A marker Mutare.Phoenix.Swoosh injects for a mailer whose use Phoenix.Swoosh line
configures a layout (layout: {LayoutView, :email}).
It is not a behaviour anyone implements. Mutare's use expansion records the behaviours a
use injects and passes them to mutators as context.behaviours; this package's extension
adds this module to that set to indicate a configured layout to Mutare.Phoenix.Swoosh.Layout.
Nothing else reads it: the marker never reaches the metamutant source, the compiled program,
or the report.
This uses Mutare's module-scoped behaviour set, with two consequences:
the marker does not apply to a nested module's calls (behaviours never inherit
inwards), and a run with --no-expand-uses drops the injected half altogether. In both cases
off is generated only when the call contains an author-written truthy layout: assign.
It exists because the use line itself is unreachable — use options are compile-time
configuration, pruned whole before any mutation could be delivered (see the "What's
deliberately out of scope" section of the README). The marker records that configuration
for use when mutating runtime calls. Without a layout in effect, suppressing one would
produce an equivalent mutant.