Use -Z rustdoc-map instead of rewriting it

[jyn514]

Closes #1177, closes #1165. Fixes #1166.

[Nemo157]

A crate to test #1166 with is futures-util-preview 0.2.2, it depends on futures-core-preview which has the lib name futures_core. (Note that futures-core-preview has an html_root_url, so the only way to check would be looking at the command line passed to rustdoc, which we don't show 😦).

[Nemo157]

Tested with this little patch applied on top:

diff --git src/docbuilder/rustwide_builder.rs src/docbuilder/rustwide_builder.rs
index 83592dc..1a80aea 100644
--- src/docbuilder/rustwide_builder.rs
+++ src/docbuilder/rustwide_builder.rs
@@ -579,6 +579,9 @@ impl RustwideBuilder {

         // Add docs.rs specific arguments
         let mut cargo_args = Vec::new();
+        cargo_args.push("--verbose".into());
+        cargo_args.push(r#"--config=doc.extern-map.registries.crates-io="https://docs.rs""#.into());
+        cargo_args.push("-Zunstable-options".into());
         if let Some(cpu_limit) = self.config.build_cpu_limit {
             cargo_args.push(format!("-j{}", cpu_limit));
         }

On master I see us passing:

"--extern-html-root-url" "futures_core_preview=https://docs.rs/futures-core-preview/0.2.3"

And on this branch I see cargo passing:

--extern-html-root-url 'futures_core=https://docs.rs/futures-core-preview/0.2.3/'

So this definitely fixes #1166

[jyn514]

Thanks, that definitely looks like the right approach since it means the hyperlinks will work even when built with old versions of nightly :)

This is ready for re-review.

[pietroalbini]

Random note on this: it'd be nice if rustdoc-map allowed using the semver pattern instead of the version, since docs.rs would then redirect to the latest semver-compatible docs automatically:

https://docs.rs/rustwide/^0.10/...

[jyn514]

That would be nice, yeah - it needs rustdoc changes first, though.
#1055