This feature is aimed at giving proc macros access to powers similar to
those used by builtin macros such as `format_args!` or `concat!`. These
macros are able to accept macros in place of string literal parameters,
such as the format string, as they perform recursive macro expansion
while being expanded.

This can be especially useful in many cases thanks to helper macros like
`concat!`, `stringify!` and `include_str!` which are often used to
construct string literals at compile-time in user code.

For now, this method only allows expanding macros which produce
literals, although more expresisons will be supported before the method
is stabilized.

… off-switch

This also fixes `x doc src/tools/clippy` when compiler-docs is disabled.

As far as I can tell, this parameter was never used, so just delete it
as unnecessary.

proc_macro: Add an expand_expr method to TokenStream

This feature is aimed at giving proc macros access to powers similar to those used by builtin macros such as `format_args!` or `concat!`. These macros are able to accept macros in place of string literal parameters, such as the format string, as they perform recursive macro expansion while being expanded.

This can be especially useful in many cases thanks to helper macros like `concat!`, `stringify!` and `include_str!` which are often used to construct string literals at compile-time in user code.

For now, this method only allows expanding macros which produce literals, although more expressions will be supported before the method is stabilized.

In earlier versions of this PR, this method exclusively returned `Literal`, and spans on returned literals were stripped of expansion context before being returned to be as conservative as possible about permission leakage. The method's naming has been generalized to eventually support arbitrary expressions, and the context stripping has been removed (rust-lang#87264 (comment)), which should allow for more general APIs like "format_args_implicits" (rust-lang#67984) to be supported as well.

## API Surface

```rust
impl TokenStream {
    pub fn expand_expr(&self) -> Result<Expr, ExpandError>;
}

#[non_exhaustive]
pub struct ExpandError;

impl Debug for ExpandError { ... }
impl Display for ExpandError { ... }
impl Error for ExpandError {}
impl !Send for ExpandError {}
impl !Sync for ExpandError {}
```

…test, r=Mark-Simulacrum

Allow to run a specific rustdoc-js* test

r? `@Mark-Simulacrum`

…crum

Make `compiler-docs` only control the default instead of being a hard off-switch

This also fixes `x doc src/tools/clippy` when compiler-docs is disabled.

See rust-lang#90354 (comment).

r? `@Mark-Simulacrum`

…ulacrum

x.py: remove fixme by deleting code

As far as I can tell, this parameter was never used, so just delete it
as unnecessary.

…s, r=davidtwco

Record more artifact sizes during self-profiling.

This PR adds artifact size recording for

- "linked artifacts" (executables, RLIBs, dylibs, static libs)
- object files
- dwo files
- assembly files
- crate metadata
- LLVM bitcode files
- LLVM IR files
- codegen unit size estimates

Currently the identifiers emitted for these are hard-coded as string literals. Is it worth adding constants to https://github.com/rust-lang/measureme/blob/master/measureme/src/rustc.rs instead? We don't do that for query names and the like -- but artifact kinds might be more stable than query names.

Better document `Box` and `alloc::alloc::box_free` connection

The internal `alloc::alloc::box_free` function requires that its signature matches the `owned_box` struct's declaration, but previously that connection was only documented on the `box_free` function.

This PR makes the documentation two-way to help anyone making theoretical changes to `Box` to see the connection, since changes are more likely to originate from `Box`.