fix the actual bug

- only skip `--target` for proc-macros
- don't try build non-host targets for proc-macros, since they're not
  supported anyway.

crates/metadata/lib.rs

@@ -197,15 +197,26 @@ impl Metadata {
     /// If `include_default_targets` is `true` and `targets` is unset, this also includes
     /// [`DEFAULT_TARGETS`]. Otherwise, if `include_default_targets` is `false` and `targets`
     /// is unset, `other_targets` will be empty.
+    ///
+    /// All of the above is ignored for proc-macros, which are always only compiled for the host.
     pub fn targets(&self, include_default_targets: bool) -> BuildTargets<'_> {
+        // Proc macros can only be compiled for the host, so just completely ignore any configured targets.
+        // It would be nice to warn about this somehow ...
+        if self.proc_macro {
+            return BuildTargets {
+                default_target: HOST_TARGET,
+                other_targets: HashSet::default(),
+            };
+        }
+
         let default_target = self
             .default_target
             .as_deref()
             // Use the first element of `targets` if `default_target` is unset and `targets` is non-empty
             .or_else(|| {
                 self.targets
                     .as_ref()
-                    .and_then(|targets| targets.iter().next().map(String::as_str))
+                    .and_then(|targets| targets.first().map(String::as_str))
             })
             .unwrap_or(HOST_TARGET);
 

src/docbuilder/rustwide_builder.rs

@@ -350,7 +350,11 @@ impl RustwideBuilder {
                     if res.result.successful {
                         if let Some(name) = res.cargo_metadata.root().library_name() {
                             let host_target = build.host_target_dir();
-                            has_docs = host_target.join("doc").join(name).is_dir();
+                            has_docs = if metadata.proc_macro {
+                                host_target.join("doc").join(name).is_dir()
+                            } else {
+                                host_target.join("doc").join(HOST_TARGET).join(name).is_dir()
+                            }
                         }
                     }
 
@@ -656,7 +660,16 @@ impl RustwideBuilder {
         if let Some(cpu_limit) = self.config.build_cpu_limit {
             cargo_args.push(format!("-j{}", cpu_limit));
         }
-        if target != HOST_TARGET {
+        // Cargo has a series of frightening bugs around cross-compiling proc-macros:
+        // - Passing `--target` causes RUSTDOCFLAGS to fail to be passed 🤦
+        // - Passing `--target` will *create* `target/{target-name}/doc` but will put the docs in `target/doc` anyway
+        // As a result, it's not possible for us to support cross-compiling proc-macros.
+        // However, all these caveats unfortunately still apply when `{target-name}` is the host.
+        // So, only pass `--target` for crates that aren't proc-macros.
+        //
+        // Originally, this had a simpler check `target != HOST_TARGET`, but *that* was buggy when `HOST_TARGET` wasn't the same as the default target.
+        // Rather than trying to keep track of it all, only special case proc-macros, which are what we actually care about.
+        if !metadata.proc_macro {
             cargo_args.push("--target".into());
             cargo_args.push(target.into());
         };
@@ -935,4 +948,66 @@ mod tests {
             Ok(())
         })
     }
+
+    #[test]
+    #[ignore]
+    fn test_proc_macro() {
+        wrapper(|env| {
+            let crate_ = "thiserror-impl";
+            let version = "1.0.26";
+            let mut builder = RustwideBuilder::init(env).unwrap();
+            assert!(builder.build_package(crate_, version, PackageKind::CratesIo)?);
+
+            let storage = env.storage();
+
+            // doc archive exists
+            let doc_archive = rustdoc_archive_path(crate_, version);
+            assert!(storage.exists(&doc_archive)?);
+
+            // source archive exists
+            let source_archive = source_archive_path(crate_, version);
+            assert!(storage.exists(&source_archive)?);
+
+            Ok(())
+        });
+    }
+
+    #[test]
+    #[ignore]
+    fn test_cross_compile_non_host_default() {
+        wrapper(|env| {
+            let crate_ = "xingapi";
+            let version = "0.3.3";
+            let mut builder = RustwideBuilder::init(env).unwrap();
+            assert!(builder.build_package(crate_, version, PackageKind::CratesIo)?);
+
+            let storage = env.storage();
+
+            // doc archive exists
+            let doc_archive = rustdoc_archive_path(crate_, version);
+            assert!(storage.exists(&doc_archive)?);
+
+            // source archive exists
+            let source_archive = source_archive_path(crate_, version);
+            assert!(storage.exists(&source_archive)?);
+
+            let target = "x86_64-unknown-linux-gnu";
+            let crate_path = crate_.replace("-", "_");
+            let target_docs_present = storage.exists_in_archive(
+                &doc_archive,
+                &format!("{}/{}/index.html", target, crate_path),
+            )?;
+
+            let web = env.frontend();
+            let target_url = format!(
+                "/{}/{}/{}/{}/index.html",
+                crate_, version, target, crate_path
+            );
+
+            assert!(target_docs_present);
+            assert_success(&target_url, web)?;
+
+            Ok(())
+        });
+    }
 }