Lexer gains scan_interp_string which replaces scan_string in the main
lex loop. When no ${ is found it behaves identically to before (single
Str token). When interpolations are present it emits a flat token
sequence — Str, Plus, (expr tokens), Plus, Str, … — that the existing
parse_binop / cg_expr BinOp-Plus-string path assembles into nested
el_str_concat calls with zero parser or codegen changes.
Key design choices:
- scan_interp_brace tracks { depth so fn(a, b) inside ${} is safe
- inner expr tokens are wrapped in ( ) so operators like + in ${n+1}
do not associate with the surrounding concat Plus tokens
- \$ escapes to a literal dollar sign; bare $ not before { passes through
- empty ${} emits an empty string segment
Will Anderson
ce9a2caff4