Add an explanatory note when suggesting to use `impl Trait` instead of a type parameter #104755

kvverti · 2022-11-23T01:56:59Z

Add a note to E308 explaining that generic type variables are chosen by the caller rather than the implementer, when suggesting to use impl Trait instead of a generic type variable.

error[E0308]: mismatched types
 --> src/lib.rs:3:5
  |
2 | fn foo<T: Clone>() -> T {
  |        -              -
  |        |              |
  |        |              expected `T` because of return type
  |        |              help: consider using an impl return type: `impl Clone`
  |        this type parameter
3 |     "hello"
  |     ^^^^^^^ expected type parameter `T`, found `&str`
  |
  = note: expected type parameter `T`
                  found reference `&'static str`
  = note: the caller chooses the value of a type parameter

r? rust-lang/diagnostics

rustbot · 2022-11-23T01:57:06Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @davidtwco (or someone else) soon.

Please see the contribution instructions for more information.

rust-log-analyzer · 2022-11-23T02:13:25Z

The job x86_64-gnu-llvm-13 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

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---
To only update this specific test, also pass `--test-args return/return-impl-trait-bad.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/return/return-impl-trait-bad.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/return/return-impl-trait-bad" "-A" "unused" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/return/return-impl-trait-bad/auxiliary"
stdout: none
--- stderr -------------------------------
  --> /checkout/src/test/ui/return/return-impl-trait-bad.rs:5:5
   |
   |
LL | fn bad_echo<T>(_t: T) -> T {
   |             -            - expected `T` because of return type
   |             this type parameter
   |             this type parameter
LL |     "this should not suggest impl Trait" //~ ERROR mismatched types
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found `&str`
   = note: expected type parameter `T`
                   found reference `&'static str`

error[E0308]: mismatched types
error[E0308]: mismatched types
  --> /checkout/src/test/ui/return/return-impl-trait-bad.rs:9:5
Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
   |
LL | fn bad_echo_2<T: Trait>(_t: T) -> T {
   |               -                   - expected `T` because of return type
   |               this type parameter
   |               this type parameter
LL |     "this will not suggest it, because that would probably be wrong" //~ ERROR mismatched types
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found `&str`
   = note: expected type parameter `T`
                   found reference `&'static str`

error[E0308]: mismatched types
error[E0308]: mismatched types
  --> /checkout/src/test/ui/return/return-impl-trait-bad.rs:17:5
   |
LL | fn other_bounds_bad<T>() -> T
   |                     -       - expected `T` because of return type
   |                     this type parameter
...
...
LL |     "don't suggest this, because Option<T> places additional constraints" //~ ERROR mismatched types
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found `&str`
   = note: expected type parameter `T`
                   found reference `&'static str`

error[E0308]: mismatched types
error[E0308]: mismatched types
  --> /checkout/src/test/ui/return/return-impl-trait-bad.rs:28:5
   |
LL | fn used_in_trait<T>() -> T
   |                  |       |
   |                  |       expected `T` because of return type
   |                  |       help: consider using an impl return type: `impl Send`
   |                  this type parameter
   |                  this type parameter
...
LL |     "don't suggest this, because the generic param is used in the bound." //~ ERROR mismatched types
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found `&str`
   = note: expected type parameter `T`
                   found reference `&'static str`
   = note: the caller chooses the value of a type parameter

---
To only update this specific test, also pass `--test-args return/return-impl-trait.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/return/return-impl-trait.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/return/return-impl-trait" "-A" "unused" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/return/return-impl-trait/auxiliary"
stdout: none
--- stderr -------------------------------
  --> /checkout/src/test/ui/return/return-impl-trait.rs:15:5
   |
   |
LL | fn bar<T: Trait + std::marker::Sync>() -> T
   |        |                                  |
   |        |                                  expected `T` because of return type
   |        this type parameter                help: consider using an impl return type: `impl Trait + std::marker::Sync + Send`
...
...
LL |     () //~ ERROR mismatched types
   |     ^^ expected type parameter `T`, found `()`
   = note: expected type parameter `T`
                   found unit type `()`
   = note: the caller chooses the value of a type parameter


error[E0308]: mismatched types
  --> /checkout/src/test/ui/return/return-impl-trait.rs:23:5
   |
LL | fn other_bounds<T>() -> T
   |                 |       |
   |                 |       expected `T` because of return type
   |                 |       help: consider using an impl return type: `impl Trait`
   |                 this type parameter
   |                 this type parameter
...
LL |     () //~ ERROR mismatched types
   |     ^^ expected type parameter `T`, found `()`
   = note: expected type parameter `T`
                   found unit type `()`
   = note: the caller chooses the value of a type parameter

fmease · 2022-11-23T06:22:54Z

You need to bless the tests locally (./x.py test <FAILING_TESTS...> --bless) and commit the resulting changes.

davidtwco · 2022-11-23T09:16:27Z

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -878,6 +878,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
            format!("impl {}", all_bounds_str),
            Applicability::MaybeIncorrect,
        );
+        err.note("the caller chooses the value of a type parameter");


Maybe something like..

the type of T is chosen by the caller

..would be clearer?

I'm not keen on describing type parameters as having a "value", I think that could lead to confusion.

apiraino · 2023-01-05T14:49:36Z

Switching to waiting on author to carry out the changes mentioned in the latest comments. @kvverti Feel free to request a review with @rustbot ready, thanks!

@rustbot author

JohnCSimon · 2023-03-17T04:10:59Z

@kvverti

Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you are going to continue please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive

…Trait Revives rust-lang#112088 and rust-lang#104755. Co-authored-by: sladynnunes <[email protected]> Co-authored-by: Thalia Nero <[email protected]>

Note that the caller chooses a type for type param ``` error[E0308]: mismatched types --> $DIR/return-impl-trait.rs:23:5 | LL | fn other_bounds<T>() -> T | - - | | | | | expected `T` because of return type | | help: consider using an impl return type: `impl Trait` | expected this type parameter ... LL | () | ^^ expected type parameter `T`, found `()` | = note: expected type parameter `T` found unit type `()` = note: the caller chooses the type of T which can be different from () ``` Tried to see if "expected this type parameter" can be replaced, but that goes all the way to `rustc_infer` so seems not worth the effort and can affect other diagnostics. Revives rust-lang#112088 and rust-lang#104755.

Rollup merge of rust-lang#122195 - jieyouxu:impl-return-note, r=fmease Note that the caller chooses a type for type param ``` error[E0308]: mismatched types --> $DIR/return-impl-trait.rs:23:5 | LL | fn other_bounds<T>() -> T | - - | | | | | expected `T` because of return type | | help: consider using an impl return type: `impl Trait` | expected this type parameter ... LL | () | ^^ expected type parameter `T`, found `()` | = note: expected type parameter `T` found unit type `()` = note: the caller chooses the type of T which can be different from () ``` Tried to see if "expected this type parameter" can be replaced, but that goes all the way to `rustc_infer` so seems not worth the effort and can affect other diagnostics. Revives rust-lang#112088 and rust-lang#104755.

Note that the caller chooses a type for type param ``` error[E0308]: mismatched types --> $DIR/return-impl-trait.rs:23:5 | LL | fn other_bounds<T>() -> T | - - | | | | | expected `T` because of return type | | help: consider using an impl return type: `impl Trait` | expected this type parameter ... LL | () | ^^ expected type parameter `T`, found `()` | = note: expected type parameter `T` found unit type `()` = note: the caller chooses the type of T which can be different from () ``` Tried to see if "expected this type parameter" can be replaced, but that goes all the way to `rustc_infer` so seems not worth the effort and can affect other diagnostics. Revives rust-lang#112088 and rust-lang#104755.

kvverti added 3 commits November 22, 2022 19:20

Add a note when suggesting to use impl Trait

9d46641

Change to non-spanned note

7092b1c

Amend wording

5628d90

rustbot assigned davidtwco Nov 23, 2022

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 23, 2022

davidtwco reviewed Nov 23, 2022

View reviewed changes

rustbot 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 Jan 5, 2023

JohnCSimon closed this Mar 17, 2023

rustbot added the S-inactive Status: Inactive and waiting on the author. This is often applied to closed PRs. label Mar 17, 2023

sladyn98 mentioned this pull request May 29, 2023

Add a note when suggesting to use Impl Trait #112088

Closed

jieyouxu mentioned this pull request Mar 8, 2024

Note that the caller chooses a type for type param #122195

Merged

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Add an explanatory note when suggesting to use `impl Trait` instead of a type parameter #104755

Add an explanatory note when suggesting to use `impl Trait` instead of a type parameter #104755

kvverti commented Nov 23, 2022

rustbot commented Nov 23, 2022

rust-log-analyzer commented Nov 23, 2022

fmease commented Nov 23, 2022 •

edited

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davidtwco Nov 23, 2022

apiraino commented Jan 5, 2023

JohnCSimon commented Mar 17, 2023

Add an explanatory note when suggesting to use impl Trait instead of a type parameter #104755

Add an explanatory note when suggesting to use impl Trait instead of a type parameter #104755

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kvverti commented Nov 23, 2022

rustbot commented Nov 23, 2022

rust-log-analyzer commented Nov 23, 2022

fmease commented Nov 23, 2022 • edited Loading

davidtwco Nov 23, 2022

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apiraino commented Jan 5, 2023

JohnCSimon commented Mar 17, 2023

Add an explanatory note when suggesting to use `impl Trait` instead of a type parameter #104755

Add an explanatory note when suggesting to use `impl Trait` instead of a type parameter #104755

fmease commented Nov 23, 2022 •

edited

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