For following:

```rust
struct A;
impl A {
    fn test4(&self) {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method
    }
```
Suggest:

```rust
impl A {
    fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method

    Ok(())
    }
}
```

For rust-lang#125997

The documentation incorrectly stated that std::env::var could return
an error for variable names containing '=' or the NUL byte. Copy the
correct documentation from var_os.

var_os was fixed in Commit 8a7a665, Pull Request rust-lang#109894, which
closed Issue rust-lang#109893.

This documentation was incorrectly added in commit f2c0f29, which
replaced a panic in var_os by returning None, but documented the
change as "May error if ...".

Reference the specific error values and link to them.

... by using `std::fs::remove_dir_all`, which handles a bunch of edge
cases including read-only files and symlinks which are extremely tricky
on Windows.

It's only valid when applied to a type or lifetime parameter
in `Drop` trait implementation.

fixes rust-lang#127576

Move ZST ABI handling to `rustc_target`

Currently, target specific handling of ZST function call ABI (specifically passing them indirectly instead of ignoring them) is handled in `rustc_ty_utils`, whereas all other target specific function call ABI handling is located in `rustc_target`. This PR moves the ZST handling to `rustc_target` so that all the target-specific function call ABI handling is in one place. In the process of doing so, this PR fixes rust-lang#125850 by ensuring that ZST arguments are always correctly ignored in the x86-64 `"sysv64"` ABI; any code which would be affected by this fix would have ICEd before this PR. Tests are also added using `#[rustc_abi(debug)]` to ensure this behaviour does not regress.

Fixes rust-lang#125850

…=Amanieu

fix: fs::remove_dir_all: treat internal ENOENT as success

fixes rust-lang#127576

Suggest adding Result return type for associated method in E0277.

Recommit rust-lang#126515 because I messed up during rebase,

Suggest adding Result return type for associated method in E0277.

For following:

```rust
struct A;
impl A {
    fn test4(&self) {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method
    }
```

Suggest:

```rust
impl A {
    fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method

    Ok(())
    }
}
```

For rust-lang#125997

r? ```@cjgillot```

…rkingjubilee

doc: std::env::var: Returns None for names with '=' or NUL byte

The documentation incorrectly stated that std::env::var could return an error for variable names containing '=' or the NUL byte. Copy the correct documentation from var_os.

var_os was fixed in Commit 8a7a665, Pull Request rust-lang#109894, which closed Issue rust-lang#109893.

This documentation was incorrectly added in commit f2c0f29, which replaced a panic in var_os by returning None, but documented the change as "May error if ...".

Reference the specific error values and link to them.

…links, r=Kobzol

bootstrap: fix clean's remove_dir_all implementation

It turns out bootstrap's `clean.rs`'s hand-rolled `rm_rf` (which probably comes before `std::fs::remove_dir_all` was stable) is very broken on native Windows around both read-only files/directories and especially symbolic links. So instead of rolling our own, just use `std::fs::remove_dir_all`.

This is a blocker for compiletest's own `rm_rf` implementation rust-lang#129155 which happens to be also buggy, which in turn is a blocker for the rmake.rs test port rust-lang#128562 that heavily exercises symlinks (I was reviewing rust-lang#128562 and testing it on native Windows which is how I found out).

I also left a FIXME for `detect_src_and_out` due to a failing assertion on native Windows (opened rust-lang#129188):

```
---- core::config::tests::detect_src_and_out stdout ----
thread 'core::config::tests::detect_src_and_out' panicked at src\core\config\tests.rs:72:13:
assertion `left == right` failed
  left: "E:\\tmp"
 right: "C:\\tmp"
```

Fixes rust-lang#112544 (because now we handle Windows symlinks properly).

try-job: x86_64-msvc
try-job: i686-mingw
try-job: test-various
try-job: armhf-gnu
try-job: aarch64-apple
try-job: aarch64-gnu

…piler-errors

Check that `#[may_dangle]` is properly applied

It's only valid when applied to a type or lifetime parameter in `Drop` trait implementation.

Tracking issue: rust-lang#34761
cc rust-lang#34761 (comment)

Typo

Fix a typo in a comment.