Add trybuild test for instrumented async fn type mismatch

tracing-attributes/Cargo.toml

@@ -45,6 +45,8 @@ tokio-test = "0.4.2"
 tracing-core = { path = "../tracing-core", version = "0.2"}
 tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", features = ["env-filter"] }
 async-trait = "0.1.56"
+trybuild = "1.0.64"
+rustversion = "1.0.9"
 
 [badges]
 maintenance = { status = "experimental" }

tracing-attributes/tests/ui.rs

@@ -0,0 +1,7 @@
+// Only test on nightly, since UI tests are bound to change over time
+#[rustversion::nightly]
+#[test]
+fn async_instrument() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/async_instrument.rs");
+}

tracing-attributes/tests/ui/async_instrument.rs

@@ -0,0 +1,31 @@
+#![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> {
+    ""
+}
+
+fn main() {
+    let _ = unit();
+    let _ = simple_mismatch();
+    let _ = opaque_unsatisfied();
+    let _ = mismatch_with_opaque();
+}

tracing-attributes/tests/ui/async_instrument.stderr

@@ -0,0 +1,56 @@
+error[E0308]: mismatched types
+ --> tests/ui/async_instrument.rs:5:5
+  |
+5 |     ""
+  |     ^^ expected `()`, found `&str`
+
+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[E0277]: `(&str,)` doesn't implement `std::fmt::Display`
+  --> tests/ui/async_instrument.rs:15:34
+   |
+15 | async fn opaque_unsatisfied() -> impl std::fmt::Display {
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^ `(&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
+
+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("")
+   |     ++++++++  +