This PR extracts the functional (lambda) passed to `brecOn` in
structural
recursion into a named `_f` helper definition (e.g. `foo._f`), similar
to
how well-founded recursion uses `._unary`. This way the functional shows
up
with a helpful name in kernel diagnostics rather than as an anonymous
lambda.
The `_f` definition is added with `.abbrev` kernel reducibility hints
and
the `@[reducible]` elaborator attribute, so the kernel unfolds it
eagerly
after `brecOn` iota-reduces. For inductive predicates, the previous
inline
lambda behavior is kept.
To ensure that parent definitions still get the correct reducibility
height
(since `getMaxHeight` ignores `.abbrev` definitions), each `_f`'s body
height is registered via a new `defHeightOverrideExt` environment
extension.
`getMaxHeight` checks this extension for all definitions, making the
height
computation transparent to the extraction.
This change improves code size (a bit). It may regress kernel reduction
times,
especially if a function defined by structural recursion is used in
kernel reduction
proofs on the hot path. Functions defined by structural recursion are
not particularly
fast to reduce anyways (due to the `.brecOn` construction), so already
now it may be
worth writing a kernel-reduction-friendly function manually (using the
recursor directly,
avoiding overloaded operations). This change will guide you in knowing
which function to
optimize.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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