improving error message when returning an opaque type inside an if else block

[vincenzopalazzo]

Code

async fn async_dummy() {}

async fn async_dummy2() {}


fn main() {
    let _ = if true {
        async_dummy()
    } else {
        async_dummy2()
    };
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0308]: `if` and `else` have incompatible types
  --> src/main.rs:10:9
   |
7  |       let _ = if true {
   |  _____________-
8  | |         async_dummy()
   | |         ------------- expected because of this
9  | |     } else {
10 | |         async_dummy2()
   | |         ^^^^^^^^^^^^^^ expected opaque type, found a different opaque type
11 | |     };
   | |_____- `if` and `else` have incompatible types

Desired output

No response

Rationale and extra context

We start a discussion on zulip regarding this that is available here where we noted that this suggestion is not helpful at all, but also we noted that it is difficult make a helpful suggestion in this case because there is the question that a user may ask to himself when reading the error message:

1. What is it an opaque type?
2. Future<Output = ()> is equal to Future<Output = ()>
3. Why two futures are not equal?a

Other cases

No response

Anything else?

A possible solution for this can be:

• Do not try to be too smart in some corner case like this, and in this case, I would like an error like You can not return two different futures inside an if-else block. rustc --explain ...
• make a specific explanation on why we can not compare these two types
• In case of the future generate a note that the user may want to call await on each branch
• change the opaque type string into future string when applicable

[vincenzopalazzo]

I had a partial patch already for this, but I think this required a little bit of discussion

@rustbot label +A-async-await +I-async-nominated
@rustbot claim

[rustbot]

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[vincenzopalazzo]

@rustbot label +E-mentor

[sladyn98]

@rustbot claim

[sladyn98]

@vincenzopalazzo Where do I add the helpful error message, a good starting point would be helpful

[vincenzopalazzo]

@sladyn98 this PR #107902 is not helping you?

the full grep of the error is not working because the message is build dynamically

[sladyn98]

@vincenzopalazzo This is my first contribution to the compiler so I am still getting used to a bit of the codebase, I can see some functions that construct messages about future in PR #107902, but I am not quite sure under which one would this new message come ? Sorry if the question does not make much sense, but I am still trying to understand the codebase

[sladyn98]

https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_middle/src/ty/error.rs#L95-L96 I found out that the error is being built from here and the if else block is here https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_infer/src/infer/error_reporting/mod.rs#L801

I think I can understand the flow, the IfExpression cause is generated here, from where the error is constructed

self.cause(
error_sp,
ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
else_id,
then_id,
then_ty,
else_ty,
outer_span,
opt_suggest_box_span,
})),
)

I am wondering if the async type checks would be done just before this line so that the message could be changed

if let Some(sp) = outer_span {
err.span_label(sp, "if and else have incompatible types");
}

CC @vincenzopalazzo

[vincenzopalazzo]

@rustbot claim

[vincenzopalazzo]

When I was trying to run this with master I found that my #107902 now fixed also this?

So closing as resolved, this is the error message that we want

➜  rust git:(macros/opaque-and-if-else) ✗ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc test.rs --edition 2021
error[E0308]: `if` and `else` have incompatible types
  --> test.rs:10:9
   |
7  |       let _ = if true {
   |  _____________-
8  | |         async_dummy()
   | |         ------------- expected because of this
9  | |     } else {
10 | |         async_dummy2()
   | |         ^^^^^^^^^^^^^^ expected future, found a different future
11 | |     };
   | |_____- `if` and `else` have incompatible types
   |
   = note: distinct uses of `impl Trait` result in different opaque types
help: consider `await`ing on both `Future`s
   |
8  ~         async_dummy().await
9  |     } else {
10 ~         async_dummy2().await
   |

error: aborting due to previous error