Very hard to read error when returning ? from closure not returning Result or Option

[jyn514]

The original code:

    // '[' expr ']' | '(' argument* ')' | '.' ID | '->' ID | '++' | '--'
    fn match_postfix_op(&mut self) -> SyntaxResult<Option<Locatable<impl Fn(Expr) -> ExprType>>> {
        let needs_id = |this: &mut Self, constructor: fn(Box<Expr>, InternedStr) -> ExprType| {
            let start = this.next_token().unwrap().location;
            let Locatable { data: id, location } = this.expect_id()?;
            let location = start.merge(&location);
            (move |expr| constructor(expr, id), location)
        };
        // prefix operator
        let (func, location) = match self.peek_token() {
            Some(Token::Dot) => needs_id(self, ExprType::Member)?,
            _ => return Ok(None),
        };
        Ok(Some(Locatable {
            data: move |e| func(Box::new(e)),
            location,
        }))
    }

error[E0277]: the `?` operator can only be used in a closure that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
   --> src/expr.rs:237:52
    |
235 |           let needs_id = |this: &mut Self, constructor: fn(Box<Expr>, InternedStr) -> ExprType| {
    |  ________________________-
236 | |             let start = this.next_token().unwrap().location;
237 | |             let Locatable { data: id, location } = this.expect_id()?;
    | |                                                    ^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a closure that returns `([closure@src/expr.rs:239:14: 239:47 constructor:_, id:_], data::lex::DefaultLocation)`
238 | |             let location = start.merge(&location);
239 | |             (move |expr| constructor(expr, id), location)
240 | |         };
    | |_________- this function should return `Result` or `Option` to accept `?`
    |
    = help: the trait `std::ops::Try` is not implemented for `([closure@src/expr.rs:239:14: 239:47 constructor:_, id:_], data::lex::DefaultLocation)`
    = note: required by `std::ops::Try::from_error`

error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
   --> src/expr.rs:243:33
    |
243 |             Some(Token::Dot) => needs_id(self, ExprType::Member)?,
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the `?` operator cannot be applied to type `([closure@src/expr.rs:239:14: 239:47 constructor:_, id:_], data::lex::DefaultLocation)`
    |
    = help: the trait `std::ops::Try` is not implemented for `([closure@src/expr.rs:239:14: 239:47 constructor:_, id:_], data::lex::DefaultLocation)`
    = note: required by `std::ops::Try::into_result`

error: aborting due to 2 previous errors

The fix was to return Ok() at the end of the closure: Ok((move |expr| constructor(expr, id), location)). It would be much more helpful if the error message suggested returning Ok() at the end instead of spouting a wall of text at me.

[jyn514]

Minimized: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=fe170e5b26850bb57a1e82f0fcae117c

fn returns_err() -> Result<(), ()> {
    Err(())
}

fn match_postfix_op() -> Result<usize, ()> {
    let closure = || {
        let _ = returns_err()?;
        1
    };
    let i = closure()?;
    Ok(i)
}

[jyn514]

Related issue: the closure types just look like noise. Maybe rustc could only print the location, not all the arguments? closure@src/expr.rs:239:14

[LeSeulArtichaut]

I think we can @rustbot modify labels: A-diagnostics D-papercut

[estebank]

I believe the current output is an improvement over the original report:

error[E0277]: the `?` operator can only be used in a closure that returns `Result` or `Option` (or another type that implements `FromResidual`)
 --> src/lib.rs:7:30
  |
6 |     let closure = || {
  |                   -- this function should return `Result` or `Option` to accept `?`
7 |         let _ = returns_err()?;
  |                              ^ cannot use the `?` operator in a closure that returns `{integer}`
  |
  = help: the trait `FromResidual<Result<Infallible, ()>>` is not implemented for `{integer}`

error[E0277]: the `?` operator can only be applied to values that implement `Try`
  --> src/lib.rs:11:13
   |
11 |     let i = closure()?;
   |             ^^^^^^^^^^ the `?` operator cannot be applied to type `{integer}`
   |
   = help: the trait `Try` is not implemented for `{integer}`

[jyn514]

it's better - "Try not implemented for integer" is the actual problem, but it's hard to spot. it would be nice if it pointed to line 8 with 1 as the reason the return type is inferred to be {integer}.

[estebank]

I think that could be done. We already walk the hir tree to point at where inference kicks in.