
-                    );
-                    ty_matches(ty::Binder::dummy(ty))
                     })
                     .map(|expr| expr.span);
-            let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } = cause;
+                let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } =
+                    cause;
 
                 interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(*span));
                 interior_extra_info = Some((*scope_span, *yield_span, *expr, from_awaited_ty));
             };
+        } else {
+            interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(span));
+        }
 
-        debug!(
-            "maybe_note_obligation_cause_for_async_await: interior_or_upvar={:?} \
-                generator_interior_types={:?}",
-            interior_or_upvar_span, typeck_results.generator_interior_types
-        );
         if let Some(interior_or_upvar_span) = interior_or_upvar_span {
             self.note_obligation_cause_for_async_await(
                 err,
@@ -1620,7 +1608,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         outer_generator: Option<DefId>,
         trait_ref: ty::TraitRef<'tcx>,
         target_ty: Ty<'tcx>,
-        typeck_results: &ty::TypeckResults<'tcx>,
+        typeck_results: Option<&ty::TypeckResults<'tcx>>,
         obligation: &PredicateObligation<'tcx>,
         next_code: Option<&ObligationCauseCode<'tcx>>,
     ) {
@@ -1831,7 +1819,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         // Look at the last interior type to get a span for the `.await`.
                         debug!(
                             "note_obligation_cause_for_async_await generator_interior_types: {:#?}",
-                            typeck_results.generator_interior_types
+                            typeck_results.as_ref().map(|t| &t.generator_interior_types)
                         );
                         explain_yield(interior_span, yield_span, scope_span);
                     }
@@ -1852,10 +1840,14 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             // ^^^^^^^ a temporary `&T` created inside this method call due to `&self`
                             // ```
                             //
-                            let is_region_borrow = typeck_results
+                            let is_region_borrow = if let Some(typeck_results) = typeck_results {
+                                typeck_results
                                     .expr_adjustments(expr)
                                     .iter()
-                                .any(|adj| adj.is_region_borrow());
+                                    .any(|adj| adj.is_region_borrow())
+                            } else {
+                                false
+                            };
 
                             // ```rust
                             // struct Foo(*const u8);
@@ -1868,7 +1860,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                     DefKind::Fn | DefKind::Ctor(..) => target_ty.is_unsafe_ptr(),
                                     _ => false,
                                 };
-
+                            if let Some(typeck_results) = typeck_results {
                                 if (typeck_results.is_method_call(e) && is_region_borrow)
                                     || is_raw_borrow_inside_fn_like_call
                                 {
@@ -1882,6 +1874,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         }
                     }
                 }
+            }
             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
                 // `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
                 let refers_to_non_sync = match target_ty.kind() {

src/test/ui/async-await/issues/issue-67893.rs

@@ -7,5 +7,5 @@ fn g(_: impl Send) {}
 
 fn main() {
     g(issue_67893::run())
-    //~^ ERROR: `MutexGuard<'_, ()>` cannot be sent between threads safely
+    //~^ ERROR generator cannot be sent between threads safely
 }

src/test/ui/async-await/issues/issue-67893.stderr

@@ -1,20 +1,10 @@
-error[E0277]: `MutexGuard<'_, ()>` cannot be sent between threads safely
+error: generator cannot be sent between threads safely
   --> $DIR/issue-67893.rs:9:5
    |
 LL |     g(issue_67893::run())
-   |     ^ `MutexGuard<'_, ()>` cannot be sent between threads safely
+   |     ^ generator is not `Send`
-   |
-  ::: $DIR/auxiliary/issue_67893.rs:7:20
-   |
-LL | pub async fn run() {
-   |                    - within this `impl Future`
    |
    = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
-   = note: required because it appears within the type `for<'r, 's, 't0, 't1, 't2, 't3> {ResumeTy, Arc<Mutex<()>>, &'r Mutex<()>, Result<MutexGuard<'s, ()>, PoisonError<MutexGuard<'t0, ()>>>, &'t1 MutexGuard<'t2, ()>, MutexGuard<'t3, ()>, (), impl Future}`
-   = note: required because it appears within the type `[static generator@run::{closure#0}]`
-   = note: required because it appears within the type `from_generator::GenFuture<[static generator@run::{closure#0}]>`
-   = note: required because it appears within the type `impl Future`
-   = note: required because it appears within the type `impl Future`
 note: required by a bound in `g`
   --> $DIR/issue-67893.rs:6:14
    |
@@ -23,4 +13,3 @@ LL | fn g(_: impl Send) {}
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.