Rollup merge of #111252 - matthewjasper:min-spec-improvements, r=compiler-errors

Min specialization improvements

- Don't allow specialization impls with no items, such implementations are probably not correct and only occur as mistakes in the compiler and standard library
- Fix a missing normalization call
- Adds spans for lifetime errors from overly general specializations

Closes #79457
Closes #109815

compiler/rustc_data_structures/src/owned_slice.rs

@@ -109,9 +109,11 @@ impl Borrow<[u8]> for OwnedSlice {
 }
 
 // Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Send`
+#[cfg(parallel_compiler)]
 unsafe impl Send for OwnedSlice {}
 
 // Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Sync`
+#[cfg(parallel_compiler)]
 unsafe impl Sync for OwnedSlice {}
 
 #[cfg(test)]