GeezerVison

w(mask, replace) << x with a Python bool, int or float x goes through
_update, a second Updater, an index resolver for `...`, a Scalar built
from x and _prep_for_assign before it reaches GrB_Vector_assign_<T> (or
GrB_Matrix_assign_<T>) over all indices. That is about 7 us of Python
for a call SuiteSparse finishes in about 0.5 us, and level-synchronous
traversals make it once per level: graphblas-algorithms marks each BFS
level visited with seen(q.S) << True.

Make that call directly when nothing needs the full path: x of type
bool, int (within INT64) or float, a non-UDT target, no accum, no opts,
no input_mask, a Mask (structural or value, complemented or not) of an
object of the target's own type and shape, or no mask without replace,
and no active Recorder. x takes the dtype the full path gives its
Scalar, and GraphBLAS casts it as before. Everything else, including
masks the full path converts or rejects, ints beyond INT64 and numpy
scalars, takes the full path unchanged.

Measured with no test suite running (load average about 5.3 from other
work), mlx, SuiteSparse 10.5.0, best of 5, two rounds each:

    seen(q.S) << True, 10000-element Vector    7.7 us -> 2.3 to 2.4 us
    bfs_edges, 10k-node road ball, 134 levels  5.1 ms -> 4.2 to 4.3 ms
    bfs_edges, 50k-node road ball, 352 levels  16.5 to 16.9 ms -> 14.7 to 14.9 ms

with the same edges. The tests compare the fast path with the full path
(forced by an active Recorder) over 7 dtypes, 9 values (bool, 0, -1,
300, 2**40, 1.5, -0.0, nan), 5 mask kinds and replace for Vector, and a
smaller grid for Matrix, plus the errors of a wrong-kind or wrong-size
mask and an int beyond INT64. Breaking the fast path's replace,
complement, structure or value each fails between 216 and 423 cases.

_assign_literal imported lookup_dtype on every call. base.py can import
it at module level (literals.py, which base.py imports, already does),
and the import was about a sixth of what the fast path still costs:
seen(q.S) << True on a 10000-element Vector, best of 5, goes from
2.28 us to 1.9 us (the GrB_Vector_assign_BOOL call itself is 0.42 us).

…ull path does

_assign_literal checks the status of GrB_*_assign_<T> itself, and no test
reached that check, since Python validates the mask and value first. A
Vector whose data was exported with give_ownership=True has no GraphBLAS
object left, so the assign fails: both the fast path and the full path
(forced by a Recorder) raise NullPointer.