Structural Interpolation

Placeholders (${VAR} and SECRET[backend.key]) in structural positions of a Vector configuration file are deprecated and will be removed in a future release. Current behavior is unchanged.

Affected patterns:

• Unquoted placeholders in non-string fields (count = ${MY_COUNT}).
• Placeholders in map keys (${KEY} = value).
• Placeholders in TOML table headers ([${SECTION}]).
• Placeholders as inline array elements that expand to multiple values (inputs = [${VECTOR_INPUTS}] where VECTOR_INPUTS=a,b).

Migration guidance will accompany the removal PR. envsubst can be used today to pre-expand env-var placeholders into a fully static config as a workaround.

See RFC: parse-first config interpolation for the full rationale.