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Suggest calling await on method call and field access
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When encountering a failing method or field resolution on a `Future`,
look at the `Output` and try the same operation on it. If successful,
suggest calling `.await` on the `Future`.

This had already been introduced in rust-lang#72784, but at some point they
stopped working.
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@estebank
estebank committed Oct 26, 2020
1 parent 9f6c670 commit 28f02fb
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Showing 5 changed files with 90 additions and 95 deletions.
2 changes: 1 addition & 1 deletion compiler/rustc_infer/src/infer/error_reporting/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1669,7 +1669,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
self.note_error_origin(diag, cause, exp_found);
}

fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
pub fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
if let ty::Opaque(def_id, substs) = ty.kind() {
let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
// Future::Output
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71 changes: 27 additions & 44 deletions compiler/rustc_typeck/src/check/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,7 @@ use rustc_middle::ty::{AdtKind, Visibility};
use rustc_span::hygiene::DesugaringKind;
use rustc_span::source_map::Span;
use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_trait_selection::traits::{self, ObligationCauseCode, SelectionContext};
use rustc_trait_selection::traits::{self, ObligationCauseCode};

use std::fmt::Display;

Expand Down Expand Up @@ -1580,51 +1580,34 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
err: &mut DiagnosticBuilder<'_>,
field_ident: Ident,
base: &'tcx hir::Expr<'tcx>,
expr: &'tcx hir::Expr<'tcx>,
def_id: DefId,
ty: Ty<'tcx>,
) {
let param_env = self.tcx().param_env(def_id);
let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
// Future::Output
let item_def_id =
self.tcx.associated_items(future_trait).in_definition_order().next().unwrap().def_id;

let projection_ty = self.tcx.projection_ty_from_predicates((def_id, item_def_id));
debug!("suggest_await_on_field_access: projection_ty={:?}", projection_ty);

let cause = self.misc(expr.span);
let mut selcx = SelectionContext::new(&self.infcx);

let mut obligations = vec![];
if let Some(projection_ty) = projection_ty {
let normalized_ty = rustc_trait_selection::traits::normalize_projection_type(
&mut selcx,
param_env,
projection_ty,
cause,
0,
&mut obligations,
);
debug!(
"suggest_await_on_field_access: normalized_ty={:?}, ty_kind={:?}",
self.resolve_vars_if_possible(&normalized_ty),
normalized_ty.kind(),
);
if let ty::Adt(def, _) = normalized_ty.kind() {
// no field access on enum type
if !def.is_enum() {
if def.non_enum_variant().fields.iter().any(|field| field.ident == field_ident)
{
err.span_suggestion_verbose(
base.span.shrink_to_hi(),
"consider awaiting before field access",
".await".to_string(),
Applicability::MaybeIncorrect,
);
}
let output_ty = match self.infcx.get_impl_future_output_ty(ty) {
Some(output_ty) => self.resolve_vars_if_possible(&output_ty),
_ => return,
};
let mut add_label = true;
if let ty::Adt(def, _) = output_ty.kind() {
// no field access on enum type
if !def.is_enum() {
if def.non_enum_variant().fields.iter().any(|field| field.ident == field_ident) {
add_label = false;
err.span_label(
field_ident.span,
"field not available in `impl Future`, but it is available in its `Output`",
);
err.span_suggestion_verbose(
base.span.shrink_to_hi(),
"consider `await`ing on the `Future` and access the field of its `Output`",
".await".to_string(),
Applicability::MaybeIncorrect,
);
}
}
}
if add_label {
err.span_label(field_ident.span, &format!("field not found in `{}`", ty));
}
}

fn ban_nonexisting_field(
Expand Down Expand Up @@ -1653,8 +1636,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
ty::Param(param_ty) => {
self.point_at_param_definition(&mut err, param_ty);
}
ty::Opaque(def_id, _) => {
self.suggest_await_on_field_access(&mut err, field, base, expr, def_id);
ty::Opaque(_, _) => {
self.suggest_await_on_field_access(&mut err, field, base, expr_t.peel_refs());
}
_ => {}
}
Expand Down
54 changes: 13 additions & 41 deletions compiler/rustc_typeck/src/check/method/suggest.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -21,7 +21,6 @@ use rustc_span::symbol::{kw, sym, Ident};
use rustc_span::{source_map, FileName, Span};
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::Obligation;
use rustc_trait_selection::traits::SelectionContext;

use std::cmp::Ordering;

Expand Down Expand Up @@ -870,46 +869,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
call: &hir::Expr<'_>,
span: Span,
) {
if let ty::Opaque(def_id, _) = *ty.kind() {
let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
// Future::Output
let item_def_id = self
.tcx
.associated_items(future_trait)
.in_definition_order()
.next()
.unwrap()
.def_id;

let projection_ty = self.tcx.projection_ty_from_predicates((def_id, item_def_id));
let cause = self.misc(span);
let mut selcx = SelectionContext::new(&self.infcx);
let mut obligations = vec![];
if let Some(projection_ty) = projection_ty {
let normalized_ty = rustc_trait_selection::traits::normalize_projection_type(
&mut selcx,
self.param_env,
projection_ty,
cause,
0,
&mut obligations,
);
debug!(
"suggest_await_before_method: normalized_ty={:?}, ty_kind={:?}",
self.resolve_vars_if_possible(&normalized_ty),
normalized_ty.kind(),
);
let method_exists = self.method_exists(item_name, normalized_ty, call.hir_id, true);
debug!("suggest_await_before_method: is_method_exist={}", method_exists);
if method_exists {
err.span_suggestion_verbose(
span.shrink_to_lo(),
"consider awaiting before this method call",
"await.".to_string(),
Applicability::MaybeIncorrect,
);
}
}
let output_ty = match self.infcx.get_impl_future_output_ty(ty) {
Some(output_ty) => self.resolve_vars_if_possible(&output_ty),
_ => return,
};
let method_exists = self.method_exists(item_name, output_ty, call.hir_id, true);
debug!("suggest_await_before_method: is_method_exist={}", method_exists);
if method_exists {
err.span_suggestion_verbose(
span.shrink_to_lo(),
"consider `await`ing on the `Future` and calling the method on its `Output`",
"await.".to_string(),
Applicability::MaybeIncorrect,
);
}
}

Expand Down
29 changes: 27 additions & 2 deletions src/test/ui/async-await/issue-61076.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,14 @@ async fn foo() -> Result<(), ()> {

async fn bar() -> Result<(), ()> {
foo()?; //~ ERROR the `?` operator can only be applied to values that implement `Try`
//~^ NOTE the `?` operator cannot be applied to type `impl Future`
//~| HELP the trait `Try` is not implemented for `impl Future`
//~| NOTE required by `into_result`
//~| HELP consider `await`ing on the `Future`
//~| NOTE in this expansion of desugaring of operator `?`
//~| NOTE in this expansion of desugaring of operator `?`
//~| NOTE in this expansion of desugaring of operator `?`
//~| NOTE in this expansion of desugaring of operator `?`
Ok(())
}

Expand All @@ -48,25 +56,42 @@ async fn struct_() -> Struct {
}

async fn tuple() -> Tuple {
//~^ NOTE the `Output` of this `async fn`'s expected opaque type
Tuple(1i32)
}

async fn baz() -> Result<(), ()> {
let t = T;
t?; //~ ERROR the `?` operator can only be applied to values that implement `Try`
//~^ NOTE the `?` operator cannot be applied to type `T`
//~| HELP the trait `Try` is not implemented for `T`
//~| NOTE required by `into_result`
//~| HELP consider `await`ing on the `Future`
//~| NOTE in this expansion of desugaring of operator `?`
//~| NOTE in this expansion of desugaring of operator `?`
//~| NOTE in this expansion of desugaring of operator `?`
//~| NOTE in this expansion of desugaring of operator `?`


let _: i32 = tuple().0; //~ ERROR no field `0`
//~^ HELP consider `await`ing on the `Future`
//~| NOTE field not available in `impl Future`

let _: i32 = struct_().a; //~ ERROR no field `a`
//~^ HELP consider `await`ing on the `Future`
//~| NOTE field not available in `impl Future`

struct_().method(); //~ ERROR no method named

//~^ NOTE method not found in `impl Future`
//~| HELP consider `await`ing on the `Future`
Ok(())
}

async fn match_() {
match tuple() {
match tuple() { //~ HELP consider `await`ing on the `Future`
Tuple(_) => {} //~ ERROR mismatched types
//~^ NOTE expected opaque type, found struct `Tuple`
//~| NOTE expected opaque type `impl Future`
}
}

Expand Down
29 changes: 22 additions & 7 deletions src/test/ui/async-await/issue-61076.stderr
View file
Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ LL | foo().await?;
| ^^^^^^

error[E0277]: the `?` operator can only be applied to values that implement `Try`
--> $DIR/issue-61076.rs:56:5
--> $DIR/issue-61076.rs:65:5
|
LL | t?;
| ^^ the `?` operator cannot be applied to type `T`
Expand All @@ -25,25 +25,40 @@ LL | t.await?;
| ^^^^^^

error[E0609]: no field `0` on type `impl Future`
--> $DIR/issue-61076.rs:58:26
--> $DIR/issue-61076.rs:76:26
|
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`
|
LL | let _: i32 = tuple().await.0;
| ^^^^^^

error[E0609]: no field `a` on type `impl Future`
--> $DIR/issue-61076.rs:60:28
--> $DIR/issue-61076.rs:80:28
|
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`
|
LL | let _: i32 = struct_().await.a;
| ^^^^^^

error[E0599]: no method named `method` found for opaque type `impl Future` in the current scope
--> $DIR/issue-61076.rs:62:15
--> $DIR/issue-61076.rs:84:15
|
LL | struct_().method();
| ^^^^^^ method not found in `impl Future`
|
help: consider `await`ing on the `Future` and calling the method on its `Output`
|
LL | struct_().await.method();
| ^^^^^^

error[E0308]: mismatched types
--> $DIR/issue-61076.rs:69:9
--> $DIR/issue-61076.rs:92:9
|
LL | async fn tuple() -> Tuple {
| ----- the `Output` of this `async fn`'s expected opaque type
Expand Down

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