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 rust-lang#27300.

Currently it only tests AST pretty-printing. This commit changes it to
run every example through both AST pretty-printing and TokenStream
pretty-printing. This makes it clear where there two pretty-printing
approaches produce different results.

By adding tests (or placeholders, or comments) for missing AST variants.

By making some case more complex, adding some new cases, tweaking
formatting, and removing unnecessary `rustfmt` attributes.

…of truth

    ---- symbols::tests::test_symbols stdout ----
    thread 'symbols::tests::test_symbols' panicked at library/proc_macro/src/bridge/client.rs:311:17:
    procedural macro API is used outside of a procedural macro

…em::swap

Invalid `?` suggestion on mismatched `Ok(T)`

fixes: rust-lang#116967

…s, r=petrochenkov

Enable cg_clif tests for riscv64gc

Cranelift now has support for riscv64 on Linux.

When expecting closure argument but finding block provide suggestion

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 rust-lang#27300. Fix rust-lang#104690.

…, r=petrochenkov

Improve `stringify.rs` test

Best reviewed one commit at a time.

r? `@petrochenkov`

Avoid repeated interning of `env!("CFG_RELEASE")`

Implements `@cjgillot's` suggestion from rust-lang#117148 (comment).

…t, r=thomcc

Explain implementation of mem::replace

This adds a comment to explain why `mem::replace` is not implemented in terms of `mem::swap` to prevent [naïfs like me](rust-lang#117189) from trying to "fix" it.