From 6b472c90e18a04c4b519ef6ff7f8ff6905364b7a Mon Sep 17 00:00:00 2001
From: Eric Huss <eric@huss.org>
Date: Fri, 4 Dec 2020 13:34:09 -0800
Subject: [PATCH] Fix panic with -Zbuild-std and no roots.

---
 src/cargo/core/compiler/unit_dependencies.rs |  6 ++++++
 tests/testsuite/standard_lib.rs              | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/cargo/core/compiler/unit_dependencies.rs b/src/cargo/core/compiler/unit_dependencies.rs
index b6ae6df45ec..f0dd340cc99 100644
--- a/src/cargo/core/compiler/unit_dependencies.rs
+++ b/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),
diff --git a/tests/testsuite/standard_lib.rs b/tests/testsuite/standard_lib.rs
index 4a091d71e5b..2e0adf92aa1 100644
--- a/tests/testsuite/standard_lib.rs
+++ b/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();
+}