Code change:
<ins> // Copyright 2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //compile-flags: -Z borrowck=mir #![feature(slice_patterns)] fn nop(_s: &[& i32]) {} fn nop_subslice(_s: &[i32]) {} fn const_index_ok(s: &mut [i32]) { if let [ref first, ref second, _, ref fourth, ..] = *s { if let [_, _, ref mut third, ..] = *s { nop(&[first, second, third, fourth]); } } } fn const_index_err(s: &mut [i32]) { if let [ref first, ref second, ..] = *s { if let [_, ref mut  second2, ref mut third, ..] = *s { //~ERROR nop(&[first, second, second2, third]); } } } fn const_index_from_end_ok(s: &mut [i32]) { if let [.., ref fourth, ref third, _, ref first] = *s { if let [.., ref mut second, _] = *s { nop(&[first, second, third, fourth]); } } } fn const_index_from_end_err(s: &mut [i32]) { if let [.., ref fourth, ref third, _, ref first] = *s { if let [.., ref mut third2, _, _] = *s { //~ERROR nop(&[first, third, third2, fourth]); } } } fn const_index_mixed(s: &mut [i32]) { if let [.., _, ref from_end4, ref from_end3, _, ref from_end1] = *s { if let [ref mut from_begin0, ..] = *s { nop(&[from_begin0, from_end1, from_end3, from_end4]); } if let [_, ref mut from_begin1, ..] = *s { //~ERROR nop(&[from_begin1, from_end1, from_end3, from_end4]); } if let [_, _, ref mut from_begin2, ..] = *s { //~ERROR nop(&[from_begin2, from_end1, from_end3, from_end4]); } if let [_, _, _, ref mut from_begin3, ..] = *s { //~ERROR nop(&[from_begin3, from_end1, from_end3, from_end4]); } } if let [ref from_begin0, ref from_begin1, _, ref from_begin3, _, ..] = *s { if let [.., ref mut from_end1] = *s { nop(&[from_begin0, from_begin1, from_begin3, from_end1]); } if let [.., ref mut from_end2, _] = *s { //~ERROR nop(&[from_begin0, from_begin1, from_begin3, from_end2]); } if let [.., ref mut from_end3, _,  _] = *s { //~ERROR nop(&[from_begin0, from_begin1, from_begin3, from_end3]); } if let [.., ref mut from_end4, _, _, _] = *s { //~ERROR nop(&[from_begin0, from_begin1, from_begin3, from_end4]); } } } fn const_index_and_subslice_ok(s: &mut [i32]) { if let [ref first, ref second, ..] = *s { if let [_, _, ref mut tail..] = *s { nop(&[first, second]); nop_subslice(tail); } } } fn const_index_and_subslice_err(s: &mut [i32]) { if let [ref first, ref second, ..] = *s { if let [_, ref mut tail..] = *s { //~ERROR nop(&[first, second]); nop_subslice(tail); } } } fn const_index_and_subslice_from_end_ok(s: &mut [i32]) { if let [.., ref second, ref first] = *s { if let [ref mut tail.., _, _] = *s { nop(&[first, second]); nop_subslice(tail); } } } fn const_index_and_subslice_from_end_err(s: &mut [i32]) { if let [.., ref second, ref first] = *s { if let [ref mut tail.., _] = *s { //~ERROR nop(&[first, second]); nop_subslice(tail); } } } fn subslices(s: &mut [i32]) { if let [_, _, _, ref s1..] = *s { if let [ref mut s2.., _, _, _] = *s { //~ERROR nop_subslice(s1); nop_subslice(s2); } } } fn main() { let mut v = [1,2,3,4]; const_index_ok(&mut v); const_index_err(&mut v); const_index_from_end_ok(&mut v); const_index_from_end_err(&mut v); const_index_and_subslice_ok(&mut v); const_index_and_subslice_err(&mut v); const_index_and_subslice_from_end_ok(&mut v); const_index_and_subslice_from_end_err(&mut v); subslices(&mut v); } </ins>

Related discussion:
text: Minimal example: This will complain about multiple mutable borrows, even though it should be valid.
Noted in (but may deserve a separate bug report?). (FYI, slice patterns are unstable for a reason; they're pretty buggy at the moment).
should be closed after MIR borrowck will be by default