message: Explain the size regression
  Summary: Explain why the size changed in the nightly. Looks like Rust was suboptimal before, and then improved.
diff: @@ -88,6 +88,10 @@ pub(crate) type AstStmt = AstStmtP<AstNoPayload>;
  // These two data structures reduced in size with nightly ~10 Sep 2022.
  // Once that date is in the distant past, we should make these assertions equality.
  +//
  +// Our best understanding of the drop in size is that previously the largest field
  +// was Literal (9 words) wrapping AstLiteral (7 words). That's one more word of padding
  +// than expected, which is fixed in later nightly.
  const _: () = assert!(mem::size_of::<AstStmt>() <= mem::size_of::<[usize; 12]>());
  const _: () = assert!(mem::size_of::<AstExpr>() <= mem::size_of::<[usize; 9]>());
  assert_eq_size!(AstAssign, [usize; 7]);
  