When expecting closure argument but finding block provide suggestion

[estebank]

Detect if there is a potential typo where the { meant to open the closure body was written before the body.

error[E0277]: expected a `FnOnce<({integer},)>` closure, found `Option<usize>`
  --> $DIR/ruby_style_closure_successful_parse.rs:3:31
   |
LL |       let p = Some(45).and_then({|x|
   |  ______________________--------_^
   | |                      |
   | |                      required by a bound introduced by this call
LL | |         1 + 1;
LL | |         Some(x * 2)
   | |         ----------- this tail expression is of type `Option<usize>`
LL | |     });
   | |_____^ expected an `FnOnce<({integer},)>` closure, found `Option<usize>`
   |
   = help: the trait `FnOnce<({integer},)>` is not implemented for `Option<usize>`
note: required by a bound in `Option::<T>::and_then`
  --> $SRC_DIR/core/src/option.rs:LL:COL
help: you might have meant to open the closure body instead of placing a closure within a block
   |
LL -     let p = Some(45).and_then({|x|
LL +     let p = Some(45).and_then(|x| {
   |

Detect the potential typo where the closure header is missing.

error[E0277]: expected a `FnOnce<(&bool,)>` closure, found `bool`
  --> $DIR/block_instead_of_closure_in_arg.rs:3:23
   |
LL |        Some(true).filter({
   |  _________________------_^
   | |                 |
   | |                 required by a bound introduced by this call
LL | |/         if number % 2 == 0 {
LL | ||             number == 0
LL | ||         } else {
LL | ||             number != 0
LL | ||         }
   | ||_________- this tail expression is of type `bool`
LL | |      });
   | |______^ expected an `FnOnce<(&bool,)>` closure, found `bool`
   |
   = help: the trait `for<'a> FnOnce<(&'a bool,)>` is not implemented for `bool`
note: required by a bound in `Option::<T>::filter`
  --> $SRC_DIR/core/src/option.rs:LL:COL
help: you might have meant to create the closure instead of a block
   |
LL |     Some(true).filter(|_| {
   |                       +++

Partially address #27300. Fix #104690.

[rustbot]

r? @petrochenkov

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[petrochenkov]

@bors r+

[bors]

📌 Commit c1bfd46 has been approved by petrochenkov

It is now in the queue for this repository.