Function getkeystring_with 

pub fn getkeystring_with(
    s: &str,
    how: u32,
    misc: Option<&mut i32>,
) -> (String, usize)

getkeystring with the C how parameter (Src/utils.c:6915). The plain getkeystring(s) shim above defaults how=0 for non-EMACS callers (zbeep, dollar-quote — they keep unknown \<char> as literal \<char>). Pass GETKEYS_PRINT to get the print/echo behavior (drop backslash on unknown). getkeystring with the C how parameter AND the GETKEY_UPDATE_OFFSET cursor-offset out-param (Src/utils.c:6915).

Port of getkeystring(char *s, int *len, int how, int *misc) from Src/utils.c:6915, specifically its GETKEY_UPDATE_OFFSET path. When how sets GETKEY_UPDATE_OFFSET and misc is Some, *misc is the C *misc completion offset — a byte index into s — and is updated as escapes positioned before the cursor collapse:

• -1 per collapsed \<char> escape whose backslash precedes the cursor, s - sstart < *misc (c:6987); undone if the backslash is kept literal in the default (non-EMACS) arm (c:7181-7183);
• additional -6 for \u, -10 (-4+-6) for \U (c:7073-7084), then +N for the N output bytes emitted (c:7123-7124). Octal/hex escapes get only the base -1, matching C’s own incomplete /* HERE: GETKEY_UPDATE_OFFSET? */ handling (c:7155).

The offset is a byte index. For ASCII $'...' content (one byte per char) it matches the C metafied-byte semantics exactly; the byte-vs- char divergence for non-ASCII content is the pre-existing shared-cursor limitation documented on set_comp_sep (compcore.rs), not new here.

Callers with no offset to track pass misc = None.