ICE in borrow checker

[eefriedman]

https://play.rust-lang.org/?gist=23de822c03921663ae66&version=nightly .

Error message:

<anon>:110:25: 110:60 error: cannot assign to `*entry` because it is borrowed
<anon>:110                         *entry = FragmentRepr::Enum(discrs);
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of if let expansion
<anon>:108:21: 111:22 note: expansion site
note: in expansion of for loop expansion
<anon>:100:5: 150:6 note: expansion site
<anon>:112:64: 112:78 note: borrow of `*entry` occurs here
<anon>:112                     let mut discrs = if let FragmentRepr::Enum(ref mut discrs) = *entry {
                                                                          ^~~~~~~~~~~~~~
note: in expansion of if let expansion
<anon>:112:38: 116:22 note: expansion site
note: in expansion of for loop expansion
<anon>:100:5: 150:6 note: expansion site
<anon>:112:64: 112:78 error: cannot borrow `entry.0` as mutable more than once at a time
<anon>:112                     let mut discrs = if let FragmentRepr::Enum(ref mut discrs) = *entry {
                                                                          ^~~~~~~~~~~~~~
note: in expansion of if let expansion
<anon>:112:38: 116:22 note: expansion site
note: in expansion of for loop expansion
<anon>:100:5: 150:6 note: expansion site
<anon>:112:64: 112:78 note: previous borrow of `entry.0` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `entry.0` until the borrow ends
<anon>:112                     let mut discrs = if let FragmentRepr::Enum(ref mut discrs) = *entry {
                                                                          ^~~~~~~~~~~~~~
note: in expansion of if let expansion
<anon>:112:38: 116:22 note: expansion site
note: in expansion of for loop expansion
<anon>:100:5: 150:6 note: expansion site
<anon>:150:6: 150:6 note: previous borrow ends here
<anon>:100     for &move_path_index in paths {
...
<anon>:150     }
               ^
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: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'assertion failed: `(left == right)` (left: `collections::vec::Vec<(u32, FragmentRepr)>`, right: `Box<FragmentRepr>`)', ../src/librustc_borrowck/borrowck/mod.rs:454

[Aatch]

Could you maybe try to reduce the case down a bit more?

[eefriedman]

Reduced:

pub enum FragmentRepr {
    Boxed(Box<FragmentRepr>),
    Enum(Vec<FragmentRepr>),
}
fn bar() -> u32 { panic!(); }
pub fn foo(mut entry: &mut FragmentRepr) {
    loop {
        match bar() {
            1 => {
                let mut discrs = if let FragmentRepr::Enum(ref mut discrs) = *entry {
                    discrs
                } else {
                    return;
                };
                entry = &mut discrs[0];
            }
            _ => {
                entry = if let FragmentRepr::Boxed(ref mut contents) = *entry {
                    contents
                } else {
                    return;
                };
            }
        }
    }
}
fn main() {}

[arielb1]

Much reduced:

pub enum FragmentRepr {
    Boxed(Box<FragmentRepr>),
    Enum(()),
}
fn unconstrained<T>() -> T { loop {} }
pub fn foo(mut entry: &mut FragmentRepr) {
    entry = if let &mut FragmentRepr::Boxed(ref mut contents) = entry {
        contents
    } else {
        unconstrained()
    };
    match *entry {
        FragmentRepr::Enum(ref mut discrs) => {},
        _ => {}
    };
}
fn main() {}

Somehow &mut &mut FragmentRepr became &mut FragmentRepr (wut?)

Seems like we don't handle deref-coercions somewhere.

[arielb1]

duplicate of #27844.