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attributes: add fake return to improve span on type error (#2270)
## Motivation
Return type errors on instrumented async functions are a bit vague,
since the type error originates within the macro itself due to the
indirection of additional `async {}` blocks generated in the proc-macro
(and due to the way that inference propagates around in Rust).
This leads to a pretty difficult to understand error. For example:
```rust
#[instrument]
async fn foo() -> String {
""
}
```
results in...
```
error[E0308]: mismatched types
--> src/main.rs:1:1
|
1 | #[tracing::instrument]
| ^^^^^^^^^^^^^^^^^^^^^^- help: try using a conversion method: `.to_string()`
| |
| expected struct `String`, found `&str`
```
## Solution
Installs a fake `return` statement as the first thing that happens in
the auto-generated block of an instrumented async function.
This causes the coercion machinery within rustc to infer the right
return type (matching the the outer function) eagerly, as opposed to
after the `async {}` block has been type-checked.
This will cause us to both be able to point out the return type span
correctly, and properly suggest fixes on the expressions that cause the
type mismatch.
After this change, the example code above compiles to:
```
error[E0308]: mismatched types
--> src/main.rs:3:5
|
3 | ""
| ^^- help: try using a conversion method: `.to_string()`
| |
| expected struct `String`, found `&str`
|
note: return type inferred to be `String` here
--> src/main.rs:2:20
|
2 | async fn foo() -> String {
| ^^^^^^
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,7 @@ | ||
| // Only test on nightly, since UI tests are bound to change over time | ||
| #[rustversion::stable] | ||
| #[test] | ||
| fn async_instrument() { | ||
| let t = trybuild::TestCases::new(); | ||
| t.compile_fail("tests/ui/async_instrument.rs"); | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,46 @@ | ||
| #![allow(unreachable_code)] | ||
|
|
||
| #[tracing::instrument] | ||
| async fn unit() { | ||
| "" | ||
| } | ||
|
|
||
| #[tracing::instrument] | ||
| async fn simple_mismatch() -> String { | ||
| "" | ||
| } | ||
|
|
||
| // FIXME: this span is still pretty poor | ||
| #[tracing::instrument] | ||
| async fn opaque_unsatisfied() -> impl std::fmt::Display { | ||
| ("",) | ||
| } | ||
|
|
||
| struct Wrapper<T>(T); | ||
|
|
||
| #[tracing::instrument] | ||
| async fn mismatch_with_opaque() -> Wrapper<impl std::fmt::Display> { | ||
| "" | ||
| } | ||
|
|
||
| #[tracing::instrument] | ||
| async fn early_return_unit() { | ||
| if true { | ||
| return ""; | ||
| } | ||
| } | ||
|
|
||
| #[tracing::instrument] | ||
| async fn early_return() -> String { | ||
| if true { | ||
| return ""; | ||
| } | ||
| String::new() | ||
| } | ||
|
|
||
| #[tracing::instrument] | ||
| async fn extra_semicolon() -> i32 { | ||
| 1; | ||
| } | ||
|
|
||
| fn main() {} |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,89 @@ | ||
| error[E0308]: mismatched types | ||
| --> tests/ui/async_instrument.rs:5:5 | ||
| | | ||
| 5 | "" | ||
| | ^^ expected `()`, found `&str` | ||
| | | ||
| note: return type inferred to be `()` here | ||
| --> tests/ui/async_instrument.rs:4:10 | ||
| | | ||
| 4 | async fn unit() { | ||
| | ^^^^ | ||
|
|
||
| error[E0308]: mismatched types | ||
| --> tests/ui/async_instrument.rs:10:5 | ||
| | | ||
| 10 | "" | ||
| | ^^- help: try using a conversion method: `.to_string()` | ||
| | | | ||
| | expected struct `String`, found `&str` | ||
| | | ||
| note: return type inferred to be `String` here | ||
| --> tests/ui/async_instrument.rs:9:31 | ||
| | | ||
| 9 | async fn simple_mismatch() -> String { | ||
| | ^^^^^^ | ||
|
|
||
| error[E0277]: `(&str,)` doesn't implement `std::fmt::Display` | ||
| --> tests/ui/async_instrument.rs:14:1 | ||
| | | ||
| 14 | #[tracing::instrument] | ||
| | ^^^^^^^^^^^^^^^^^^^^^^ `(&str,)` cannot be formatted with the default formatter | ||
| | | ||
| = help: the trait `std::fmt::Display` is not implemented for `(&str,)` | ||
| = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead | ||
| = note: this error originates in the attribute macro `tracing::instrument` (in Nightly builds, run with -Z macro-backtrace for more info) | ||
|
|
||
| error[E0308]: mismatched types | ||
| --> tests/ui/async_instrument.rs:23:5 | ||
| | | ||
| 23 | "" | ||
| | ^^ expected struct `Wrapper`, found `&str` | ||
| | | ||
| = note: expected struct `Wrapper<_>` | ||
| found reference `&'static str` | ||
| note: return type inferred to be `Wrapper<_>` here | ||
| --> tests/ui/async_instrument.rs:22:36 | ||
| | | ||
| 22 | async fn mismatch_with_opaque() -> Wrapper<impl std::fmt::Display> { | ||
| | ^^^^^^^ | ||
| help: try wrapping the expression in `Wrapper` | ||
| | | ||
| 23 | Wrapper("") | ||
| | ++++++++ + | ||
|
|
||
| error[E0308]: mismatched types | ||
| --> tests/ui/async_instrument.rs:29:16 | ||
| | | ||
| 29 | return ""; | ||
| | ^^ expected `()`, found `&str` | ||
| | | ||
| note: return type inferred to be `()` here | ||
| --> tests/ui/async_instrument.rs:27:10 | ||
| | | ||
| 27 | async fn early_return_unit() { | ||
| | ^^^^^^^^^^^^^^^^^ | ||
|
|
||
| error[E0308]: mismatched types | ||
| --> tests/ui/async_instrument.rs:36:16 | ||
| | | ||
| 36 | return ""; | ||
| | ^^- help: try using a conversion method: `.to_string()` | ||
| | | | ||
| | expected struct `String`, found `&str` | ||
| | | ||
| note: return type inferred to be `String` here | ||
| --> tests/ui/async_instrument.rs:34:28 | ||
| | | ||
| 34 | async fn early_return() -> String { | ||
| | ^^^^^^ | ||
|
|
||
| error[E0308]: mismatched types | ||
| --> tests/ui/async_instrument.rs:42:35 | ||
| | | ||
| 42 | async fn extra_semicolon() -> i32 { | ||
| | ___________________________________^ | ||
| 43 | | 1; | ||
| | | - help: remove this semicolon | ||
| 44 | | } | ||
| | |_^ expected `i32`, found `()` |