This commit fixes a bug introduced in #12706, where the behavior of the
lint has been changed, to avoid suggestions that introduce a move. The
motivation in the commit message is quite poor (if the detection for
significant drops is not sufficient because it's not transitive, the
proper fix would be to make it transitive). However, #12454, the linked
issue, provides a good reason for the change — if the value being
borrowed is bound to a variable, then moving it will only introduce
friction into future refactorings.
Thus #12706 changes the logic so that the lint triggers if the value
being borrowed is Copy, or is the result of a function call, simplifying
the logic to the point where analysing "is this the only use of this
value" isn't necessary.
However, said PR also introduces an undocumented carveout, where
referents that themselves are mutable references are treated as Copy,
to catch some cases that we do want to lint against. However, that is
not sound — it's possible to consume a mutable reference by moving it.
To avoid emitting false suggestions, this PR reintroduces the
referent_used_exactly_once logic and runs that check for referents that
are themselves mutable references.
Thinking about the code shape of &mut x, where x: &mut T, raises the
point that while removing the &mut outright won't work, the extra
indirection is still undesirable, and perhaps instead we should suggest
reborrowing: &mut *x. That, however, is left as possible future work.
Fixes#12856
Current implementation looks for significant drops, that can change the
behavior, but that's not enough - value might not have a Drop itself but
one of its children might have it.
A good example is passing a reference to `PathBuf` to `std::fs::File::open`.
There's no benefits to pass `PathBuf` by value, but since clippy can't
see `Drop` on `Vec` several layers down it complains forcing pass by
value and making it impossible to use the same name later.
New implementation only looks at copy values or values created inplace
so existing variable will never be moved but things that take a string
reference created and value is created inplace `&"".to_owned()` will
make it to suggest to use `"".to_owned()` still.
Fixes https://github.com/rust-lang/rust-clippy/issues/12454
Before this fix, the lint had a false positive, namely when a reference
was taken to a field when the field operand implements a custom Drop.
The compiler will refuse to partially move a type that implements Drop,
because that would put the operand in a weird state. See added
regression test.
