Auto merge of #9834 - ehuss:fix-build-std-proc-macro-only, r=alexcrichton

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

If you try to run `cargo build -Zbuild-std` in a proc-macro project, cargo would panic in [`check_collisions`](https://github.com/rust-lang/cargo/blob/835d5576e1f0b917a42ba18a9556242204377cd1/src/cargo/core/compiler/context/mod.rs#L427). This is because it iterates over every Unit in the build graph checking the `outputs` for filenames.  However, [`outputs`](https://github.com/rust-lang/cargo/blob/835d5576e1f0b917a42ba18a9556242204377cd1/src/cargo/core/compiler/context/compilation_files.rs#L109-L110) was missing the outputs for standard library units. That is because `outputs` is computed by walking the graph starting from the roots.

The bug here is that `attach_std_deps` was adding the standard library units to graph, even though they aren't reachable from the roots, thus creating orphans.

The solution is to avoid adding the standard library units if they are not needed (as is the case when building just a proc-macro).

Fixes #9828

Author: bors

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();
+}