Fix control flow in assert_default_hashing_controls.

Calling `match` on a struct is a really weird thing to do. As the name
suggests, it's an assert, so let's write it as one. Also clarify the
comment a little.

Author: nnethercote

compiler/rustc_span/src/hygiene.rs

@@ -132,19 +132,21 @@ impl !PartialOrd for LocalExpnId {}
 /// to maintain separate versions of `ExpnData` hashes for each permutation
 /// of `HashingControls` settings.
 fn assert_default_hashing_controls(ctx: &impl HashStableContext, msg: &str) {
-    match ctx.hashing_controls() {
-        // Note that we require that `hash_spans` be set according to the global
-        // `-Z incremental-ignore-spans` option. Normally, this option is disabled,
-        // which will cause us to require that this method always be called with `Span` hashing
-        // enabled.
-        //
-        // Span hashing can also be disabled without `-Z incremental-ignore-spans`.
-        // This is the case for instance when building a hash for name mangling.
-        // Such configuration must not be used for metadata.
-        HashingControls { hash_spans }
-            if hash_spans != ctx.unstable_opts_incremental_ignore_spans() => {}
-        other => panic!("Attempted hashing of {msg} with non-default HashingControls: {other:?}"),
-    }
+    let hashing_controls = ctx.hashing_controls();
+    let HashingControls { hash_spans } = hashing_controls;
+
+    // Note that we require that `hash_spans` be the inverse of the global
+    // `-Z incremental-ignore-spans` option. Normally, this option is disabled,
+    // in which case `hash_spans` must be true.
+    //
+    // Span hashing can also be disabled without `-Z incremental-ignore-spans`.
+    // This is the case for instance when building a hash for name mangling.
+    // Such configuration must not be used for metadata.
+    assert_eq!(
+        hash_spans,
+        !ctx.unstable_opts_incremental_ignore_spans(),
+        "Attempted hashing of {msg} with non-default HashingControls: {hashing_controls:?}"
+    );
 }
 
 /// A unique hash value associated to an expansion.