Test that all interned symbols are referenced in Clippy sources (rust-lang#14842)

This test checks that all symbols defined in Clippy's `sym.rs` file are
used in Clippy. Otherwise, it will fail with a list of symbols which are
unused.

This test only runs when the `internal` feature is set, to allow
developers to temporarily define new, yet unused symbols without failing
the tests. Also, it will not run as part of the compiler test suite, as
an extra symbol should never be seen as a fatal problem in the compiler
repo.

changelog: none

Author: Alexendoo

clippy_utils/src/sym.rs

@@ -132,7 +132,6 @@ generate! {
     enum_glob_use,
     enumerate,
     err,
-    error,
     exp,
     expect_err,
     expn_data,

tests/symbols-used.rs

@@ -0,0 +1,81 @@
+// This test checks that all symbols defined in Clippy's `sym.rs` file
+// are used in Clippy. Otherwise, it will fail with a list of symbols
+// which are unused.
+//
+// This test is a no-op if run as part of the compiler test suite
+// and will always succeed.
+
+use std::collections::HashSet;
+use std::ffi::OsStr;
+use std::fs;
+
+use regex::Regex;
+use walkdir::{DirEntry, WalkDir};
+
+const SYM_FILE: &str = "clippy_utils/src/sym.rs";
+
+type Result<T, E = AnyError> = std::result::Result<T, E>;
+type AnyError = Box<dyn std::error::Error>;
+
+#[test]
+#[allow(clippy::case_sensitive_file_extension_comparisons)]
+fn all_symbols_are_used() -> Result<()> {
+    if option_env!("RUSTC_TEST_SUITE").is_some() {
+        return Ok(());
+    }
+
+    // Load all symbols defined in `SYM_FILE`.
+    let content = fs::read_to_string(SYM_FILE)?;
+    let content = content
+        .split_once("generate! {")
+        .ok_or("cannot find symbols start")?
+        .1
+        .split_once("\n}\n")
+        .ok_or("cannot find symbols end")?
+        .0;
+    let mut interned: HashSet<String> = Regex::new(r"(?m)^    (\w+)")
+        .unwrap()
+        .captures_iter(content)
+        .map(|m| m[1].to_owned())
+        .collect();
+
+    // Remove symbols used as `sym::*`.
+    let used_re = Regex::new(r"\bsym::(\w+)\b").unwrap();
+    let rs_ext = OsStr::new("rs");
+    for dir in ["clippy_lints", "clippy_lints_internal", "clippy_utils", "src"] {
+        for file in WalkDir::new(dir)
+            .into_iter()
+            .flatten()
+            .map(DirEntry::into_path)
+            .filter(|p| p.extension() == Some(rs_ext))
+        {
+            for cap in used_re.captures_iter(&fs::read_to_string(file)?) {
+                interned.remove(&cap[1]);
+            }
+        }
+    }
+
+    // Remove symbols used as part of paths.
+    let paths_re = Regex::new(r"path!\(([\w:]+)\)").unwrap();
+    for path in [
+        "clippy_utils/src/paths.rs",
+        "clippy_lints_internal/src/internal_paths.rs",
+    ] {
+        for cap in paths_re.captures_iter(&fs::read_to_string(path)?) {
+            for sym in cap[1].split("::") {
+                interned.remove(sym);
+            }
+        }
+    }
+
+    let mut extra = interned.iter().collect::<Vec<_>>();
+    if !extra.is_empty() {
+        extra.sort_unstable();
+        eprintln!("Unused symbols defined in {SYM_FILE}:");
+        for sym in extra {
+            eprintln!("  - {sym}");
+        }
+        Err(format!("extra symbols found — remove them {SYM_FILE}"))?;
+    }
+    Ok(())
+}