When .await is called on a non-Future expression, suggest removal

Keep track of the origin of a `T: Future` obligation when caused by an
`.await` expression.

Address #66731.

Author: estebank

Diff for: compiler/rustc_ast_lowering/src/expr.rs

@@ -607,6 +607,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     /// }
     /// ```
     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
+        let dot_await_span = expr.span.shrink_to_hi().to(await_span);
         match self.generator_kind {
             Some(hir::GeneratorKind::Async(_)) => {}
             Some(hir::GeneratorKind::Gen) | None => {
@@ -623,7 +624,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 err.emit();
             }
         }
-        let span = self.mark_span_with_reason(DesugaringKind::Await, await_span, None);
+        let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
         let gen_future_span = self.mark_span_with_reason(
             DesugaringKind::Await,
             await_span,
@@ -682,7 +683,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let break_x = self.with_loop_scope(loop_node_id, move |this| {
                 let expr_break =
                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
-                this.arena.alloc(this.expr(await_span, expr_break, ThinVec::new()))
+                this.arena.alloc(this.expr(span, expr_break, ThinVec::new()))
             });
             self.arm(ready_pat, break_x)
         };

Diff for: compiler/rustc_middle/src/traits/mod.rs

@@ -348,6 +348,8 @@ pub enum ObligationCauseCode<'tcx> {
     /// If `X` is the concrete type of an opaque type `impl Y`, then `X` must implement `Y`
     OpaqueType,
 
+    AwaitableExpr,
+
     /// 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.

Diff for: compiler/rustc_parse/src/parser/expr.rs

@@ -2838,8 +2838,8 @@ impl<'a> Parser<'a> {
         ExprKind::Call(f, args)
     }
 
-    fn mk_await_expr(&mut self, self_arg: P<Expr>, lo: Span) -> P<Expr> {
-        let span = lo.to(self.prev_token.span);
+    fn mk_await_expr(&mut self, self_arg: P<Expr>, _lo: Span) -> P<Expr> {
+        let span = self.prev_token.span;
         let await_expr = self.mk_expr(span, ExprKind::Await(self_arg), AttrVec::new());
         self.recover_from_await_method_call();
         await_expr

Diff for: compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -439,6 +439,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         self.suggest_remove_reference(&obligation, &mut err, trait_ref);
                         self.suggest_semicolon_removal(&obligation, &mut err, span, trait_ref);
                         self.note_version_mismatch(&mut err, &trait_ref);
+                        self.suggest_remove_await(&obligation, &mut err);
 
                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
                             self.suggest_await_before_try(&mut err, &obligation, trait_ref, span);

Diff for: compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -89,6 +89,12 @@ pub trait InferCtxtExt<'tcx> {
         trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
     );
 
+    fn suggest_remove_await(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut DiagnosticBuilder<'_>,
+    );
+
     fn suggest_change_mut(
         &self,
         obligation: &PredicateObligation<'tcx>,
@@ -873,6 +879,25 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
     }
 
+    fn suggest_remove_await(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut DiagnosticBuilder<'_>,
+    ) {
+        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?
+            err.span_suggestion_verbose(
+                span,
+                "do not `.await` the expression",
+                String::new(),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+
     /// Check if the trait bound is implemented for a different mutability and note it in the
     /// final error.
     fn suggest_change_mut(
@@ -1935,6 +1960,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             | ObligationCauseCode::ReturnType
             | ObligationCauseCode::ReturnValue(_)
             | ObligationCauseCode::BlockTailExpression(_)
+            | ObligationCauseCode::AwaitableExpr
             | ObligationCauseCode::LetElse => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");

Diff for: compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -804,7 +804,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let ty = item_ty.subst(self.tcx, substs);
 
         self.write_resolution(hir_id, Ok((def_kind, def_id)));
-        self.add_required_obligations(span, def_id, &substs);
+        self.add_required_obligations_with_code(
+            span,
+            def_id,
+            &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`.
+                _ => traits::ItemObligation(def_id),
+            },
+        );
         (Res::Def(def_kind, def_id), ty)
     }
 
@@ -1486,12 +1496,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     /// Add all the obligations that are required, substituting and normalized appropriately.
-    #[tracing::instrument(level = "debug", skip(self, span, def_id, substs))]
     crate fn add_required_obligations(&self, span: Span, def_id: DefId, substs: &SubstsRef<'tcx>) {
+        self.add_required_obligations_with_code(
+            span,
+            def_id,
+            substs,
+            traits::ItemObligation(def_id),
+        )
+    }
+
+    #[tracing::instrument(level = "debug", skip(self, span, def_id, substs))]
+    fn add_required_obligations_with_code(
+        &self,
+        span: Span,
+        def_id: DefId,
+        substs: &SubstsRef<'tcx>,
+        code: ObligationCauseCode<'tcx>,
+    ) {
         let (bounds, _) = self.instantiate_bounds(span, def_id, &substs);
 
         for obligation in traits::predicates_for_generics(
-            traits::ObligationCause::new(span, self.body_id, traits::ItemObligation(def_id)),
+            traits::ObligationCause::new(span, self.body_id, code),
             self.param_env,
             bounds,
         ) {

Diff for: src/test/ui/async-await/async-error-span.stderr

@@ -14,10 +14,10 @@ LL |     let a;
    |         ^ cannot infer type
    |
 note: the type is part of the `async fn` body because of this `await`
-  --> $DIR/async-error-span.rs:14:5
+  --> $DIR/async-error-span.rs:14:17
    |
 LL |     get_future().await;
-   |     ^^^^^^^^^^^^^^^^^^
+   |                 ^^^^^^
 
 error: aborting due to 2 previous errors
 

Diff for: src/test/ui/async-await/async-fn-nonsend.stderr

@@ -6,13 +6,13 @@ LL |     assert_send(local_dropped_before_await());
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/async-fn-nonsend.rs:24:5
+  --> $DIR/async-fn-nonsend.rs:24:10
    |
 LL |     let x = non_send();
    |         - has type `impl Debug` which is not `Send`
 LL |     drop(x);
 LL |     fut().await;
-   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+   |          ^^^^^^ await occurs here, with `x` maybe used later
 LL | }
    | - `x` is later dropped here
 note: required by a bound in `assert_send`
@@ -29,12 +29,12 @@ LL |     assert_send(non_send_temporary_in_match());
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/async-fn-nonsend.rs:33:20
+  --> $DIR/async-fn-nonsend.rs:33:25
    |
 LL |     match Some(non_send()) {
    |                ---------- has type `impl Debug` which is not `Send`
 LL |         Some(_) => fut().await,
-   |                    ^^^^^^^^^^^ await occurs here, with `non_send()` maybe used later
+   |                         ^^^^^^ await occurs here, with `non_send()` maybe used later
 ...
 LL | }
    | - `non_send()` is later dropped here
@@ -52,13 +52,13 @@ LL |     assert_send(non_sync_with_method_call());
    |
    = help: the trait `Send` is not implemented for `dyn std::fmt::Write`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/async-fn-nonsend.rs:42:9
+  --> $DIR/async-fn-nonsend.rs:42:14
    |
 LL |     let f: &mut std::fmt::Formatter = panic!();
    |         - has type `&mut Formatter<'_>` which is not `Send`
 LL |     if non_sync().fmt(f).unwrap() == () {
 LL |         fut().await;
-   |         ^^^^^^^^^^^ await occurs here, with `f` maybe used later
+   |              ^^^^^^ await occurs here, with `f` maybe used later
 LL |     }
 LL | }
    | - `f` is later dropped here

Diff for: src/test/ui/async-await/await-keyword/incorrect-syntax-suggestions.stderr

@@ -162,52 +162,52 @@ LL |     let _ = (await bar())?;
    |              ^^^^^^^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
-  --> $DIR/incorrect-syntax-suggestions.rs:71:13
+  --> $DIR/incorrect-syntax-suggestions.rs:71:19
    |
 LL | fn foo13() -> Result<(), ()> {
    |    ----- this is not `async`
 LL |     let _ = bar().await();
-   |             ^^^^^^^^^^^ only allowed inside `async` functions and blocks
+   |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
-  --> $DIR/incorrect-syntax-suggestions.rs:76:13
+  --> $DIR/incorrect-syntax-suggestions.rs:76:19
    |
 LL | fn foo14() -> Result<(), ()> {
    |    ----- this is not `async`
 LL |     let _ = bar().await()?;
-   |             ^^^^^^^^^^^ only allowed inside `async` functions and blocks
+   |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
-  --> $DIR/incorrect-syntax-suggestions.rs:81:13
+  --> $DIR/incorrect-syntax-suggestions.rs:81:19
    |
 LL | fn foo15() -> Result<(), ()> {
    |    ----- this is not `async`
 LL |     let _ = bar().await;
-   |             ^^^^^^^^^^^ only allowed inside `async` functions and blocks
+   |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
-  --> $DIR/incorrect-syntax-suggestions.rs:85:13
+  --> $DIR/incorrect-syntax-suggestions.rs:85:19
    |
 LL | fn foo16() -> Result<(), ()> {
    |    ----- this is not `async`
 LL |     let _ = bar().await?;
-   |             ^^^^^^^^^^^ only allowed inside `async` functions and blocks
+   |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
-  --> $DIR/incorrect-syntax-suggestions.rs:90:17
+  --> $DIR/incorrect-syntax-suggestions.rs:90:23
    |
 LL |     fn foo() -> Result<(), ()> {
    |        --- this is not `async`
 LL |         let _ = bar().await?;
-   |                 ^^^^^^^^^^^ only allowed inside `async` functions and blocks
+   |                       ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
-  --> $DIR/incorrect-syntax-suggestions.rs:97:17
+  --> $DIR/incorrect-syntax-suggestions.rs:97:23
    |
 LL |     let foo = || {
    |               -- this is not `async`
 LL |         let _ = bar().await?;
-   |                 ^^^^^^^^^^^ only allowed inside `async` functions and blocks
+   |                       ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/incorrect-syntax-suggestions.rs:113:17

Diff for: src/test/ui/async-await/issue-64130-1-sync.stderr

@@ -6,12 +6,12 @@ LL |     is_sync(bar());
    |
    = help: within `impl Future<Output = ()>`, the trait `Sync` is not implemented for `Foo`
 note: future is not `Sync` as this value is used across an await
-  --> $DIR/issue-64130-1-sync.rs:15:5
+  --> $DIR/issue-64130-1-sync.rs:15:10
    |
 LL |     let x = Foo;
    |         - has type `Foo` which is not `Sync`
 LL |     baz().await;
-   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+   |          ^^^^^^ await occurs here, with `x` maybe used later
 LL | }
    | - `x` is later dropped here
 note: required by a bound in `is_sync`

Diff for: src/test/ui/async-await/issue-64130-2-send.stderr

@@ -6,12 +6,12 @@ LL |     is_send(bar());
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Foo`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/issue-64130-2-send.rs:15:5
+  --> $DIR/issue-64130-2-send.rs:15:10
    |
 LL |     let x = Foo;
    |         - has type `Foo` which is not `Send`
 LL |     baz().await;
-   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+   |          ^^^^^^ await occurs here, with `x` maybe used later
 LL | }
    | - `x` is later dropped here
 note: required by a bound in `is_send`

Diff for: src/test/ui/async-await/issue-64130-3-other.stderr

@@ -8,12 +8,12 @@ LL |     is_qux(bar());
    |            ^^^^^ within `impl Future<Output = ()>`, the trait `Qux` is not implemented for `Foo`
    |
 note: future does not implement `Qux` as this value is used across an await
-  --> $DIR/issue-64130-3-other.rs:18:5
+  --> $DIR/issue-64130-3-other.rs:18:10
    |
 LL |     let x = Foo;
    |         - has type `Foo` which does not implement `Qux`
 LL |     baz().await;
-   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+   |          ^^^^^^ await occurs here, with `x` maybe used later
 LL | }
    | - `x` is later dropped here
 note: required by a bound in `is_qux`

Diff for: src/test/ui/async-await/issue-64130-4-async-move.stderr

@@ -6,13 +6,13 @@ LL | pub fn foo() -> impl Future + Send {
    |
    = help: the trait `Sync` is not implemented for `(dyn Any + Send + 'static)`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/issue-64130-4-async-move.rs:21:26
+  --> $DIR/issue-64130-4-async-move.rs:21:31
    |
 LL |         match client.status() {
    |               ------ has type `&Client` which is not `Send`
 LL |             200 => {
 LL |                 let _x = get().await;
-   |                          ^^^^^^^^^^^ await occurs here, with `client` maybe used later
+   |                               ^^^^^^ await occurs here, with `client` maybe used later
 ...
 LL |     }
    |     - `client` is later dropped here

Diff for: src/test/ui/async-await/issue-64130-non-send-future-diags.stderr

@@ -6,12 +6,12 @@ LL |     is_send(foo());
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, u32>`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/issue-64130-non-send-future-diags.rs:17:5
+  --> $DIR/issue-64130-non-send-future-diags.rs:17:10
    |
 LL |     let g = x.lock().unwrap();
    |         - has type `MutexGuard<'_, u32>` which is not `Send`
 LL |     baz().await;
-   |     ^^^^^^^^^^^ await occurs here, with `g` maybe used later
+   |          ^^^^^^ await occurs here, with `g` maybe used later
 LL | }
    | - `g` is later dropped here
 note: required by a bound in `is_send`

Diff for: src/test/ui/async-await/issue-67252-unnamed-future.stderr

@@ -6,12 +6,12 @@ LL |     spawn(async {
    |
    = help: within `impl Future<Output = [async output]>`, the trait `Send` is not implemented for `*mut ()`
 note: future is not `Send` as this value is used across an await
-  --> $DIR/issue-67252-unnamed-future.rs:20:9
+  --> $DIR/issue-67252-unnamed-future.rs:20:16
    |
 LL |         let _a = std::ptr::null_mut::<()>(); // `*mut ()` is not `Send`
    |             -- has type `*mut ()` which is not `Send`
 LL |         AFuture.await;
-   |         ^^^^^^^^^^^^^ await occurs here, with `_a` maybe used later
+   |                ^^^^^^ await occurs here, with `_a` maybe used later
 LL |     });
    |     - `_a` is later dropped here
 note: required by a bound in `spawn`

Diff for: src/test/ui/async-await/issue-70594.rs

@@ -6,6 +6,7 @@ async fn fun() {
     //~| error: `.await` is not allowed in a `const`
     //~| error: `.await` is not allowed in a `const`
     //~| error: `()` is not a future
+    //~| error: `()` is not a future
 }
 
 fn main() {}