Diagnostic about `try` could be better on edition 2018

[Centril]

Given:

fn main() -> Result<(), ()> {
    let x = try!(Ok(1u8));
    Ok(())
}

the compiler emits the standard "reserved keyword error":

error: expected expression, found reserved keyword `try`
 --> src/main.rs:2:13
  |
2 |     let x = try!(Ok(1u8));
  |             ^^^ expected expression

However, since try! was once a macro people used (wherefore it stands to reason that some old documentation may folk astray), it might be prudent to catch this pattern somehow and refer the user to the documentation about ? somewhere.

cc @scottmcm, @estebank

[zackmdavis]

refer the user to the documentation about ? somewhere.

Or write a cargo fix-able lint for the rust-2018-compatibility lint group.

[estebank]

Is the macro still available in 2018 as r#try? If it is we should also suggest it, but otherwise just

error: expected expression, found reserved keyword `try`
 --> src/main.rs:2:13
  |
2 |     let x = try!(Ok(1u8));
  |             ^^^ expected expression
  help: the `try!` macro has been removed from the language, use the `?` operator instead
  |
2 |     let x = Ok(1u8)?;
  |             ^^^^^^^^

The actual implementation of the MachineApplicable suggestion needs to attempt to parse try!(.*), and if successful suggest accounting for operator precedence (injecting braces as needed).

As a stretch goal, if we encounter any keyword being used and there's an r#{keyword} binding in scope, or if we're in the name of a let statement, suggest using the r#{keyword} version.

error: expected expression, found reserved keyword `try`
 --> src/main.rs:2:13
  |
2 |     let try = 42;
  |         ^^^ 
  |         |
  |         expected expression
  |         help: you can use the raw identifier format: `r#try`

error: expected expression, found reserved keyword `try`
 --> src/main.rs:2:13
  |
2 |     let x = try;
  |             ^^^ 
  |             |
  |             expected expression
  |             help: a binding with that name using the raw identifier format is available: `r#try`

[Centril]

Is the macro still available in 2018 as r#try?

Yes; it has to be and this is what cargo fix --edition will change try!(..) to.

help: the try! macro has been removed from the language, use the ? operator instead

This seems good; tho technically it hasn't been removed so perhaps emit:

if you were trying to use the try! macro, use the ? operator instead or refer to the macro with r#try!.

We should try to nudge folks towards ? here in some way that is.

As a stretch goal, if we encounter any keyword being used and there's an r#{keyword} binding in scope, or if we're in the name of a let statement, suggest using the r#{keyword} version.

This seems like a great idea.

[estebank]

Given that r#try!() is still available in 2018, then the error should be closer to the following in that case

error: expected expression, found reserved keyword `try`
 --> src/main.rs:2:13
  |
2 |     let x = try!(Ok(1u8));
  |             ^^^ expected expression
  help: `try` is a reserved keyword in the 2018 edition, use the `?` operator instead
  |
2 |     let x = Ok(1u8)?;
  |             ^^^^^^^^
  help: alternatively, you can still access the `try!` macro using the raw identifier format
  |
2 |     let x = r#try!(Ok(1u8));
  |             ^^

[Centril]

That seems better / quite good :)

[philippludwig]

Is there any documentation somewhere why try! was removed? All I can find is this issue.

[Centril]

@philippludwig it was decided here: #53686 (comment).

[scottmcm]

Looks like the improved version happened, so I'll close this:

error: use of deprecated `try` macro
 --> src/main.rs:2:13
  |
2 |     let x = try!(Ok(1u8));
  |             ^^^^^^^^^^^^^
  |
  = note: in the 2018 edition `try` is a reserved keyword, and the `try!()` macro is deprecated
help: you can use the `?` operator instead
  |
2 |     let x = Ok(1u8)?;
  |            --      ^
help: alternatively, you can still access the deprecated `try!()` macro using the "raw identifier" syntax
  |
2 |     let x = r#try!(Ok(1u8));
  |             ^^