Double-indirection with Cow causes inconsistent behavior of the borrow checker

[darkwisebear]

UPDATE: This needs a test! Mentoring instructions can be found here.

---

While working on some code using std::borrow::Cow I encountered some strange behavior. Here is a simplified version of the code I was using:

use std::borrow::Cow;

#[derive(Clone, Debug)]
struct S<'a> {
    name: Cow<'a, str>
}

#[derive(Clone, Debug)]
struct T<'a> {
    s: Cow<'a, [S<'a>]>
}

fn main() {
    let s1 = [S { name: Cow::Borrowed("Test1") }, S { name: Cow::Borrowed("Test2") }];
    let b1 = T { s: Cow::Borrowed(&s1) };
    let s2 = [S { name: Cow::Borrowed("Test3") }, S { name: Cow::Borrowed("Test4") }];
    let b2 = T { s: Cow::Borrowed(&s2) };
    
    let mut v = Vec::new();
    v.push(b1);
    v.push(b2);
    
    println!("{:?}", v);
}

This fails with

error[E0597]: s2 does not live long enough

Removing b1,s1 or b2,s2 from the code fixes the error. However, since the very same pattern isused for both variable pairs it seems to be inconsistent behavior of the borrow checker.

Meta

Versions checked:
rustc 1.18.0 (03fc9d6 2017-06-06)
binary: rustc
commit-hash: 03fc9d6
commit-date: 2017-06-06
host: x86_64-pc-windows-gnu
release: 1.18.0
LLVM version: 3.9

rustc 1.19.0-beta.2 (a175ee5 2017-06-15)
rustc 1.20.0-nightly (734c836 2017-07-03)

[kennytm]

The problem is that the lifetime of s1 is strictly longer than s2. (I think this is because Cow::Owned(<B as ToOwned>::Owned) somehow causes B i.e. [S<'a>] to be invariant.) Not sure if non-lexical lifetime can fix this.

This could be fixed by moving the let s2 = line above let b1 =.

[arielb1]

@kennytm

This is because Cow is invariant in its lifetime.

NLL should fix this. I tagged this as NLL-prototype so we can have a test and demonstration that this is indeed fixed.

However, compiling this with NLL currently causes an ICE, which I think should be fixed before the prototype:

thread 'rustc' panicked at 'region constraints already solved', /tmp/tmp.gJ7nOH7fpQ/rust/src/libcore/option.rs:874:4
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/sys_common/backtrace.rs:68
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/sys_common/backtrace.rs:57
   2: std::panicking::default_hook::{{closure}}
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/panicking.rs:381
   3: std::panicking::default_hook
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/panicking.rs:391
   4: std::panicking::rust_panic_with_hook
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/panicking.rs:577
   5: std::panicking::begin_panic
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/panicking.rs:538
   6: std::panicking::begin_panic_fmt
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/panicking.rs:522
   7: rust_begin_unwind
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/panicking.rs:498
   8: core::panicking::panic_fmt
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libcore/panicking.rs:71
   9: core::option::expect_failed
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libcore/option.rs:874
  10: rustc::infer::InferCtxt::start_snapshot
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libcore/option.rs:302
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/infer/mod.rs:1537
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libcore/cell.rs:1090
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/infer/mod.rs:1535
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/infer/mod.rs:790
  11: rustc::infer::InferCtxt::probe
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/infer/mod.rs:904
  12: rustc::traits::project::opt_normalize_projection_type
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:1002
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:807
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:541
  13: rustc::traits::project::normalize_projection_type
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:398
  14: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:318
  15: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::fold_with
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/structural_impls.rs:695
  16: rustc::traits::project::normalize
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:266
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:225
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/project.rs:209
  17: rustc::traits::fully_normalize
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/traits/mod.rs:605
  18: rustc_mir::transform::nll::constraint_generation::ConstraintGeneration::add_drop_live_constraint
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/transform/nll/constraint_generation.rs:158
  19: rustc_mir::transform::nll::constraint_generation::ConstraintGeneration::add_liveness_constraints::{{closure}}
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/transform/nll/constraint_generation.rs:81
  20: rustc_mir::util::liveness::LivenessResult::simulate_block
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/util/liveness.rs:151
  21: rustc_mir::transform::nll::constraint_generation::generate_constraints
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/transform/nll/constraint_generation.rs:76
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/transform/nll/constraint_generation.rs:54
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/transform/nll/constraint_generation.rs:35
  22: rustc_mir::transform::nll::compute_regions
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/transform/nll/mod.rs:78
  23: rustc_mir::borrow_check::do_mir_borrowck
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/borrow_check.rs:116
  24: <std::thread::local::LocalKey<T>>::with
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/borrow_check.rs:65
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/infer/mod.rs:439
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/context.rs:1453
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/thread/local.rs:377
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/thread/local.rs:288
  25: rustc::ty::context::tls::enter
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/context.rs:1450
  26: rustc::ty::context::GlobalCtxt::enter_local
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/context.rs:1261
  27: rustc::infer::InferCtxtBuilder::enter
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/infer/mod.rs:439
  28: rustc_mir::borrow_check::mir_borrowck
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_mir/borrow_check.rs:63
  29: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_borrowck<'tcx>>::compute_result   
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:382
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/dep_graph/graph.rs:273
  31: rustc::dep_graph::graph::DepGraph::with_task
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/dep_graph/graph.rs:189
  32: rustc_errors::Handler::track_diagnostics
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:472
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_errors/lib.rs:564
  33: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:465
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:120
  34: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_borrowck<'tcx>>::force
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:464
  35: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_borrowck<'tcx>>::try_get
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:300
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:506
  36: rustc::ty::maps::TyCtxtAt::mir_borrowck
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:545
  37: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_borrowck
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/maps/plumbing.rs:538
  38: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::{{closure}}
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_driver/driver.rs:1072
  39: rustc::util::common::time
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/util/common.rs:120
  40: <std::thread::local::LocalKey<T>>::with
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc_driver/driver.rs:1070
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/librustc/ty/context.rs:1453
             at /tmp/tmp.gJ7nOH7fpQ/rust/src/libstd/thread/local.rs:377

[nikomatsakis]

Compiling with nll-master currently gives this:

lunch-box. rustc --stage1 ~/tmp/issue-43058.rs  -Znll -Zborrowck=mir
error[E0597]: borrowed value does not live long enough
  --> /home/nmatsakis/tmp/issue-43058.rs:24:2
   |
16 |     let s2 = [S { name: Cow::Borrowed("Test3") }, S { name: Cow::Borrowed("Test4") }];
   |         -- temporary value created here
...
24 | }
   |  ^ temporary value dropped here while still borrowed
   |
   = note: consider using a `let` binding to increase its lifetime

error: aborting due to previous error

[nikomatsakis]

Update: This code is currently accepted:

https://play.rust-lang.org/?gist=0afae43679e86fef1dcedc90b4035ef3&version=nightly

[nikomatsakis]

Marking as E-needstest and E-mentor:

Since this code works now, what we need to do is add it to our test-suite. To do that, you would create a file in src/test/ui/nll named issue-43058.rs containing the example I linked to here. There are general instructions for adding new tests here.

[pnkfelix]

Re-triaging for #56754. NLL-fixed-by-NLL, so leaving open. Assigning to self for the main remaining task of adding the test. But also tagging as P-low since I don't think that is a high priority task.

[pnkfelix]

Ah, the test was added in PR #49634

[pnkfelix]

NLL (migrate mode) is enabled in all editions as of PR #59114. Verified that test case compiles in Nightly 2015 edition; closing as fixed.