Module expand 

Port of internal/variables/expand.go: shell-like ${VAR}/$VAR expansion of config values and variable definitions.

The Go original parses each string as a mvdan.cc/sh/v3 shell document word and walks/rewrites that AST. There is no equivalent Rust crate with this exact operator set, so this is a hand-rolled scanner/parser scoped exactly to the grammar caproni uses: $VAR/${VAR} references, the ${VAR:-default}/${VAR:?message}/${VAR:+alternate} (and their unset-only ${VAR-x}/${VAR?x}/${VAR+x} variants) operators for variable definitions, ${#VAR} length, ${!VAR} indirection, ${VAR:off:len} slicing, ${VAR/pat/repl}/${VAR//pat/repl} replacement, and ${VAR#pat}/${VAR##pat}/${VAR%pat}/${VAR%%pat} prefix/suffix removal – plus verbatim (never evaluated) preservation of $(...), `...`, $((...)) and <(...)/>(...).

Pattern arguments to /, # and % are matched as literal substrings, not shell globs, because nothing in caproni’s config surface uses glob metacharacters there; this is a deliberate scope reduction from the Go original’s full glob matching.

Structs

Unresolved

Records the references in one config value that resolved to nothing.

Enums

ExpandError

Functions

expand_definition

Expands a variable definition, where the ${VAR:-default}, ${VAR:?message} and ${VAR:+alternate} operators are available on top of the bare references config values take.

expand_map

Walks a parsed config mapping and expands references in every string value, in place. The top-level "variables" subtree is left untouched.

expand_string

Expands ${VAR}/$VAR references in s. A reference that resolves to nothing becomes the empty string.