Add an unstable option to build proc macros for both the host and the target

src/cargo/core/compiler/context/compilation_files.rs

@@ -276,7 +276,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
         let out_dir = self.out_dir(unit);
         let file_stem = self.file_stem(unit);
         let link_stem = self.link_stem(unit);
-        let info = if unit.target.for_host() {
+        let info = if unit.kind == Kind::Host {
             &bcx.host_info
         } else {
             &bcx.target_info

src/cargo/core/compiler/context/unit_dependencies.rs

@@ -180,8 +180,19 @@ fn compute_deps<'a, 'cfg, 'tmp>(
         };
         let mode = check_or_build_mode(unit.mode, lib);
         let dep_unit_for = unit_for.with_for_host(lib.for_host());
-        let unit = new_unit(bcx, pkg, lib, dep_unit_for, unit.kind.for_target(lib), mode);
-        ret.push((unit, dep_unit_for));
+
+        if bcx.config.cli_unstable().dual_proc_macros
+            && lib.proc_macro()
+            && unit.kind == Kind::Target
+        {
+            let unit = new_unit(bcx, pkg, lib, dep_unit_for, Kind::Target, mode);
+            ret.push((unit, dep_unit_for));
+            let unit = new_unit(bcx, pkg, lib, dep_unit_for, Kind::Host, mode);
+            ret.push((unit, dep_unit_for));
+        } else {
+            let unit = new_unit(bcx, pkg, lib, dep_unit_for, unit.kind.for_target(lib), mode);
+            ret.push((unit, dep_unit_for));
+        }
     }
 
     // If this target is a build script, then what we've collected so far is

src/cargo/core/features.rs

@@ -320,6 +320,7 @@ pub struct CliUnstable {
     pub package_features: bool,
     pub advanced_env: bool,
     pub config_profile: bool,
+    pub dual_proc_macros: bool,
     pub mtime_on_use: bool,
 }
 
@@ -357,6 +358,7 @@ impl CliUnstable {
             "package-features" => self.package_features = true,
             "advanced-env" => self.advanced_env = true,
             "config-profile" => self.config_profile = true,
+            "dual-proc-macros" => self.dual_proc_macros = true,
             "mtime-on-use" => self.mtime_on_use = true,
             _ => failure::bail!("unknown `-Z` flag specified: {}", k),
         }

src/cargo/core/manifest.rs

@@ -212,6 +212,7 @@ pub struct Target {
     doctest: bool,
     harness: bool, // whether to use the test harness (--test)
     for_host: bool,
+    proc_macro: bool,
     edition: Edition,
 }
 
@@ -352,6 +353,7 @@ compact_debug! {
                 doctest
                 harness
                 for_host
+                proc_macro
                 edition
             )]
         }
@@ -585,6 +587,7 @@ impl Target {
             doctest: false,
             harness: true,
             for_host: false,
+            proc_macro: false,
             edition,
             tested: true,
             benched: true,
@@ -735,6 +738,9 @@ impl Target {
     pub fn for_host(&self) -> bool {
         self.for_host
     }
+    pub fn proc_macro(&self) -> bool {
+        self.proc_macro
+    }
     pub fn edition(&self) -> Edition {
         self.edition
     }
@@ -860,6 +866,10 @@ impl Target {
         self.for_host = for_host;
         self
     }
+    pub fn set_proc_macro(&mut self, proc_macro: bool) -> &mut Target {
+        self.proc_macro = proc_macro;
+        self
+    }
     pub fn set_edition(&mut self, edition: Edition) -> &mut Target {
         self.edition = edition;
         self

src/cargo/util/toml/targets.rs

@@ -764,6 +764,7 @@ fn configure(features: &Features, toml: &TomlTarget, target: &mut Target) -> Car
         .set_doctest(toml.doctest.unwrap_or_else(|| t2.doctested()))
         .set_benched(toml.bench.unwrap_or_else(|| t2.benched()))
         .set_harness(toml.harness.unwrap_or_else(|| t2.harness()))
+        .set_proc_macro(toml.proc_macro.unwrap_or_else(|| t2.proc_macro()))
         .set_for_host(match (toml.plugin, toml.proc_macro()) {
             (None, None) => t2.for_host(),
             (Some(true), _) | (_, Some(true)) => true,