lint-docs: Don't hard-code the valid editions

This removes the hard-coded list of edition support in the lint-docs
tool, and instead just assumes the edition attribute is something valid.
There isn't a real reason to have this, as rustc will error if given a
wrong number. This should be easier to maintain going forward.

Author: ehuss

src/tools/lint-docs/src/lib.rs

@@ -444,21 +444,15 @@ impl<'a> LintExtractor<'a> {
         fs::write(&tempfile, source)
             .map_err(|e| format!("failed to write {}: {}", tempfile.display(), e))?;
         let mut cmd = Command::new(self.rustc_path);
-        if options.contains(&"edition2024") {
-            cmd.arg("--edition=2024");
-            cmd.arg("-Zunstable-options");
-        } else if options.contains(&"edition2021") {
-            cmd.arg("--edition=2021");
-        } else if options.contains(&"edition2018") {
-            cmd.arg("--edition=2018");
-        } else if options.contains(&"edition2015") {
-            cmd.arg("--edition=2015");
-        } else if options.contains(&"edition") {
-            panic!("lint-docs: unknown edition");
-        } else {
+        let edition = options
+            .iter()
+            .filter_map(|opt| opt.strip_prefix("edition"))
+            .next()
             // defaults to latest edition
-            cmd.arg("--edition=2021");
-        }
+            .unwrap_or("2021");
+        cmd.arg(format!("--edition={edition}"));
+        // Just in case this is an unstable edition.
+        cmd.arg("-Zunstable-options");
         cmd.arg("--error-format=json");
         cmd.arg("--target").arg(self.rustc_target);
         if let Some(target_linker) = self.rustc_linker {