Auto merge of #51384 - QuietMisdreavus:extern-version, r=GuillaumeGomez

rustdoc: add flag to control the html_root_url of dependencies

The `--extern-html-root-url` flag in this PR allows one to override links to crates whose docs are not already available locally in the doc bundle. Docs.rs currently uses a version of this to make sure links to other crates go into that crate's docs.rs page. See the included test for intended use, but the idea is as follows:

Calling rustdoc with `--extern-html-root-url crate=https://some-url.com` will cause rustdoc to override links that point to that crate to instead be replaced with a link rooted at `https://some-url.com/`. (e.g. for docs.rs this would be `https://docs.rs/crate/0.1.0` or the like.) Cheekily, rustup could use these options to redirect links to std/core/etc to instead point to locally-downloaded docs, if it so desired.

Fixes https://github.com/rust-lang/rust/issues/19603

src/test/rustdoc/extern-html-root-url.rs

@@ -0,0 +1,18 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-tidy-linelength
+
+// compile-flags:-Z unstable-options --extern-html-root-url core=https://example.com/core/0.1.0
+
+// @has extern_html_root_url/index.html
+// @has - '//a/@href' 'https://example.com/core/0.1.0/core/iter/index.html'
+#[doc(no_inline)]
+pub use std::iter;