pub const MAX_DEPTH: u32 = 64;Expand description
How deep a macro may expand before the expander decides it is not going to terminate.
This counts expansions — a macro whose output is another call to itself — and nothing else.
Walking into a form’s arguments is not an expansion, and counting it here is what used to make
a 65-level-deep expression with no macros in it report that macro expansion had not terminated.
The structural walk is bounded by [Nesting] against the ceiling the whole front end shares.