Rollup merge of #131549 - compiler-errors:try-in-sync, r=spastorino

Add a note for `?` on a `impl Future<Output = Result<..>>` in sync function

It's confusing to `?` a future of a result in a sync function. We have a suggestion to `.await` it if we're in an async function, but not a sync function. Note that this is the case for sync functions, at least.

Let's be a bit more vague about a fix, since it's somewhat context dependent. For example, you could block on it, or you could make your function asynchronous. 🤷

compiler/rustc_hir_typeck/src/expr.rs

@@ -10,7 +10,8 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_data_structures::unord::UnordMap;
 use rustc_errors::codes::*;
 use rustc_errors::{
-    Applicability, Diag, ErrorGuaranteed, StashKey, Subdiagnostic, pluralize, struct_span_code_err,
+    Applicability, Diag, ErrorGuaranteed, MultiSpan, StashKey, Subdiagnostic, pluralize,
+    struct_span_code_err,
 };
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -2763,12 +2764,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             field_ident.span,
             "field not available in `impl Future`, but it is available in its `Output`",
         );
-        err.span_suggestion_verbose(
+        match self.tcx.coroutine_kind(self.body_id) {
-            base.span.shrink_to_hi(),
+            Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) => {
-            "consider `await`ing on the `Future` and access the field of its `Output`",
+                err.span_suggestion_verbose(
-            ".await",
+                    base.span.shrink_to_hi(),
-            Applicability::MaybeIncorrect,
+                    "consider `await`ing on the `Future` to access the field",
-        );
+                    ".await",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            _ => {
+                let mut span: MultiSpan = base.span.into();
+                span.push_span_label(self.tcx.def_span(self.body_id), "this is not `async`");
+                err.span_note(
+                    span,
+                    "this implements `Future` and its output type has the field, \
+                    but the future cannot be awaited in a synchronous function",
+                );
+            }
+        }
     }
 
     fn ban_nonexisting_field(

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

@@ -3594,52 +3594,64 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         trait_pred: ty::PolyTraitPredicate<'tcx>,
         span: Span,
     ) {
-        if let Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) =
+        let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
-            self.tcx.coroutine_kind(obligation.cause.body_id)
+
+        let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
+        let impls_future = self.type_implements_trait(
+            future_trait,
+            [self.tcx.instantiate_bound_regions_with_erased(self_ty)],
+            obligation.param_env,
+        );
+        if !impls_future.must_apply_modulo_regions() {
+            return;
+        }
+
+        let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
+        // `<T as Future>::Output`
+        let projection_ty = trait_pred.map_bound(|trait_pred| {
+            Ty::new_projection(
+                self.tcx,
+                item_def_id,
+                // Future::Output has no args
+                [trait_pred.self_ty()],
+            )
+        });
+        let InferOk { value: projection_ty, .. } =
+            self.at(&obligation.cause, obligation.param_env).normalize(projection_ty);
+
+        debug!(
+            normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
+        );
+        let try_obligation = self.mk_trait_obligation_with_new_self_ty(
+            obligation.param_env,
+            trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
+        );
+        debug!(try_trait_obligation = ?try_obligation);
+        if self.predicate_may_hold(&try_obligation)
+            && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+            && snippet.ends_with('?')
         {
-            let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
+            match self.tcx.coroutine_kind(obligation.cause.body_id) {
-
+                Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) => {
-            let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
+                    err.span_suggestion_verbose(
-            let impls_future = self.type_implements_trait(
+                        span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
-                future_trait,
+                        "consider `await`ing on the `Future`",
-                [self.tcx.instantiate_bound_regions_with_erased(self_ty)],
+                        ".await",
-                obligation.param_env,
+                        Applicability::MaybeIncorrect,
-            );
+                    );
-            if !impls_future.must_apply_modulo_regions() {
+                }
-                return;
+                _ => {
-            }
+                    let mut span: MultiSpan = span.with_lo(span.hi() - BytePos(1)).into();
-
+                    span.push_span_label(
-            let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
+                        self.tcx.def_span(obligation.cause.body_id),
-            // `<T as Future>::Output`
+                        "this is not `async`",
-            let projection_ty = trait_pred.map_bound(|trait_pred| {
+                    );
-                Ty::new_projection(
+                    err.span_note(
-                    self.tcx,
+                        span,
-                    item_def_id,
+                        "this implements `Future` and its output type supports \
-                    // Future::Output has no args
+                        `?`, but the future cannot be awaited in a synchronous function",
-                    [trait_pred.self_ty()],
+                    );
-                )
+                }
-            });
-            let InferOk { value: projection_ty, .. } =
-                self.at(&obligation.cause, obligation.param_env).normalize(projection_ty);
-
-            debug!(
-                normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
-            );
-            let try_obligation = self.mk_trait_obligation_with_new_self_ty(
-                obligation.param_env,
-                trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
-            );
-            debug!(try_trait_obligation = ?try_obligation);
-            if self.predicate_may_hold(&try_obligation)
-                && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
-                && snippet.ends_with('?')
-            {
-                err.span_suggestion_verbose(
-                    span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
-                    "consider `await`ing on the `Future`",
-                    ".await",
-                    Applicability::MaybeIncorrect,
-                );
             }
         }
     }

tests/ui/async-await/field-in-sync.rs

@@ -0,0 +1,13 @@
+//@ edition: 2021
+
+struct S {
+    field: (),
+}
+
+async fn foo() -> S { todo!() }
+
+fn main() -> Result<(), ()> {
+    foo().field;
+    //~^ ERROR no field `field` on type `impl Future<Output = S>`
+    Ok(())
+}

tests/ui/async-await/field-in-sync.stderr

@@ -0,0 +1,17 @@
+error[E0609]: no field `field` on type `impl Future<Output = S>`
+  --> $DIR/field-in-sync.rs:10:11
+   |
+LL |     foo().field;
+   |           ^^^^^ field not available in `impl Future`, but it is available in its `Output`
+   |
+note: this implements `Future` and its output type has the field, but the future cannot be awaited in a synchronous function
+  --> $DIR/field-in-sync.rs:10:5
+   |
+LL | fn main() -> Result<(), ()> {
+   | --------------------------- this is not `async`
+LL |     foo().field;
+   |     ^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0609`.

tests/ui/async-await/issue-61076.stderr

@@ -28,7 +28,7 @@ error[E0609]: no field `0` on type `impl Future<Output = Tuple>`
 LL |     let _: i32 = tuple().0;
    |                          ^ field not available in `impl Future`, but it is available in its `Output`
    |
-help: consider `await`ing on the `Future` and access the field of its `Output`
+help: consider `await`ing on the `Future` to access the field
    |
 LL |     let _: i32 = tuple().await.0;
    |                         ++++++
@@ -39,7 +39,7 @@ error[E0609]: no field `a` on type `impl Future<Output = Struct>`
 LL |     let _: i32 = struct_().a;
    |                            ^ field not available in `impl Future`, but it is available in its `Output`
    |
-help: consider `await`ing on the `Future` and access the field of its `Output`
+help: consider `await`ing on the `Future` to access the field
    |
 LL |     let _: i32 = struct_().await.a;
    |                           ++++++

tests/ui/async-await/try-in-sync.rs

@@ -0,0 +1,9 @@
+//@ edition: 2021
+
+async fn foo() -> Result<(), ()> { todo!() }
+
+fn main() -> Result<(), ()> {
+    foo()?;
+    //~^ ERROR the `?` operator can only be applied to values that implement `Try`
+    Ok(())
+}

tests/ui/async-await/try-in-sync.stderr

@@ -0,0 +1,18 @@
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/try-in-sync.rs:6:5
+   |
+LL |     foo()?;
+   |     ^^^^^^ the `?` operator cannot be applied to type `impl Future<Output = Result<(), ()>>`
+   |
+   = help: the trait `Try` is not implemented for `impl Future<Output = Result<(), ()>>`
+note: this implements `Future` and its output type supports `?`, but the future cannot be awaited in a synchronous function
+  --> $DIR/try-in-sync.rs:6:10
+   |
+LL | fn main() -> Result<(), ()> {
+   | --------------------------- this is not `async`
+LL |     foo()?;
+   |          ^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.