pub const MAX_AST_SERIALIZE_DEPTH: usize = 512 /* 512usize */;Expand description
Greatest AST depth AstNode will serialize.
An AstNode tree is as deep as the parsed expression nesting, which a
caller controls directly — 100 000 levels fit in a 200 KB payload, well
inside bca-web’s body cap. serde cannot emit a tree without one
native stack frame per level, and overflowing that stack aborts the
process rather than raising a catchable panic (#1056), so the depth is
bounded: a deeper tree fails serialization with an ordinary serializer
error naming the limit.
The bound is set well clear of real source: the deepest AST across the
~8 000-file corpus under tests/repositories (TensorFlow, serde,
DeepSpeech, …) is 188 levels. It is also set well clear of the stack:
the earliest measured overflow of any emitted format was 2 000 levels
on a debug build’s default 2 MiB thread.
Read that 188 as a corpus figure, not a conversion rate. The depth
counted here is the AstNode tree after Alterator::alterate,
and how many levels one level of source nesting costs is a
per-language property of what that alterator flattens. The Tcl family
is the outlier: since #1381 a script body keeps its children, so one
brace level spans braced_word → command → word_list and costs
three levels here — 196 levels over 64 nested eval braces, measured
by a_tcl_brace_level_costs_three_ast_levels below — putting the
ceiling near 170 nested braces rather than 512. Nothing in the corpus
approaches either number — it holds no Tcl at all — but a caller
re-deriving this bound from the 188 alone would be reasoning about
the wrong quantity.