Rollup merge of #84942 - jyn514:channel-replace, r=Manishearth

rustdoc: link to stable/beta docs consistently in documentation

This is an alternative to https://github.com/rust-lang/rust/pull/84941 which fixes the problem consistently by linking to stable/beta for *all* items, not just for primitives.

 ## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel

cc Mark-Simulacrum - I know [you were dubious about this in the past](231223124), but I'm not quite sure why? I see this as "just a bugfix", I don't know why rustdoc should unconditionally link to nightly.
cc dtolnay who commented in https://github.com/rust-lang/rust/issues/30693:

>  I would welcome a PR to solve this permanently if anyone has ideas for how. I don't believe we need an RFC.

Fixes https://github.com/rust-lang/rust/issues/30693 (note that issue is marked as feature-accepted, although I don't see where it was discussed).

src/librustdoc/clean/types.rs

@@ -499,7 +499,7 @@ impl Item {
                                     format!("{}/std/", s.trim_end_matches('/'))
                                 }
                                 Some(ExternalLocation::Unknown) | None => {
-                                    "https://doc.rust-lang.org/nightly/std/".to_string()
+                                    format!("{}/std/", crate::DOC_RUST_LANG_ORG_CHANNEL)
                                 }
                             };
                             // This is a primitive so the url is done "by hand".

src/librustdoc/clean/utils.rs

@@ -556,3 +556,8 @@ crate fn has_doc_flag(attrs: ty::Attributes<'_>, flag: Symbol) -> bool {
             && attr.meta_item_list().map_or(false, |l| rustc_attr::list_contains_name(&l, flag))
     })
 }
+
+/// A link to `doc.rust-lang.org` that includes the channel name.
+///
+/// Set by `bootstrap::Builder::doc_rust_lang_org_channel` in order to keep tests passing on beta/stable.
+crate const DOC_RUST_LANG_ORG_CHANNEL: &'static str = env!("DOC_RUST_LANG_ORG_CHANNEL");