suggest return type for main()

[llogiq]

This adds a suggestion for main() methods with default () return if the resulting expression has a type that implements std::process::Termination.

This fixes #61277.

r? @estebank

[Centril]

Can you please also add tests to exercise this behavior?

[estebank]

LGTM, please add tests.

[estebank]

I think the whitespace in the code suggestion might be subtly wrong. I believe this should be " -> {} ", but without the stderr` test output I can't say for sure.

[llogiq]

I just added a UI test, but the suggestion doesn't appear. Any idea why?

[estebank]

I'm not sure. Could you add some debug statements to verify the contents of fn_decl.output, found.is_suggestable(), can_suggest, expected and ret_ty?

[llogiq]

Sure thing.

[estebank]

tidy error: /checkout/src/librustc_typeck/check/mod.rs:3943: line longer than 100 chars

[llogiq]

It appears that the on_unimplemented check is executed before it can get to the return type suggestion – at least that's what the debug logs show (I never got a debug message from rustc_typeck, I had to RUSTC_LOG=debug to find out). So I probably need to defer that check (or even better omit it if I can prove that the later suggestion will help). If I get rid of the ?, I still get the error without the typeck-debug messages. However, I get the following logs:

 INFO (timestamp elided): rustc::traits::error_reporting: skipping ErrorDescriptor { predicate: Binder(TraitPredicate(<std::result::Result<[type error], ()> as std::marker::Sized>)), index: Some(3) } (implied by ErrorDescriptor { predicate: Binder(TraitPredicate(<[type error] as std::marker::Sized>)), index: Some(1) })
 INFO (timestamp elided): rustc::traits::error_reporting: skipping ErrorDescriptor { predicate: Binder(TraitPredicate(<std::result::Result<(), [type error]> as std::marker::Sized>)), index: Some(2) } (implied by ErrorDescriptor { predicate: Binder(TraitPredicate(<[type error] as std::marker::Sized>)), index: Some(0) })

[estebank]

I see. The problem is that found is already poisoned with TyErr which makes predicate_for_trait_def not do what you expect it to do. I'll try to take a look at this during the weekend.

[llogiq]

Thanks, you're a hero!

[estebank]

@llogiq the reason the suggestion doesn't trigger is because the found type references FreshTy. If you change the code to use a fully qualified type, the suggestion triggers:

error[E0308]: mismatched types
 --> src/test/ui/suggestions/missing_return_type.rs:6:5
  |
6 |     Result::<(), ()>::Ok(())
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found enum `std::result::Result`
  |
  = note: expected type `()`
             found type `std::result::Result<(), ()>`
help: try adding a semicolon
  |
6 |     Result::<(), ()>::Ok(());
  |                             ^
help: try adding a return type
  |
3 | fn main() -> std::result::Result<(), ()> {
  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I see two options: 1) modify the machinery in traits/select.rs to have a way to eagerly resolve in this case or 2) modify found in-place to replace all its FreshTy to some "safe" type, probably something along the lines of std::result::Result<(), impl std::fmt::Debug>. This should only be done in the case of Result, and hope that users' implement Termination trait in non-generic structs 😬

This later suggestion would end up causing an error like the following though:

error[E0282]: type annotations needed
 --> src/test/ui/suggestions/missing_return_type.rs:3:25
  |
3 | fn main() -> Result<(), impl std::fmt::Debug>{
  |                         ^^^^^^^^^^^^^^^^^^^^ cannot infer type

Maybe suggesting replacing all the FreshTy type args with () would be enough for the intended resolution.

[estebank]

Another pattern that would be interesting to support is

error[E0308]: mismatched types
 --> src/test/ui/suggestions/missing_return_type.rs:7:5
  |
3 | fn main() {
  |           - expected `()` because of default return type
...
7 |     Ok(Err(())?)
  |     ^^^^^^^^^^^^- help: try adding a semicolon: `;`
  |     |
  |     expected (), found enum `std::result::Result`
  |
  = note: expected type `()`
             found type `std::result::Result<_, _>`

[wirelessringo]

Ping from triage. @llogiq any updates on this?

[llogiq]

I'm afraid I have a lot of code reading and understanding to do before I can implement @estebank's proposal. If you like, you can close the PR, and I'll reopen it when I have something working.

[JohnTitor]

Ping from triage: @llogiq, I'll close this PR to keep things tidy. But don't worry, you can re-open this if you find a time to read and understand code to implement, we'll be happy to review it again! Thanks for taking the time to contribute.