[naga wgsl-in] Restore accidentally removed recursion depth check.

[jimblandy]

In naga::front::wgsl::parse, restore an incorrectly removed call to Parser::track_recursion, moving it from Parser::unary_expression to Parser::expression.

To prevent pathological shaders from overflowing the Rust stack, 34cfee6 introduced the Parser::track_recursion function to track how deeply the parser has recursed, and throw an uncategorized error if it has far exceeded the depth we expect to encounter while processing any reasonable input. These checks were added to a few strategically chosen functions, such that any recursive execution would need to go through at least one check.

When fca3c2e rewrote Parser::unary_expression to use a loop instead of recursing, it removed that function's call to track_recursion. While it was true that unary_expression itself no longer recursed, its call to track_recursion was also serving as a choke point for the rest of the expression parser; removing the check reintroduced the potential for unbounded recursion.

This commit reintroduces the recursion depth check, but this time places it in Parser::expression, a more natural location that is adequate now that unary_expression no longer recurses on its own.

In local testing, a nesting depth limit of 256 still allowed the Rust stack to overflow, so this patch also lowers the depth limit to 200.

[jimblandy]

This bug was not our highest priority, but the process of just filing it was most of the work of fixing it, so I just went ahead and wrote a fix.

[teoxoy]

The only functions that indirectly recurse:

• enclosed_expression
• template_elaborated_ident
• arguments