Add fake dependency roots for all transitive proc-macros (#2749)

See the extensive comment for details, but tl;dr: when cross-compiling,
just doing top-level resolves of direct dependencies can miss resolving
platform-specific features for proc-macros which are only used on
certain platforms.

crate_universe/private/srcs.bzl

@@ -50,5 +50,6 @@ CARGO_BAZEL_SRCS = [
     Label("//crate_universe:src/utils/starlark/select_set.rs"),
     Label("//crate_universe:src/utils/starlark/serialize.rs"),
     Label("//crate_universe:src/utils/starlark/target_compatible_with.rs"),
+    Label("//crate_universe:src/utils/symlink.rs"),
     Label("//crate_universe:src/utils/target_triple.rs"),
 ]

crate_universe/src/cli/generate.rs

@@ -89,15 +89,19 @@ pub fn generate(opt: GenerateOptions) -> Result<()> {
     }
 
     // Ensure Cargo and Rustc are available for use during generation.
-    let cargo_bin = Cargo::new(match opt.cargo {
-        Some(bin) => bin,
-        None => bail!("The `--cargo` argument is required when generating unpinned content"),
-    });
     let rustc_bin = match &opt.rustc {
         Some(bin) => bin,
         None => bail!("The `--rustc` argument is required when generating unpinned content"),
     };
 
+    let cargo_bin = Cargo::new(
+        match opt.cargo {
+            Some(bin) => bin,
+            None => bail!("The `--cargo` argument is required when generating unpinned content"),
+        },
+        rustc_bin.clone(),
+    );
+
     // Ensure a path to a metadata file was provided
     let metadata_path = match &opt.metadata {
         Some(path) => path,

crate_universe/src/cli/query.rs

@@ -67,7 +67,7 @@ pub fn query(opt: QueryOptions) -> Result<()> {
         &lockfile,
         &config,
         &splicing_manifest,
-        &Cargo::new(opt.cargo),
+        &Cargo::new(opt.cargo, opt.rustc.clone()),
         &opt.rustc,
     )?;
 

crate_universe/src/cli/splice.rs

@@ -77,26 +77,25 @@ pub fn splice(opt: SpliceOptions) -> Result<()> {
     // Generate a splicer for creating a Cargo workspace manifest
     let splicer = Splicer::new(splicing_dir, splicing_manifest)?;
 
+    let cargo = Cargo::new(opt.cargo, opt.rustc.clone());
+
     // Splice together the manifest
     let manifest_path = splicer
-        .splice_workspace(&opt.cargo)
+        .splice_workspace(&cargo)
         .context("Failed to splice workspace")?;
 
-    let cargo = Cargo::new(opt.cargo);
-
     // Generate a lockfile
     let cargo_lockfile = generate_lockfile(
         &manifest_path,
         &opt.cargo_lockfile,
         cargo.clone(),
-        &opt.rustc,
         &opt.repin,
     )
     .context("Failed to generate lockfile")?;
 
     let config = Config::try_from_path(&opt.config).context("Failed to parse config")?;
 
-    let resolver_data = TreeResolver::new(cargo.clone(), opt.rustc.clone())
+    let resolver_data = TreeResolver::new(cargo.clone())
         .generate(
             manifest_path.as_path_buf(),
             &config.supported_platform_triples,

crate_universe/src/cli/vendor.rs

@@ -126,26 +126,25 @@ pub fn vendor(opt: VendorOptions) -> Result<()> {
     let splicer = Splicer::new(PathBuf::from(temp_dir.as_ref()), splicing_manifest)
         .context("Failed to create splicer")?;
 
+    let cargo = Cargo::new(opt.cargo, opt.rustc.clone());
+
     // Splice together the manifest
     let manifest_path = splicer
-        .splice_workspace(&opt.cargo)
+        .splice_workspace(&cargo)
         .context("Failed to splice workspace")?;
 
-    let cargo = Cargo::new(opt.cargo);
-
     // Gather a cargo lockfile
     let cargo_lockfile = generate_lockfile(
         &manifest_path,
         &opt.cargo_lockfile,
         cargo.clone(),
-        &opt.rustc,
         &opt.repin,
     )?;
 
     // Load the config from disk
     let config = Config::try_from_path(&opt.config)?;
 
-    let resolver_data = TreeResolver::new(cargo.clone(), opt.rustc.clone()).generate(
+    let resolver_data = TreeResolver::new(cargo.clone()).generate(
         manifest_path.as_path_buf(),
         &config.supported_platform_triples,
     )?;