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)

........................................................................................ 10472/13887
........................................................................................ 10560/13887
........................................................................................ 10648/13887
........................................................................................ 10736/13887
.................................................F...F.................................. 10824/13887
........................................................iiiii...i....i.i................ 11000/13887
........................................................................................ 11088/13887
.......................................................i................................ 11176/13887
.................................................................iiiiii.i..ii..iiiiiiiii 11264/13887
---
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

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

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

Uh oh!

kvverti commented Nov 23, 2022

Uh oh!

rustbot commented Nov 23, 2022

Uh oh!

rust-log-analyzer commented Nov 23, 2022

Uh oh!

fmease commented Nov 23, 2022 •

edited

Loading

Uh oh!

davidtwco Nov 23, 2022

Uh oh!

apiraino commented Jan 5, 2023

Uh oh!

JohnCSimon commented Mar 17, 2023

Uh oh!

Uh oh!

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

Uh oh!

Conversation

kvverti commented Nov 23, 2022

Uh oh!

rustbot commented Nov 23, 2022

Uh oh!

rust-log-analyzer commented Nov 23, 2022

Uh oh!

fmease commented Nov 23, 2022 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

davidtwco Nov 23, 2022

Choose a reason for hiding this comment

Uh oh!

apiraino commented Jan 5, 2023

Uh oh!

JohnCSimon commented Mar 17, 2023

Uh oh!

Uh oh!

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

Loading