Provide appropriate types in turbofish suggestions #76043

estebank · 2020-08-28T21:58:17Z

When encountering an expression that is missing type information, provide a structured suggestion to use a turbofish or local binding type with an appropriate type.

rust-highfive · 2020-08-28T21:58:20Z

r? @matthewjasper

(rust_highfive has picked a reviewer for you, use r? to override)

src/test/ui/suggestions/appropriate-type-param-turbofish.stderr

eddyb · 2020-08-29T03:09:42Z

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

+                        // FIXME: ideally we would use `can_coerce` here instead, but `typeck`
+                        // comes *after* in the dependency graph.


You could make it a query (using canonicalization to allow inference variables to pass through). In fact, we've been meaning to make coercions more integrated into the trait system for many years now, cc @nikomatsakis

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

src/librustc_infer/infer/error_reporting/need_type_info.rs

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

src/test/ui/inference/cannot-infer-async-enabled-impl-trait-bindings.stderr

src/test/ui/issues/issue-72690.stderr

src/test/ui/issues/issue-7813.stderr

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

estebank · 2020-10-13T23:55:35Z

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

+        self.tcx.for_each_relevant_impl(data.trait_ref.def_id, self_ty, |impl_def_id| {
+            self.probe(|_| {
+                let trait_args = InternalSubsts::identity_for_item(self.tcx, data.trait_ref.def_id);
+                let impl_args = traits::specialize::fulfill_implication(
+                    self,
+                    obligation.param_env,
+                    data.trait_ref,
+                    impl_def_id,
+                );
+                let substs = match impl_args {
+                    Ok(impl_args) => trait_args.rebase_onto(self.tcx, impl_def_id, impl_args),
+                    _ => return,
+                };
+                let trait_ref = self.tcx.impl_trait_ref(impl_def_id).unwrap();
+
+                let cause = traits::ObligationCause::dummy();
+                let mut selcx = SelectionContext::new(&self);
+                let param_env = obligation.param_env.clone();
+                let (target_trait_ref, obligations) = traits::util::impl_trait_ref_and_oblig(
+                    &mut selcx,
+                    param_env,
+                    impl_def_id,
+                    substs,
+                );
+                self.resolve_vars_if_possible(&target_trait_ref);
+                if let Ok(eval_result) = selcx.evaluate_predicates(
+                    Some(Obligation::new(
+                        cause.clone(),
+                        param_env,
+                        target_trait_ref.without_const().to_predicate(self.tcx),
+                    ))
+                    .into_iter()
+                    .chain(obligations),
+                ) {
+                    debug!("get_turbofish_suggestions result {:?}", eval_result);
+                    if eval_result.must_apply_modulo_regions()
+                        && !matches!(trait_ref.self_ty().kind(), ty::Infer(_))
+                    {
+                        turbofish_suggestions.insert(trait_ref.self_ty());
+                    }
+                };
+            })
+        });


@eddyb @nikomatsakis could you take a look at this to see if it makes sense?

When encountering a situation where extra type annotations are needed, we sometimes suggest using turbofish. When we do so, previously we used either the `fn`'s type parameter names or `_` as placeholders. Now, we probe for all types that implement the traits the type parameter is bounded by and provide a structured suggestion for them.

…tions

Specifically avoid non-sense suggestion when calling `panic!` with something `!Send`.

bors · 2020-10-24T18:29:55Z

☔ The latest upstream changes (presumably #78319) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

Dylan-DPC-zz · 2020-12-19T20:49:02Z

closing this due to inactivity

rust-highfive assigned matthewjasper Aug 28, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 28, 2020

estebank commented Aug 28, 2020

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estebank force-pushed the needs_type_info_sugg branch 2 times, most recently from 5155ff4 to bbb7114 Compare August 30, 2020 21:30

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estebank force-pushed the needs_type_info_sugg branch from bbb7114 to dc910f9 Compare September 14, 2020 18:46

jyn514 added A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 15, 2020

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camelid added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 7, 2020

estebank force-pushed the needs_type_info_sugg branch from dc910f9 to 4df705e Compare October 11, 2020 02:27

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estebank force-pushed the needs_type_info_sugg branch from 4df705e to b888626 Compare October 11, 2020 06:00

estebank commented Oct 13, 2020

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estebank force-pushed the needs_type_info_sugg branch 3 times, most recently from 948a06c to c6171be Compare October 14, 2020 05:42

estebank added 12 commits October 23, 2020 12:55

Add tests for turbofish suggestions

4edf33e

Suggest specifying local binding types

bb83e15

Change type suggestion sorting heuristic and other cleanups

3f34590

Tweak wording for "cannot infer type" main label

0d90b59

Use string sorting exclusively

669a126

Defer ?Sized local binding error to get better suggestions

40d36b1

Modify turbofish suggestion probing logic

af13f34

collect now triggers E0283 instad of E0282

e918879

Remove unnecessary condition

8ec1c38

Avoid ?Sized local E0282 errors to provide better output and sugges…

ae5a239

…tions

Avoid suggesting inside macro call

660480a

Specifically avoid non-sense suggestion when calling `panic!` with something `!Send`.

estebank force-pushed the needs_type_info_sugg branch from 3cde317 to eedaf5d Compare October 23, 2020 20:41

estebank added 5 commits October 23, 2020 14:03

Do not skip "type annotation needed"/Sized error

15c74d5

Skip lifetimes in turbofish suggestions

0d3f077

Fix new ICE caused by derred arg Sized check

1ae1a13

Suggest &[_] instead of &[_; N] on inference error

f26b748

Fix rebase

654a412

estebank force-pushed the needs_type_info_sugg branch from eedaf5d to 654a412 Compare October 23, 2020 21:04

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Dylan-DPC-zz closed this Dec 19, 2020

Dylan-DPC-zz added S-inactive Status: Inactive and waiting on the author. This is often applied to closed PRs. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Dec 19, 2020

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Provide appropriate types in turbofish suggestions #76043

Provide appropriate types in turbofish suggestions #76043

estebank commented Aug 28, 2020

rust-highfive commented Aug 28, 2020

eddyb Aug 29, 2020

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estebank Oct 13, 2020

bors commented Oct 24, 2020

Dylan-DPC-zz commented Dec 19, 2020

		// FIXME: ideally we would use `can_coerce` here instead, but `typeck`
		// comes after in the dependency graph.

Provide appropriate types in turbofish suggestions #76043

Provide appropriate types in turbofish suggestions #76043

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estebank commented Aug 28, 2020

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bors commented Oct 24, 2020

Dylan-DPC-zz commented Dec 19, 2020