Stop using old version of syn in rustc-workspace-hack

None of the tools seem to need syn 0.15.35, so we can just build syn
1.0.

This was causing an issue with clippy's `compile-test` program: since
multiple versions of `syn` would exist in the build directory, we would
non-deterministically pick one based on filesystem iteration order. If
the pre-1.0 version of `syn` was picked, a strange build error would
occur (see
#73594 (comment))

To prevent this kind of issue from happening again, we now panic if we
find multiple versions of a crate in the build directly, instead of
silently picking the first version we find.

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",

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;
                 }
             }

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]