Fix panic with build-std of a proc-macro.

src/cargo/core/compiler/unit_dependencies.rs

@@ -142,6 +142,7 @@ fn attach_std_deps(
     std_unit_deps: UnitGraph,
 ) {
     // Attach the standard library as a dependency of every target unit.
+    let mut found = false;
     for (unit, deps) in state.unit_dependencies.iter_mut() {
         if !unit.kind.is_host() && !unit.mode.is_run_custom_build() {
             deps.extend(std_roots[&unit.kind].iter().map(|unit| UnitDep {
@@ -152,12 +153,16 @@ fn attach_std_deps(
                 public: true,
                 noprelude: true,
             }));
+            found = true;
         }
     }
-    // And also include the dependencies of the standard library itself.
-    for (unit, deps) in std_unit_deps.into_iter() {
-        if let Some(other_unit) = state.unit_dependencies.insert(unit, deps) {
-            panic!("std unit collision with existing unit: {:?}", other_unit);
+    // And also include the dependencies of the standard library itself. Don't
+    // include these if no units actually needed the standard library.
+    if found {
+        for (unit, deps) in std_unit_deps.into_iter() {
+            if let Some(other_unit) = state.unit_dependencies.insert(unit, deps) {
+                panic!("std unit collision with existing unit: {:?}", other_unit);
+            }
         }
     }
 }

tests/testsuite/standard_lib.rs

@@ -662,3 +662,31 @@ fn no_roots() {
         .with_stderr_contains("[FINISHED] [..]")
         .run();
 }
+
+#[cargo_test]
+fn proc_macro_only() {
+    // Checks for a bug where it would panic if building a proc-macro only
+    let setup = match setup() {
+        Some(s) => s,
+        None => return,
+    };
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "pm"
+                version = "0.1.0"
+
+                [lib]
+                proc-macro = true
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+    p.cargo("build")
+        .build_std(&setup)
+        .target_host()
+        .with_stderr_contains("[FINISHED] [..]")
+        .run();
+}