Rollup merge of #102468 - RalfJung:tidy, r=jyn514

tidy: make rustc dependency error less confusing

The current wording leads to very confusing messages:
```
tidy error: Dependencies for main workspace not explicitly permitted:
* unicode-ident 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)
```
Miri is part of that workspace, and there never was a problem adding Miri dependencies. The actual error is that due to a crate bump this now showed up as a rustc dependency, and *those* are restricted.

Author: notriddle

src/tools/tidy/src/deps.rs

@@ -73,14 +73,11 @@ const EXCEPTIONS_BOOTSTRAP: &[(&str, &str)] = &[
 /// these and all their dependencies *must not* be in the exception list.
 const RUNTIME_CRATES: &[&str] = &["std", "core", "alloc", "test", "panic_abort", "panic_unwind"];
 
-/// Crates whose dependencies must be explicitly permitted.
-const RESTRICTED_DEPENDENCY_CRATES: &[&str] = &["rustc_driver", "rustc_codegen_llvm"];
-
 /// Crates rustc is allowed to depend on. Avoid adding to the list if possible.
 ///
 /// This list is here to provide a speed-bump to adding a new dependency to
 /// rustc. Please check with the compiler team before adding an entry.
-const PERMITTED_DEPENDENCIES: &[&str] = &[
+const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "addr2line",
     "adler",
     "ahash",
@@ -307,7 +304,7 @@ const PERMITTED_CRANELIFT_DEPENDENCIES: &[&str] = &[
 ];
 
 const FORBIDDEN_TO_HAVE_DUPLICATES: &[&str] = &[
-    // These two crates take quite a long time to build, so don't allow two versions of them
+    // This crate takes quite a long time to build, so don't allow two versions of them
     // to accidentally sneak into our dependency graph, in order to ensure we keep our CI times
     // under control.
     "cargo",
@@ -324,12 +321,12 @@ pub fn check(root: &Path, cargo: &Path, bad: &mut bool) {
         .features(cargo_metadata::CargoOpt::AllFeatures);
     let metadata = t!(cmd.exec());
     let runtime_ids = compute_runtime_crates(&metadata);
-    check_exceptions(&metadata, EXCEPTIONS, runtime_ids, bad);
-    check_dependencies(
+    check_license_exceptions(&metadata, EXCEPTIONS, runtime_ids, bad);
+    check_permitted_dependencies(
         &metadata,
-        "main workspace",
-        PERMITTED_DEPENDENCIES,
-        RESTRICTED_DEPENDENCY_CRATES,
+        "rustc",
+        PERMITTED_RUSTC_DEPENDENCIES,
+        &["rustc_driver", "rustc_codegen_llvm"],
         bad,
     );
     check_crate_duplicate(&metadata, FORBIDDEN_TO_HAVE_DUPLICATES, bad);
@@ -342,8 +339,8 @@ pub fn check(root: &Path, cargo: &Path, bad: &mut bool) {
         .features(cargo_metadata::CargoOpt::AllFeatures);
     let metadata = t!(cmd.exec());
     let runtime_ids = HashSet::new();
-    check_exceptions(&metadata, EXCEPTIONS_CRANELIFT, runtime_ids, bad);
-    check_dependencies(
+    check_license_exceptions(&metadata, EXCEPTIONS_CRANELIFT, runtime_ids, bad);
+    check_permitted_dependencies(
         &metadata,
         "cranelift",
         PERMITTED_CRANELIFT_DEPENDENCIES,
@@ -358,13 +355,13 @@ pub fn check(root: &Path, cargo: &Path, bad: &mut bool) {
         .features(cargo_metadata::CargoOpt::AllFeatures);
     let metadata = t!(cmd.exec());
     let runtime_ids = HashSet::new();
-    check_exceptions(&metadata, EXCEPTIONS_BOOTSTRAP, runtime_ids, bad);
+    check_license_exceptions(&metadata, EXCEPTIONS_BOOTSTRAP, runtime_ids, bad);
 }
 
 /// Check that all licenses are in the valid list in `LICENSES`.
 ///
-/// Packages listed in `EXCEPTIONS` are allowed for tools.
-fn check_exceptions(
+/// Packages listed in `exceptions` are allowed for tools.
+fn check_license_exceptions(
     metadata: &Metadata,
     exceptions: &[(&str, &str)],
     runtime_ids: HashSet<&PackageId>,
@@ -434,11 +431,11 @@ fn check_exceptions(
     }
 }
 
-/// Checks the dependency of `RESTRICTED_DEPENDENCY_CRATES` at the given path. Changes `bad` to
+/// Checks the dependency of `restricted_dependency_crates` at the given path. Changes `bad` to
 /// `true` if a check failed.
 ///
-/// Specifically, this checks that the dependencies are on the `PERMITTED_DEPENDENCIES`.
-fn check_dependencies(
+/// Specifically, this checks that the dependencies are on the `permitted_dependencies`.
+fn check_permitted_dependencies(
     metadata: &Metadata,
     descr: &str,
     permitted_dependencies: &[&'static str],