rustc panic due to reference passed instead of mut reference

[daniel-tud]

The following code causes a panic in rustc nightly and stable 2018 Edition:

use std::collections::HashMap;
use std::marker::PhantomData;
use std::str::FromStr;

struct Value<'a, T : FromStr + ToString + Clone, P : HasTree> {
    subkey : String,
    parent : &'a mut P,
    default : T,
    phantom : PhantomData<T>
}

pub trait HasTree {
    fn get_tree(&mut self) -> &mut Tree;
}

impl<'a, T : FromStr + ToString + Clone, P : HasTree> Value<'a, T, P> {
    fn from (subkey : String, parent : &'a mut P, default : T) -> Value<'a, T, P> {
        Value {
            subkey : subkey,
            parent : parent,
            default : default,
            phantom : PhantomData {}
        }
    }

    fn get(&self) -> T {
        let default = || { self.default.clone() };
        self.parent.get_tree().enter(&self.subkey).map_or_else(default, |subkey|
            subkey.data.as_ref().map_or_else(default, |str|
            str.parse::<T>().ok().unwrap_or_else(default)))
    }
}


#[derive(Clone)]
pub struct Tree {
    branches : HashMap<String, Tree>,
    pub data : Option<String>
}

impl Tree {
    pub fn new() -> Tree {
        Tree {
            branches : HashMap::new(),
            data : None
        }
    }

    fn enter(&self, s : &String) -> Option<&Tree> {
        self.branches.get(s)
    }
}

I do not expect this to compile, as get_tree requires a mut reference, which is not provided by get. When changing get_tree to fn get_tree(&self) -> &Tree;, the code compiles properly.

Backtrace

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'no entry found for key', src/libcore/option.rs:1036:5
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:212
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:479
   8: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:382
   9: rust_begin_unwind
             at src/libstd/panicking.rs:309
  10: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  11: core::option::expect_failed
             at src/libcore/option.rs:1036
  12: rustc_mir::borrow_check::path_utils::each_borrow_involving_path
  13: rustc_mir::borrow_check::MirBorrowckCtxt::access_place
  14: ::visit_statement_entry
  15: rustc_mir::borrow_check::do_mir_borrowck
  16: rustc::ty::context::GlobalCtxt::enter_local
  17: rustc_mir::borrow_check::mir_borrowck
  18: rustc::ty::query::__query_compute::mir_borrowck
  19: rustc::ty::query::::compute
  20: rustc::dep_graph::graph::DepGraph::with_task_impl
  21: rustc::ty::query::plumbing::::get_query
  22: rustc::ty::::par_body_owners
  23: rustc::util::common::time
  24: rustc_interface::passes::analysis
  25: rustc::ty::query::__query_compute::analysis
  26: rustc::dep_graph::graph::DepGraph::with_task_impl
  27: rustc::ty::query::plumbing::::get_query
  28: rustc::ty::context::tls::enter_global
  29: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  30: rustc_interface::passes::create_global_ctxt::{{closure}}
  31: rustc_interface::interface::run_compiler_in_existing_thread_pool
  32: std::thread::local::LocalKey::with
  33: scoped_tls::ScopedKey::set
  34: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
#0 [mir_borrowck] processing `Value::<'a, T, P>::get`
#1 [analysis] running analysis passes on this crate
end of query stack
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: rustc 1.37.0-nightly (5eeb567a2 2019-06-06) running on x86_64-unknown-linux-gnu
note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib
note: some of the compiler flags provided by cargo are hidden
error: Could not compile playground.
To learn more, run the command again with --verbose.

[Centril]

Reduction:

fn f1<'a>(_: &'a mut ()) {}

fn f2<P>(_: P, _: ()) {}

fn f3<'a>(x: &'a ((), &'a mut ())) {
    f2(|| x.0, f1(x.1))
}

[Centril]

cc @matthewjasper

[matthewjasper]

The issue is here:

rust/src/librustc_mir/borrow_check/mod.rs

Line 1075 in c1c60d2

let bi = this.borrow_set.location_map[&location];

if location_map doesn't have location we should not take this arm, and should take the next one.

Another couple of test cases:

fn f1<'a>(_: &'a mut ()) {}

unsafe fn f3<'a>(x: *mut ((), &'a mut ())) {
    (&x, f1((*x).1));
}

trait A {
    fn f1<'a>(&'a mut self) {}
}

impl<T> A for T {}

unsafe fn f3<'a>(x: *mut ()) {
    (&x, (*x).f1());
}

[pnkfelix]

triage: P-high, assigning to @matthewjasper , removing I-nominated label.