760170bdaf
## 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 {
| ^^^^^^
```
tracing-attributes
Macro attributes for application-level tracing.
Overview
tracing is a framework for instrumenting Rust programs to collect
structured, event-based diagnostic information. This crate provides the
#[instrument] attribute for automatically instrumenting functions using
tracing.
Note that this macro is also re-exported by the main tracing crate.
Compiler support: requires rustc 1.49+
Usage
First, add this to your Cargo.toml:
[dependencies]
tracing-attributes = "0.1.22"
This crate provides the #[instrument] attribute for instrumenting a function
with a tracing span. For example:
use tracing_attributes::instrument;
#[instrument]
pub fn my_function(my_arg: usize) {
// ...
}
Supported Rust Versions
Tracing is built against the latest stable release. The minimum supported version is 1.49. The current Tracing version is not guaranteed to build on Rust versions earlier than the minimum supported version.
Tracing follows the same compiler support policies as the rest of the Tokio project. The current stable Rust compiler and the three most recent minor versions before it will always be supported. For example, if the current stable compiler version is 1.45, the minimum supported version will not be increased past 1.42, three minor versions prior. Increasing the minimum supported compiler version is not considered a semver breaking change as long as doing so complies with this policy.
License
This project is licensed under the MIT license.
Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tokio by you, shall be licensed as MIT, without any additional terms or conditions.