Cheat Sheets

Ownership Mutability and Operation sheet {own-mut-op-methods}

Table 1: Updated ownership/mutability/operation combinations with compiler results

Ownership	Mutability	Operation	Result
own	mutable	mutate	valid with Copy, but doesn’t mutate the original — mut self consumes a copy1
own	mutable	transform	valid, but mut is unnecessary — the method never writes through self
own	immutable	mutate	no — contradiction (immutable + mutate E0594)
own	immutable	transform	valid, but E0382 if reused without Copy
borrow	mutable	mutate	valid — mutates original in place (test_mutation passes)2
borrow	mutable	transform	valid — creates new Self, original unchanged; &mut self doesn’t change transformation behaviour
borrow	immutable	mutate	no — contradiction (immutable + mutate)
borrow	immutable	transform	valid

Footnotes

1. This needs Copy/Clone to compile, but won’t cause a persistent change in the original object because it doesn’t actually touch the original object.

2. Keep in mind that the return type has to be &mut Self for this to work.