fix: better spans in #[tokio::main] output

[evertedsphere]

Fix #5518.

Motivation

The #[tokio::main] macro in its current state produces output that degrades IDE functionality — code generated by the proc macro is annotated with spans that point back to the end of the (user-provided) contents of async fn main, resulting in IDE plugins (correctly) treating the last binding in main as a call to a Tokio function.

As an example, if one adds a typo to the generated code, the error produced on master points to user code:

error[E0599]: no method named `enable_al` found for struct `tokio::runtime::Builder` in the current scope
 --> src/main.rs:5:13
  |
5 |     variable;
  |             ^ help: there is a method with a similar name: `enable_all`

Solution

We replace the span attached to the Tokio-generated code with one that points to the invocation site of the #[tokio::main] macro itself:

error[E0599]: no method named `enable_al` found for struct `tokio::runtime::Builder` in the current scope
 --> src/main.rs:1:1
  |
1 | #[tokio::main]
  | ^^^^^^^^^^^^^^ help: there is a method with a similar name: `enable_all`

[evertedsphere]

There are a few other quote! invocations in parse_knobs that also point back to user code; perhaps it's a good idea to also make the same change for those?

[evertedsphere]

Looks like my patch breaks errors which should point to user code, e.g. when the body of async fn main is an expression that returns the wrong type, the following

error[E0308]: mismatched types
  --> $DIR/macros_type_mismatch.rs:32:5
   |
30 | async fn issue_4635() {
   |                       - help: try adding a return type: `-> i32`
31 |     return 1;
32 |     ;
   |     ^ expected `()`, found integer

becomes

error[E0308]: mismatched types
  --> tests/fail/macros_type_mismatch.rs:29:1
   |
29 | #[tokio::main]
   | ^^^^^^^^^^^^^^ expected `()`, found integer
30 | async fn issue_4635() {
   |                       - help: try adding a return type: `-> i32`

This can be fixed by asserting the right type for the body earlier within a span that points back to user code.

---

One thing I've tried is changing the quote! here to a quote_spanned!:

tokio/tokio-macros/src/entry.rs

Lines 415 to 417 in e10ffa2

	quote! {
	let body = async #body;
	}

and annotating the type there via let body: ??? = async #body, so that typechecking fails there instead of inside the block_on.

However, I can't figure out what type to put there: rustc complains about my variations on &mut dyn Future<..> not being Unpin. If I use the code in the is_test branch with the Unpin, it works just fine and produces decent spans (although there's some noise because the type mismatch occurs at a pinned future type instantiated at the output type, rather than at the output type itself):

error[E0308]: mismatched types
  --> $DIR/macros_type_mismatch.rs:32:5
   |
30 | async fn issue_4635() {
   |                       - help: try adding a return type: `-> i32`
31 |     return 1;
32 |     ;
   |     ^ expected `()`, found integer

is what you get on master, which becomes

error[E0271]: expected `[async block@$DIR/tests/fail/macros_type_mismatch.rs:30:23: 33:2]` to be a future that resolves to `()`, but it resolves to `{integer}`
  --> tests/fail/macros_type_mismatch.rs:32:5
   |
32 |     ;
   |     ^ expected `()`, found integer
   |
   = note: required for the cast from `[async block@$DIR/tests/fail/macros_type_mismatch.rs:30:23: 33:2]` to the object type `dyn Future<Output = ()>`

[evertedsphere]

I figured it out: doing

let body = async { let x: #output_type = body; x };

solves the problem — although it does complain about unreachable code when body is, say, return Ok(());.

[evertedsphere]

I think this is ready for review.

Remaining issues:

1. I'll need some help with this test that calls #[tokio::test] inside an MBE macro:

error[E0425]: cannot find value `body` in this scope
  --> tokio/tests/macros_test.rs:79:13
   |
79 |               #[::tokio::test]
   |               ^^^^^^^^^^^^^^^^ not found in this scope
...
85 | /     mac!(
86 | |         async fn foo() {}
87 | |     );
   | |_____- in this macro invocation
   |

I'm clearly doing something wrong around identifiers/quoting here, but I can't spot the issue.

2. Not sure what to do for

error[E0658]: the `!` type is experimental
  --> tokio/tests/macros_test.rs:38:37
   |
38 | pub async fn issue_4175_main_1() -> ! {
   |                                     ^
   |

since the type is already there in the test's source. (I suppose annotating a value as x: ! isn't supported?)

3. This abomination:

  --> tests/fail/macros_type_mismatch.rs:10:12
   |
10 |     return Ok(());
   |            ^^^^^^
help: try wrapping the expression in a variant of `Result`
   |
10 |     return Ok(())Ok(;)
   |                  +++ +
10 |     return Ok(())Err(;)
   |                  ++++ +

[evertedsphere]

I'm unable to run tests etc on the MSRV (1.56.0 currently) because I get:

$ TRYBUILD=overwrite cargo +1.56.0 test -p tests-build  --all-features -j4
    Updating crates.io index
error: failed to select a version for the requirement `predicates = "=2.1.5"`
candidate versions found which didn't match: 2.1.1, 2.1.0, 2.0.3, ...
location searched: crates.io index
required by package `mockall v0.11.4`
    ... which satisfies dependency `mockall = "=0.11.4"` of package `tokio v1.27.0 (/home/k/c/tokio/tokio)`

[taiki-e]

Thanks for the PR.

1. I'll need some help with this test that calls #[tokio::test] inside an MBE macro:

See #5244, which fixed the same issue in the past.

error[E0658]: the ! type is experimental

If the return type contains an unstable type such as impl trait or never type, the type annotation needs to be omitted.
See also #3039 (comment).

I'm unable to run tests etc on the MSRV (1.56.0 currently) because I get:

This is due to rust-lang/cargo#10623 (predicates uses weak and namespaced features).

[Darksonn]

I just merged #5621, which conflicts with your PR. Sorry about that.

[Darksonn]

Would you like to continue this PR?

[Darksonn]

Superseded by #5762.