Confusing lifetime error: expected `(&Location<'_>,)`, found `(&Location<'_>,)`

[camelid]

For one, I'm not sure why this isn't working since Location is Copy, but you can fix this by just storing self.start.line and self.end.line in locals and adding move before the closure. One of the weird things about this error is it says

expected (&Location<'_>,), found (&Location<'_>,)

but those are the same! Also, why are they 1-tuples? The error is a bit different on stable, the message I showed is on nightly:

expected &Location<'_>, found &Location<'_>

---

Code

#[derive(Debug, Clone, Copy)]
pub struct Location<'a> {
    pub filename: &'a str,
    pub start: LocationHalf,
    pub end: LocationHalf,
}

#[derive(Debug, Clone, Copy)]
pub struct LocationHalf {
    pub line: u32,
    pub column: u32,
}

impl Location<'_> {
    /// Returns an iterator over the line numbers of this location.
    pub fn line_numbers(self) -> impl Iterator<Item = u32> {
        self.start.line..=self.end.line
    }

    /// Returns an iterator over the line numbers and lines of this location.
    pub fn lines<'a>(self, source: &'a str) -> impl Iterator<Item = (u32, &'a str)> {
        let lines = source.split('\n');
        lines.enumerate().filter_map(|(i, line)| {
            if self.start.line as usize <= i && i <= self.end.line as usize {
                Some((i as u32 + 1, line))
            } else {
                None
            }
        })
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> src/lib.rs:23:38
   |
23 |           lines.enumerate().filter_map(|(i, line)| {
   |  ______________________________________^
24 | |             if self.start.line as usize <= i && i <= self.end.line as usize {
25 | |                 Some((i as u32 + 1, line))
26 | |             } else {
27 | |                 None
28 | |             }
29 | |         })
   | |_________^
   |
note: first, the lifetime cannot outlive the lifetime `'_` as defined on the impl at 14:15...
  --> src/lib.rs:14:15
   |
14 | impl Location<'_> {
   |               ^^
note: ...so that the types are compatible
  --> src/lib.rs:23:38
   |
23 |           lines.enumerate().filter_map(|(i, line)| {
   |  ______________________________________^
24 | |             if self.start.line as usize <= i && i <= self.end.line as usize {
25 | |                 Some((i as u32 + 1, line))
26 | |             } else {
27 | |                 None
28 | |             }
29 | |         })
   | |_________^
   = note: expected `(&Location<'_>,)`
              found `(&Location<'_>,)`
note: but, the lifetime must be valid for the lifetime `'a` as defined on the method body at 21:18...
  --> src/lib.rs:21:18
   |
21 |     pub fn lines<'a>(self, source: &'a str) -> impl Iterator<Item = (u32, &'a str)> {
   |                  ^^
note: ...so that return value is valid for the call
  --> src/lib.rs:21:48
   |
21 |     pub fn lines<'a>(self, source: &'a str) -> impl Iterator<Item = (u32, &'a str)> {
   |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

[camelid]

I marked this as a regression since the error has gotten worse since stable and beta.

[jyn514]

This looks like another duplicate of #41078.

[camelid]

Adding type annotations to the closure doesn't fix it though:

#[derive(Debug, Clone, Copy)]
pub struct Location<'a> {
    pub filename: &'a str,
    pub start: LocationHalf,
    pub end: LocationHalf,
}

#[derive(Debug, Clone, Copy)]
pub struct LocationHalf {
    pub line: u32,
    pub column: u32,
}

impl Location<'_> {
    /// Returns an iterator over the line numbers of this location.
    pub fn line_numbers(self) -> impl Iterator<Item = u32> {
        self.start.line..=self.end.line
    }

    /// Returns an iterator over the line numbers and lines of this location.
    pub fn lines<'a>(self, source: &'a str) -> impl Iterator<Item = (u32, &'a str)> {
        let lines = source.split('\n');
        lines.enumerate().filter_map(|(i, line): (usize, &'a str)| {
            if self.start.line as usize <= i && i <= self.end.line as usize {
                Some((i as u32 + 1, line))
            } else {
                None
            }
        })
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> src/lib.rs:23:38
   |
23 |           lines.enumerate().filter_map(|(i, line): (usize, &'a str)| {
   |  ______________________________________^
24 | |             if self.start.line as usize <= i && i <= self.end.line as usize {
25 | |                 Some((i as u32 + 1, line))
26 | |             } else {
27 | |                 None
28 | |             }
29 | |         })
   | |_________^
   |
note: first, the lifetime cannot outlive the lifetime `'_` as defined on the impl at 14:15...
  --> src/lib.rs:14:15
   |
14 | impl Location<'_> {
   |               ^^
note: ...so that the types are compatible
  --> src/lib.rs:23:38
   |
23 |           lines.enumerate().filter_map(|(i, line): (usize, &'a str)| {
   |  ______________________________________^
24 | |             if self.start.line as usize <= i && i <= self.end.line as usize {
25 | |                 Some((i as u32 + 1, line))
26 | |             } else {
27 | |                 None
28 | |             }
29 | |         })
   | |_________^
   = note: expected `(&Location<'_>,)`
              found `(&Location<'_>,)`
note: but, the lifetime must be valid for the lifetime `'a` as defined on the method body at 21:18...
  --> src/lib.rs:21:18
   |
21 |     pub fn lines<'a>(self, source: &'a str) -> impl Iterator<Item = (u32, &'a str)> {
   |                  ^^
note: ...so that return value is valid for the call
  --> src/lib.rs:21:48
   |
21 |     pub fn lines<'a>(self, source: &'a str) -> impl Iterator<Item = (u32, &'a str)> {
   |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

[camelid]

Also, notice that the error note shows tuples on nightly but not stable – that seems like a bug. And, why does the error say:

note: first, the lifetime cannot outlive the lifetime `'_` as defined on the impl at 14:15...
  --> src/lib.rs:14:15
   |
14 | impl Location<'_> {
   |               ^^
note: ...so that the types are compatible
  --> src/lib.rs:23:38
   |
23 |           lines.enumerate().filter_map(|(i, line): (usize, &'a str)| {
   |  ______________________________________^
24 | |             if self.start.line as usize <= i && i <= self.end.line as usize {
25 | |                 Some((i as u32 + 1, line))
26 | |             } else {
27 | |                 None
28 | |             }
29 | |         })
   | |_________^
   = note: expected `(&Location<'_>,)`
              found `(&Location<'_>,)`

I don't understand why it must match the Location's lifetime and also why adding move before the closure doesn't fix the problem.

[jyn514]

Here is a fixed version: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3d9156ed9de67e4bf402f54243f5f3e8
Here is a fixed version that still captures self in the closure (although that's rarely what you want): https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=8dd9c0596d4399be5bbdacb0629d7189

So the error is correct, it's just confusing.

[spastorino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.