NLL erroneous Warning: variable does not need to be mutable

[rukai]

#![feature(nll)]
struct Foo {
    pub value: i32
}

// triggers bug
fn use_foo_mut(mut foo: Foo) {
    foo = foo;
    println!("{}", foo.value);
}

// does not trigger bug
fn use_i32_mut(mut i: i32) {
    i = i;
    println!("{}", i);
}

fn main() {
    use_foo_mut(Foo { value: 413 });
    use_i32_mut(413);
}

gives:

warning: variable does not need to be mutable
 --> src/main.rs:8:16
  |
8 | fn use_foo_mut(mut foo: Foo) {
  |                ----^^^
  |                |
  |                help: remove this `mut`
  |
  = note: #[warn(unused_mut)] on by default

If I remove the mut then I get a compile error.

Compiled with nightly-2018-05-05

This is not related to #50343 as that has been fixed in the nightly I am using.

[pnkfelix]

this is probably related to the recent changes to do the use-of-mut checks without requiring AST-borrowck.

[pnkfelix]

i.e. this is probably related to PR #48605

[KiChjang]

cc me

[KiChjang]

Hmm... I think this bug is highlighted by @pnkfelix a couple of days ago, and it's regarding this branch:

rust/src/librustc_mir/borrow_check/mod.rs

Lines 1711 to 1716 in 8ff4b42

	Reservation(WriteKind::MutableBorrow(BorrowKind::Unique))
	| Write(WriteKind::MutableBorrow(BorrowKind::Unique)) => {
	if let Err(_place_err) = self.is_mutable(place, LocalMutationIsAllowed::Yes) {
	span_bug!(span, "&unique borrow for {:?} should not fail", place);
	}
	}

Or perhaps in the branch further down as well:

rust/src/librustc_mir/borrow_check/mod.rs

Lines 1805 to 1810 in 8ff4b42

	Reservation(WriteKind::Move)
	| Reservation(WriteKind::StorageDeadOrDrop)
	| Reservation(WriteKind::MutableBorrow(BorrowKind::Shared))
	| Write(WriteKind::Move)
	| Write(WriteKind::StorageDeadOrDrop)
	| Write(WriteKind::MutableBorrow(BorrowKind::Shared)) => {

I suspect it's the latter, because the MIR generated by the code above contains two move assignment statements. Either way, these two branches should really contain the used mut variables logic.