diff --git a/Cargo.lock b/Cargo.lock
index 0c1c533f395..9b406d4a1f6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3532,7 +3532,6 @@ dependencies = [
  "serde_json",
  "smallvec 0.6.10",
  "smallvec 1.4.0",
- "syn 0.15.35",
  "syn 1.0.11",
  "url 2.1.0",
  "winapi 0.3.8",
diff --git a/src/tools/clippy/tests/compile-test.rs b/src/tools/clippy/tests/compile-test.rs
index f28aedbf0ab..368fa6a98c5 100644
--- a/src/tools/clippy/tests/compile-test.rs
+++ b/src/tools/clippy/tests/compile-test.rs
@@ -49,7 +49,9 @@ fn third_party_crates() -> String {
         if let Some(name) = path.file_name().and_then(OsStr::to_str) {
             for dep in CRATES {
                 if name.starts_with(&format!("lib{}-", dep)) && name.ends_with(".rlib") {
-                    crates.entry(dep).or_insert(path);
+                    if let Some(old) = crates.insert(dep, path.clone()) {
+                        panic!("Found multiple rlibs for crate `{}`: `{:?}` and `{:?}", dep, old, path);
+                    }
                     break;
                 }
             }
diff --git a/src/tools/rustc-workspace-hack/Cargo.toml b/src/tools/rustc-workspace-hack/Cargo.toml
index 1b1f4447966..351e2d4481c 100644
--- a/src/tools/rustc-workspace-hack/Cargo.toml
+++ b/src/tools/rustc-workspace-hack/Cargo.toml
@@ -69,8 +69,7 @@ serde = { version = "1.0.82", features = ['derive'] }
 serde_json = { version = "1.0.31", features = ["raw_value"] }
 smallvec-0_6 = { package = "smallvec", version = "0.6", features = ['union', 'may_dangle'] }
 smallvec = { version = "1.0", features = ['union', 'may_dangle'] }
-syn = { version = "0.15", features = ['full', 'extra-traits'] }
-syn-1 = { package = "syn", version = "1", features = ['fold', 'full', 'extra-traits', 'visit'] }
+syn = { version = "1", features = ['fold', 'full', 'extra-traits', 'visit'] }
 url = { version = "2.0", features = ['serde'] }
 
 [target.'cfg(not(windows))'.dependencies]