Function complete 

pub fn complete(
    source: &str,
    cursor: usize,
    aliases: &QueryAliases,
    values: Option<&dyn StructureValues>,
) -> CompletionResult

Computes completion using a UTF-8 byte cursor.

cursor is a byte offset, not a character index. A cursor in the middle of a UTF-8 scalar is rejected rather than silently moving to a different replacement range. The returned range always points into source.

After a value column such as chain, resname, or name, candidates come exclusively from values when it is supplied; language keywords are never mixed with structure identifiers. $ references are completed exclusively from aliases. At a fresh token after a finished term, no candidates are offered until the caller types a character.