Issue #46589 - Kill borrows on a local variable whenever we assign ov…

[Yoric]

…er this variable

This is a first patch for the issue, handling the simple case while I figure out the data structures involved in the more complex cases.

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[bjorn3]

Issue #46589 (just to link it)

Could you add a test?

[pnkfelix]

The code looks fine to me; just add a test and we should be good.

[Yoric]

I have added the test of issue #46589 and much to my surprise, it still fails, although it looks like it should be covered by my patch. I must be missing something obvious:

error[E0506]: cannot assign to `list` because it is borrowed
  --> /Users/david/Documents/Code/rust-nll/src/test/run-pass/nll-iterating-and-updating.rs:25:13
   |
23 |         result.push(&mut list.value);
   |                     --------------- borrow of `list` occurs here
24 |         if let Some(n) = list.next.as_mut() {
25 |             list = n;
   |             ^^^^^^^^ assignment to borrowed `list` occurs here

error: aborting due to previous error

[Yoric]

Note: problem appears even if we add a

let mut list = list;

so it's probably not caused by list being an argument.

[arielb1]

@Yoric

I think that's because you haven't fixed this:

rust/src/librustc_mir/borrow_check/mod.rs

Lines 335 to 355 in a9f047c

	StatementKind::Assign(ref lhs, ref rhs) => {
	// NOTE: NLL RFC calls for *shallow* write; using Deep
	// for short-term compat w/ AST-borrowck. Also, switch
	// to shallow requires to dataflow: "if this is an
	// assignment `place = <rvalue>`, then any loan for some
	// path P of which `place` is a prefix is killed."
	self.mutate_place(
	ContextKind::AssignLhs.new(location),
	(lhs, span),
	Deep,
	JustWrite,
	flow_state,
	);
	self.consume_rvalue(
	ContextKind::AssignRhs.new(location),
	(rhs, span),
	location,
	flow_state,
	);
	}

[Yoric]

Ok, thanks to @arielb1, this now seems to work.

[Yoric]

(oops, forgot to remove the obsolete test, this should now be fixed)

[arielb1]

@bors r+

[bors]

📌 Commit 5e0e633 has been approved by arielb1

[bors]

⌛ Testing commit 5e0e633d1f780f865b4c8298f7116b274824994a with merge 627022b2338aeb9e7d416bb1c6fee34125d8ec32...

[bors]

💔 Test failed - status-appveyor

[Yoric]

As far as I can tell, the single failure is a timeout, which I imagine is not caused by this PR.

[kennytm]

@bors retry — 3 hour timeout.

Build number	1.0.5591	1.0.5765
make-prepare	43.49	49.60
pytest/bootstrap	0.00	0.00
stage0-std	112.40	120.96
stage0-tidy	6.55	7.46
bootstrap	5.73	5.92
stage0-test	11.26	12.68
llvm	557.90	642.91
stage0-rustc	1080.53	1471.46
stage1-std	54.30	75.27
stage1-test	16.18	21.58
stage1-rustc	1357.72	2055.58
stage0-compiletest	54.95	103.39
test/ui	73.17	188.49
test/run-pass	1380.91	1874.05
test/compile-fail	573.45	568.06
test/parse-fail	26.47	32.96
test/run-fail	30.84	37.61
test/run-pass-valgrind	4.08	4.66
test/mir-opt	19.00	33.14
test/codegen	9.00	10.73
test/codegen-units	12.81	10.29
test/incremental	81.99	78.64
test/debuginfo	31.47	31.93
test/ui-fulldeps	17.09	32.71
test/run-pass-fulldeps	223.00	297.01
test/run-fail-fulldeps	3.08	4.81
test/compile-fail-fulldeps	69.19	91.50
stage2-rustdoc	228.74	301.32
test/run-make	144.75	171.07
test/rustdoc	121.66	175.18
stage1-test-libstd	178.30	236.88
test/libstd	790.40	914.47
stage1-test-libtest	12.46	13.91
test/libtest	13.82	18.38
stage1-test-librustc	309.59	394.63
test/librustc	314.23	400.63
stage2-test-rustdoc	45.49	54.72
test/libtools	46.40	55.56
stage0-unstable-book-gen	8.60	10.47
stage0-rustbook	177.15	181.70
doc/std	27.87	36.66
doc/compiler	1.42	3.33
stage2-error_index_generator	22.21	28.20
stage0-linkchecker	2.51	2.92
test/linkchecker	44.02	48.44
test/docs	241.15
stage2-test-error-index	61.73

 

[Yoric]

Fwiw, SYS_BITS=32, RUST_CONFIGURE_ARGS=--build=i686-pc-windows-gnu, SCRIPT=python x.py test, MINGW_URL=https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rust-ci-mirror, MINGW_ARCHIVE=i686-6.3.0-release-posix-dwarf-rt_v5-rev2.7z, MINGW_DIR=mingw32 timed out after 3 hr

[bors]

☔ The latest upstream changes (presumably #46904) made this pull request unmergeable. Please resolve the merge conflicts.

[kennytm]

@bors r=arielb1

[bors]

📌 Commit fcb1090 has been approved by arielb1

[bors]

⌛ Testing commit fcb1090 with merge 264af16...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: arielb1
Pushing 264af16 to master...