Rollup merge of #149061 - jdonszelmann:fixed-size-encoding-assert, r=oli-obk

debug-assert FixedSizeEncoding invariant

Something like this? It asserts during encoding that for that type, decoding 0 would give the default.
Preferably, I'd either somehow statically/in const assert it once, instead of every time, but I see no easy way to do so. It'd require us to iterate all types that implement the trait or something. Let me know what you think

No types currently violate this invariant.

r? `@oli-obk`

Author: matthiaskrgr

compiler/rustc_metadata/src/rmeta/table.rs

@@ -55,8 +55,10 @@ impl IsDefault for UnusedGenericParams {
 
 /// Helper trait, for encoding to, and decoding from, a fixed number of bytes.
 /// Used mainly for Lazy positions and lengths.
-/// Unchecked invariant: `Self::default()` should encode as `[0; BYTE_LEN]`,
+///
+/// Invariant: `Self::default()` should encode as `[0; BYTE_LEN]`,
 /// but this has no impact on safety.
+/// In debug builds, this invariant is checked in `[TableBuilder::set]`
 pub(super) trait FixedSizeEncoding: IsDefault {
     /// This should be `[u8; BYTE_LEN]`;
     /// Cannot use an associated `const BYTE_LEN: usize` instead due to const eval limitations.
@@ -432,6 +434,13 @@ impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]>> TableBui
     /// arises in the future then a new method (e.g. `clear` or `reset`) will need to be introduced
     /// for doing that explicitly.
     pub(crate) fn set(&mut self, i: I, value: T) {
+        #[cfg(debug_assertions)]
+        {
+            debug_assert!(
+                T::from_bytes(&[0; N]).is_default(),
+                "expected all-zeroes to decode to the default value, as per the invariant of FixedSizeEncoding"
+            );
+        }
         if !value.is_default() {
             // FIXME(eddyb) investigate more compact encodings for sparse tables.
             // On the PR @michaelwoerister mentioned: