Fix panic with -Zbuild-std and no roots.

src/cargo/core/compiler/unit_dependencies.rs

@@ -62,6 +62,12 @@ pub fn build_unit_dependencies<'a, 'cfg>(
     profiles: &'a Profiles,
     interner: &'a UnitInterner,
 ) -> CargoResult<UnitGraph> {
+    if roots.is_empty() {
+        // If -Zbuild-std, don't attach units if there is nothing to build.
+        // Otherwise, other parts of the code may be confused by seeing units
+        // in the dep graph without a root.
+        return Ok(HashMap::new());
+    }
     let (std_resolve, std_features) = match std_resolve {
         Some((r, f)) => (Some(r), Some(f)),
         None => (None, None),

tests/testsuite/standard_lib.rs

@@ -677,3 +677,23 @@ fn different_features() {
         .target_host()
         .run();
 }
+
+#[cargo_test]
+fn no_roots() {
+    // Checks for a bug where it would panic if there are no roots.
+    let setup = match setup() {
+        Some(s) => s,
+        None => return,
+    };
+    let p = project().file("tests/t1.rs", "").build();
+    p.cargo("build")
+        .build_std(&setup)
+        .target_host()
+        .with_stderr(
+            "\
+[UPDATING] [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+}