unhelpful error when missing `move` in nested closure

[lcnr]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1cb8bf9552639abd2a8178105cd3f001

fn foo(s: &str) -> impl Iterator<Item = String> + '_ {
    None.into_iter()
        .flat_map(move |()| s.chars().map(|c| format!("{}{}", c, s)))
}

The current output is:

error: captured variable cannot escape `FnMut` closure body
 --> src/lib.rs:3:29
  |
1 | fn foo(s: &str) -> impl Iterator<Item = String> + '_ {
  |        - variable defined here
2 |     None.into_iter()
3 |         .flat_map(move |()| s.chars().map(|c| format!("{}{}", c, s)))
  |                           - -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                           | |
  |                           | returns a reference to a captured variable which escapes the closure body
  |                           | variable captured here
  |                           inferred to be a `FnMut` closure
  |
  = note: `FnMut` closures only have access to their captured variables while they are executing...
  = note: ...therefore, they cannot allow references to captured variables to escape

error: could not compile `playground` due to previous error

The actually needed fix here is the following, which should ideally be recommended

fn foo(s: &str) -> impl Iterator<Item = String> + '_ {
    None.into_iter()
        .flat_map(move |()| s.chars().map(move |c| format!("{}{}", c, s)))
}

[lcnr]

slightly more minimal:

fn foo(s: &str) -> impl Sized + '_ {
    move |()| s.chars().map(|c| format!("{}{}", c, s))
}

[yanchen4791]

@rustbot claim

[yanchen4791]

I came up with a solution of printing an extra line of help: to give user a recommendation of fixing the error by adding 'move' keyword before the closure parameter of map(). The following is the output for the first testing code. Is this good enough?

error: captured variable cannot escape `FnMut` closure body
 --> main_95079_1.rs:3:29
  |
1 | fn foo(s: &str) -> impl Iterator<Item = String> + '_ {
  |        - variable defined here
2 |     None.into_iter()
3 |         .flat_map(move |()| s.chars().map(|c| format!("{}{}", c, s)))
  |                           - -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                           | |
  |                           | returns a reference to a captured variable which escapes the closure body
  |                           | variable captured here
  |                           inferred to be a `FnMut` closure
  |
  = note: `FnMut` closures only have access to their captured variables while they are executing...
  = note: ...therefore, they cannot allow references to captured variables to escape
  = help: consider adding 'move' keyword before the parameter of closure in `Map<Chars<'_>, [closure@main_95079_1.rs:3:43: 3:68]>`

error: aborting due to previous error

This is the output from the second testing code (slightly more minimal):

error: lifetime may not live long enough
 --> main_95079_2.rs:2:15
  |
2 |     move |()| s.chars().map(|c| format!("{}{}", c, s))
  |     --------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
  |     |       |
  |     |       return type of closure `Map<Chars<'_>, [closure@main_95079_2.rs:2:29: 2:54]>` contains a lifetime `'2`
  |     lifetime `'1` represents this closure's body
  |
  = note: closure implements `Fn`, so references to captured variables can't escape the closure
  = help: consider adding 'move' keyword before the parameter of closure in `Map<Chars<'_>, [closure@main_95079_2.rs:2:29: 2:54]>`

error: aborting due to previous error

[yanchen4791]

Refined the fix with a verbose suggestion of where to add move keyword. The output of the first test case is:

error: captured variable cannot escape `FnMut` closure body
 --> /home/ychen/hello_world/src/main_95079_1.rs:3:29
  |
1 | fn foo(s: &str) -> impl Iterator<Item = String> + '_ {
  |        - variable defined here
2 |     None.into_iter()
3 |         .flat_map(move |()| s.chars().map(|c| format!("{}{}", c, s)))
  |                           - -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                           | |
  |                           | returns a reference to a captured variable which escapes the closure body
  |                           | variable captured here
  |                           inferred to be a `FnMut` closure
  |
  = note: `FnMut` closures only have access to their captured variables while they are executing...
  = note: ...therefore, they cannot allow references to captured variables to escape
help: consider adding 'move' keyword before the nested closure
  |
3 |         .flat_map(move |()| s.chars().map(move |c| format!("{}{}", c, s)))
  |                                           ++++

error: aborting due to previous error

The output of the second test case is:

error: lifetime may not live long enough
 --> /home/ychen/hello_world/src/main_95079_2.rs:2:15
  |
2 |     move |()| s.chars().map(|c| format!("{}{}", c, s))
  |     --------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
  |     |       |
  |     |       return type of closure `Map<Chars<'_>, [closure@/home/ychen/hello_world/src/main_95079_2.rs:2:29: 2:32]>` contains a lifetime `'2`
  |     lifetime `'1` represents this closure's body
  |
  = note: closure implements `Fn`, so references to captured variables can't escape the closure
help: consider adding 'move' keyword before the nested closure
  |
2 |     move |()| s.chars().map(move |c| format!("{}{}", c, s))
  |                             ++++

error: aborting due to previous error

In addition, the fix will also add the suggestion for nested closure block like this test case:

fn main() {
    // Field from upvar
    {
        let mut x = 0;
        || {
            let y = &mut x;
            &mut x; //~ ERROR cannot borrow `x` as mutable more than once at a time
            *y = 1;
        };
    }
    // Field from upvar nested
    {
        let mut x = 0;
           || {
               || { //~ ERROR captured variable cannot escape `FnMut` closure body
                   let y = &mut x;
                   &mut x; //~ ERROR cannot borrow `x` as mutable more than once at a time
                   *y = 1;
                   drop(y);
                }
           };
    }
    {
        let f = move || {};
        let _action = move || {
            || f() // The `nested` closure
            //~^ ERROR lifetime may not live long enough
        };
    }
}

The output of the above test case is:

error[E0499]: cannot borrow `x` as mutable more than once at a time
 --> /home/ychen/hello_world/src/main_closue_body.rs:7:13
  |
6 |             let y = &mut x;
  |                     ------ first mutable borrow occurs here
7 |             &mut x; //~ ERROR cannot borrow `x` as mutable more than once at a time
  |             ^^^^^^ second mutable borrow occurs here
8 |             *y = 1;
  |             ------ first borrow later used here

error[E0499]: cannot borrow `x` as mutable more than once at a time
  --> /home/ychen/hello_world/src/main_closue_body.rs:17:20
   |
16 |                    let y = &mut x;
   |                            ------ first mutable borrow occurs here
17 |                    &mut x; //~ ERROR cannot borrow `x` as mutable more than once at a time
   |                    ^^^^^^ second mutable borrow occurs here
18 |                    *y = 1;
   |                    ------ first borrow later used here

error: captured variable cannot escape `FnMut` closure body
  --> /home/ychen/hello_world/src/main_closue_body.rs:15:16
   |
13 |           let mut x = 0;
   |               ----- variable defined here
14 |              || {
   |               - inferred to be a `FnMut` closure
15 | /                || { //~ ERROR captured variable cannot escape `FnMut` closure body
16 | |                    let y = &mut x;
   | |                                 - variable captured here
17 | |                    &mut x; //~ ERROR cannot borrow `x` as mutable more than once at a time
18 | |                    *y = 1;
19 | |                    drop(y);
20 | |                 }
   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape
help: consider adding 'move' keyword before the nested closure
   |
15 |                move || { //~ ERROR captured variable cannot escape `FnMut` closure body
   |                ++++

error: lifetime may not live long enough
  --> /home/ychen/hello_world/src/main_closue_body.rs:26:13
   |
25 |         let _action = move || {
   |                       -------
   |                       |     |
   |                       |     return type of closure `[closure@/home/ychen/hello_world/src/main_closue_body.rs:26:13: 26:15]` contains a lifetime `'2`
   |                       lifetime `'1` represents this closure's body
26 |             || f() // The `nested` closure
   |             ^^^^^^ returning this value requires that `'1` must outlive `'2`
   |
   = note: closure implements `Fn`, so references to captured variables can't escape the closure
help: consider adding 'move' keyword before the nested closure
   |
26 |             move || f() // The `nested` closure
   |             ++++

error: aborting due to 4 previous errors