use the code generation parameter -Clinker (same parameter as rustc)
to control what linker to use for building the rustdoc test executables.
closes: #63816
Migrate some books to mdbook version 0.2
There are 3 books still using old version but they need more effort so I hope to do them in subsequent PR if I find the time.
Document the `html_root_url` doc attribute value.
I'm not sure if this was intentionally not documented, but I think it would be good to include. This was added in #9691. `--extern-html-root-url` is unstable, but I don't think it hurts to mention it.
Modify doctest's auto-`fn main()` to allow `Result`s
This lets the default `fn main()` ~~return `impl Termination`~~ unwrap Results, which allows the use of `?` in most tests without adding it manually. This fixes#56260
~~Blocked on `std::process::Termination` stabilization.~~
Using `Termination` would have been cleaner, but this should work OK.
set cfg(rustdoc) when rustdoc is running on a crate
When using `#[doc(cfg)]` to document platform-specific items, it's a little cumbersome to get all the platforms' items to appear all at once. For example, the standard library adds `--cfg dox` to rustdoc's command line whenever it builds docs, and the documentation for `#![feature(doc_cfg)]` suggests using a Cargo feature to approximate the same thing. This is a little awkward, because you always need to remember to set `--features dox` whenever you build documentation.
This PR proposes making rustdoc set `#[cfg(rustdoc)]` whenever it runs on a crate, to provide an officially-sanctioned version of this that is set automatically. This way, there's a standardized way to declare that a certain version of an item is specifically when building docs.
To try to prevent the spread of this feature from happening too quickly, this PR also restricts the use of this flag to whenever `#![feature(doc_cfg)]` is active. I'm sure there are other uses for this, but right now i'm tying it to this feature. (If it makes more sense to give this its own feature, i can easily do that.)
