Function generate_field_syntax_helpers 

fn generate_field_syntax_helpers(
    type_name: &str,
    pub_slots: &[&SidecarSlot],
    field_error: &[String],
    no_field_hint: Option<&str>,
    every_read: bool,
) -> String

Generate the R helpers the $ / [[ field methods of r_data_accessors share on S3, S4 and env classes (#1848): the field names, a getter and a setter switch() over the fields’ .Call()s, the classed miniextendr_no_field error naming the fields, the classes the methods put in front of their own errors (field_error, NULL without them, #1899), and the liveness check of restored(refuse = "every_read") with the flag that gates it (FALSE and a stub without it, #1901).

Prepared for every type with R accessors, whatever #[externalptr(...)] names, and bundled with its internal accessor rendering. The writer selects that rendering only when an impl enables field syntax, so the impl’s option never depends on the derive attribute. The helpers are named by the Rust type (crate::naming::rdata_helper_name); the field names and the switch() labels are the fields’ R names, in declared order, computed fields included (#1891, #1883).

Every arm passes TRUE as the accessor’s require_live flag, so the field methods refuse a pointer restored from a saved session (the standalone accessors pass FALSE and keep reading its Sidecar values). The accessors return a failed conversion or a panic as a tagged condition value with no call; the calling method raises it with its own frame as the fallback, so the condition reports x$keys <- value. A computed field’s setter arm raises miniextendr_read_only_field itself, with the calling method’s frame (parent.frame()) resolved the same way. The liveness check returns the same kind of value, which the method raises.

field_error(no_field_hint = "...") adds a sentence after the field list of the miniextendr_no_field message, each %s in it replaced by the name asked for (#2112). The hint is split on %s here and pasted around the name, so any other % or \ in it, or in the name, stays as written.