Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Misleading Suggestion when Returning a Reference to a Bare Trait from a Function #127689

Closed
veera-sivarajan opened this issue Jul 13, 2024 · 3 comments
Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-trait-objects Area: trait objects, vtable layout D-incorrect Diagnostics: A diagnostic that is giving misleading or incorrect information. D-verbose Diagnostics: Too much output caused by a single piece of incorrect code. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

Comments

@veera-sivarajan
Copy link
Contributor

veera-sivarajan commented Jul 13, 2024

Code

// edition: 2021
trait Trait {}

fn fun() -> &Trait {
    todo!()
}

fn main() {}

Current output

error[E0106]: missing lifetime specifier
 --> <source>:5:13
  |
5 | fn fun() -> &Trait {
  |             ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
  |
5 | fn fun() -> &'static Trait {
  |              +++++++

error[E0782]: trait objects must include the `dyn` keyword
 --> <source>:5:14
  |
5 | fn fun() -> &Trait {
  |              ^^^^^
  |
help: add `dyn` keyword before this trait
  |
5 | fn fun() -> &dyn Trait {
  |              +++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0106, E0782.
For more information about an error, try `rustc --explain E0106`.

Desired output

error[E0782]: trait objects must include the `dyn` keyword
 --> <source>:5:14
  |
5 | fn fun() -> &Trait {
  |              ^^^^^
  |
help: add `dyn` keyword before this trait
  |
5 | fn fun() -> &dyn Trait {
  |              +++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0106, E0782.
For more information about an error, try `rustc --explain E0106`.

Rationale and extra context

The error E0106 suggests to add a static lifetime but this is incorrect. If and when #127692 is merged, the error E0782 can provide accurate and helpful messages and the former should be suppressed in favor of E0782.

But this can be tricky as E0106 is emitted at rustc_resolve which doesn't have any context about type of values.

Rust Version

rustc 1.79.0

Anything else?

No response

@veera-sivarajan veera-sivarajan 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 Jul 13, 2024
@veera-sivarajan
Copy link
Contributor Author

@rustbot label +D-verbose +D-incorrect +A-trait-objects

@rustbot rustbot added A-trait-objects Area: trait objects, vtable layout D-incorrect Diagnostics: A diagnostic that is giving misleading or incorrect information. D-verbose Diagnostics: Too much output caused by a single piece of incorrect code. labels Jul 13, 2024
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Sep 2, 2024
…stebank

Suggest `impl Trait` for References to Bare Trait in Function Header

Fixes rust-lang#125139

This PR suggests `impl Trait` when `&Trait` is found as a function parameter type or return type. This makes use of existing diagnostics by adding `peel_refs()` when checking for type equality.

Additionaly, it makes a few other improvements:
1. Checks if functions inside impl blocks have bare trait in their headers.
2. Introduces a trait `NextLifetimeParamName` similar to the existing `NextTypeParamName` for suggesting a lifetime name. Also, abstracts out the common logic between the two trait impls.

### Related Issues
I ran into a bunch of related diagnostic issues but couldn't fix them within the scope of this PR. So, I have created the following issues:
1. [Misleading Suggestion when Returning a Reference to a Bare Trait from a Function](rust-lang#127689)
2. [Verbose Error When a Function Takes a Bare Trait as Parameter](rust-lang#127690)
3. [Incorrect Suggestion when Returning a Bare Trait from a Function](rust-lang#127691)

r​? `@estebank` since you implemented  rust-lang#119148
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Sep 3, 2024
…stebank

Suggest `impl Trait` for References to Bare Trait in Function Header

Fixes rust-lang#125139

This PR suggests `impl Trait` when `&Trait` is found as a function parameter type or return type. This makes use of existing diagnostics by adding `peel_refs()` when checking for type equality.

Additionaly, it makes a few other improvements:
1. Checks if functions inside impl blocks have bare trait in their headers.
2. Introduces a trait `NextLifetimeParamName` similar to the existing `NextTypeParamName` for suggesting a lifetime name. Also, abstracts out the common logic between the two trait impls.

### Related Issues
I ran into a bunch of related diagnostic issues but couldn't fix them within the scope of this PR. So, I have created the following issues:
1. [Misleading Suggestion when Returning a Reference to a Bare Trait from a Function](rust-lang#127689)
2. [Verbose Error When a Function Takes a Bare Trait as Parameter](rust-lang#127690)
3. [Incorrect Suggestion when Returning a Bare Trait from a Function](rust-lang#127691)

r​? ``@estebank`` since you implemented  rust-lang#119148
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Sep 3, 2024
…stebank

Suggest `impl Trait` for References to Bare Trait in Function Header

Fixes rust-lang#125139

This PR suggests `impl Trait` when `&Trait` is found as a function parameter type or return type. This makes use of existing diagnostics by adding `peel_refs()` when checking for type equality.

Additionaly, it makes a few other improvements:
1. Checks if functions inside impl blocks have bare trait in their headers.
2. Introduces a trait `NextLifetimeParamName` similar to the existing `NextTypeParamName` for suggesting a lifetime name. Also, abstracts out the common logic between the two trait impls.

### Related Issues
I ran into a bunch of related diagnostic issues but couldn't fix them within the scope of this PR. So, I have created the following issues:
1. [Misleading Suggestion when Returning a Reference to a Bare Trait from a Function](rust-lang#127689)
2. [Verbose Error When a Function Takes a Bare Trait as Parameter](rust-lang#127690)
3. [Incorrect Suggestion when Returning a Bare Trait from a Function](rust-lang#127691)

r​? ```@estebank``` since you implemented  rust-lang#119148
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Sep 5, 2024
Rollup merge of rust-lang#127692 - veera-sivarajan:bugfix-125139, r=estebank

Suggest `impl Trait` for References to Bare Trait in Function Header

Fixes rust-lang#125139

This PR suggests `impl Trait` when `&Trait` is found as a function parameter type or return type. This makes use of existing diagnostics by adding `peel_refs()` when checking for type equality.

Additionaly, it makes a few other improvements:
1. Checks if functions inside impl blocks have bare trait in their headers.
2. Introduces a trait `NextLifetimeParamName` similar to the existing `NextTypeParamName` for suggesting a lifetime name. Also, abstracts out the common logic between the two trait impls.

### Related Issues
I ran into a bunch of related diagnostic issues but couldn't fix them within the scope of this PR. So, I have created the following issues:
1. [Misleading Suggestion when Returning a Reference to a Bare Trait from a Function](rust-lang#127689)
2. [Verbose Error When a Function Takes a Bare Trait as Parameter](rust-lang#127690)
3. [Incorrect Suggestion when Returning a Bare Trait from a Function](rust-lang#127691)

r​? ```@estebank``` since you implemented  rust-lang#119148
@GrigorenkoPV
Copy link
Contributor

The message has changed a bit, maybe due to #131239.

The 'static suggestion is still here.

But now it also suggests &impl Trait, which is cool.

The &dyn Trait suggestion got changed to Box<dyn Trait>, which probably makes more sense, considering the function has no input lifetimes.

error[E0106]: missing lifetime specifier
 --> src/main.rs:4:13
  |
4 | fn fun() -> &Trait {
  |             ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
  |
4 | fn fun() -> &'static Trait {
  |              +++++++

error[E0782]: expected a type, found a trait
 --> src/main.rs:4:14
  |
4 | fn fun() -> &Trait {
  |              ^^^^^
  |
help: use `impl Trait` to return an opaque type, as long as you return a single underlying type
  |
4 | fn fun() -> &impl Trait {
  |              ++++
help: alternatively, you can return an owned trait object
  |
4 | fn fun() -> Box<dyn Trait> {
  |             ~~~~~~~~~~~~~~

@veera-sivarajan
Copy link
Contributor Author

veera-sivarajan commented Oct 13, 2024

Yeah, it's hard to prevent the 'static suggestion because its from the resolver. Anyways, this looks good to me. Thank you for updating.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-trait-objects Area: trait objects, vtable layout D-incorrect Diagnostics: A diagnostic that is giving misleading or incorrect information. D-verbose Diagnostics: Too much output caused by a single piece of incorrect code. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
Projects
None yet
Development

No branches or pull requests

3 participants