message: Add {target} to the extern-html-root-url
  This avoids cross-crate intra-doc links that go to the wrong platform,
  and possibly to items that don't exist.
diff: @@ -580,7 +580,17 @@ impl RustwideBuilder {
  // We know that `metadata` unconditionally passes `-Z rustdoc-map`.
  // Don't copy paste this, since that fact is not stable and may change in the future.
  "-Zunstable-options".into(),
  - r#"--config=doc.extern-map.registries.crates-io="https://docs.rs""#.into(),
  + // Add `target` so that if a dependency has target-specific docs, this links to them properly.
  + //
  + // Note that this includes the target even if this is the default, since the dependency
  + // may have a different default (and the web backend will take care of redirecting if
  + // necessary).
  + //
  + // FIXME: host-only crates like proc-macros should probably not have this passed? but #1417 should make it OK
  + format!(
  + r#"--config=doc.extern-map.registries.crates-io="https://docs.rs/{{pkg_name}}/{{version}}/{}""#,
  + target
  + ),
  ];
  if let Some(cpu_limit) = self.config.build_cpu_limit {
  cargo_args.push(format!("-j{}", cpu_limit));
  