tamer: xir::parse::ele: Increase parser nesting depth

This sets the maximum depth to 64, which is still arbitrary, but
unfortunately the sum types introduce multiple levels of nesting, in
particular for template applications, so nested applications can result in a
fairly large stack.

I have various ideas to improve upon that---limited a bit in that repetition
as it is current implemented inhibits tail calls---but they're not worth
doing just yet relative to other priorities.  The impact of this change is
not significant.

DEV-7145

tamer/src/xir/parse/ele.rs

@@ -37,25 +37,19 @@ use crate::{ele_parse, parse::Parser};
 /// A parser accepting a single element.
 pub trait EleParseState: ParseState {}
 
-/// Maximum level of nesting for source XML trees.
+/// Maximum level of parser nesting.
 ///
-/// Technically this is the maximum level of nesting for _parsing_ those
-///   trees,
-///     which may end up being less than this value.
-///
-/// This should be set to something reasonable,
-///   but is not an alternative to coming up with code conventions that
-///   disallow ridiculous levels of nesting.
-/// TAME does have a lot of nesting with primitives,
-///   but that nesting is easily abstracted with templates.
-/// Templates may expand into ridiculous levels of nesting---this
-///   has no impact on the template expansion phase.
+/// Unfortunately,
+///   this limit _does not_ correspond to the level of XML nesting;
+///     parsers composed of Sum NTs,
+///       in particular,
+///       push multiple parsers onto the stack for a single element.
 ///
 /// Note that this is assuming that this parser is used only for TAME
 ///   sources.
 /// If that's not the case,
 ///   this can be made to be configurable like XIRF.
-pub const MAX_DEPTH: usize = 16;
+pub const MAX_DEPTH: usize = 64;
 
 /// Parser stack for trampoline.
 ///
@@ -131,7 +125,7 @@ impl<S: ClosedParseState> StateStack<S> {
                 vec![],
                 "maximum parsing depth of {} exceeded while attempting \
                    to push return state {} \
-                   (expected XIRF configuration to prevent this error)",
+                   (try reducing XML nesting as a workaround)",
                 MAX_DEPTH,
                 TtQuote::wrap(ret),
             );