The foreach pattern was only matched against using instructions, but
the compiler emits no using/try-finally at all when the enumerator's
static type can never require disposal: a struct or a sealed class
that does not implement IDisposable (SerializationInfoEnumerator in
the issue's example). Such loops stayed while loops.
Recognize the bare 'enumerator = x.GetEnumerator(); while
(enumerator.MoveNext())' shape during statement building and reuse the
existing foreach transformation core for it. The transformation is
restricted to exactly the cases where recompilation would produce the
same IL: the enumerator type rules above (ref structs are excluded
because of pattern-based disposal), a single-store enumerator variable
unused outside the loop, and synchronous enumeration only, since async
enumerators are always IAsyncDisposable.
Assisted-by: Claude:claude-fable-5:Claude Code
Siegfried Pammer