Code change:
<ins> error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:28:20 | LL |     if let [ref first, ref second, ..] = *s { |                        ---------- immutable borrow occurs here LL |         if let [_, ref mut  second2, ref mut third, ..] = *s { //~ERROR |                    ^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[first, second, second2, third]); |                          ------ borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:44:21 | LL |     if let [.., ref fourth, ref third, _, ref first] = *s { |                             --------- immutable borrow occurs here LL |         if let [.., ref mut third2, _, _] = *s { //~ERROR |                     ^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[first, third, third2, fourth]); |                          ----- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:55:20 | LL |     if let [.., _, ref from_end4, ref from_end3, _, ref from_end1] = *s { |                    ------------- immutable borrow occurs here ... LL |         if let [_, ref mut from_begin1, ..] = *s { //~ERROR |                    ^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[from_begin1, from_end1, from_end3, from_end4]); |                                                      --------- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:58:23 | LL |     if let [.., _, ref from_end4, ref from_end3, _, ref from_end1] = *s { |                                   ------------- immutable borrow occurs here ... LL |         if let [_, _, ref mut from_begin2, ..] = *s { //~ERROR |                       ^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[from_begin2, from_end1, from_end3, from_end4]); |                                           --------- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:61:26 | LL |     if let [.., _, ref from_end4, ref from_end3, _, ref from_end1] = *s { |                                   ------------- immutable borrow occurs here ... LL |         if let [_, _, _, ref mut from_begin3, ..] = *s { //~ERROR |                          ^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[from_begin3, from_end1, from_end3, from_end4]); |                                           --------- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:69:21 | LL |     if let [ref from_begin0, ref from_begin1, _, ref from_begin3, _, ..] = *s { |                                                  --------------- immutable borrow occurs here ... LL |         if let [.., ref mut from_end2, _] = *s { //~ERROR |                     ^^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[from_begin0, from_begin1, from_begin3, from_end2]); |                                             ----------- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:72:21 | LL |     if let [ref from_begin0, ref from_begin1, _, ref from_begin3, _, ..] = *s { |                                                  --------------- immutable borrow occurs here ... LL |         if let [.., ref mut from_end3, _,  _] = *s { //~ERROR |                     ^^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[from_begin0, from_begin1, from_begin3, from_end3]); |                                             ----------- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:75:21 | LL |     if let [ref from_begin0, ref from_begin1, _, ref from_begin3, _, ..] = *s { |                              --------------- immutable borrow occurs here ... LL |         if let [.., ref mut from_end4, _, _, _] = *s { //~ERROR |                     ^^^^^^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[from_begin0, from_begin1, from_begin3, from_end4]); |                                ----------- borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:92:20 | LL |     if let [ref first, ref second, ..] = *s { |                        ---------- immutable borrow occurs here LL |         if let [_, ref mut tail..] = *s { //~ERROR |                    ^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[first, second]); |                          ------ borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:110:17 | LL |     if let [.., ref second, ref first] = *s { |                 ---------- immutable borrow occurs here LL |         if let [ref mut tail.., _] = *s { //~ERROR |                 ^^^^^^^^^^^^ mutable borrow occurs here LL |             nop(&[first, second]); |                          ------ borrow later used here error[E0502]: cannot borrow `s[..]` as mutable because it is also borrowed as immutable --> $DIR/borrowck-slice-pattern-element-loan.rs:119:17 | LL |     if let [_, _, _, ref s1..] = *s { |                      ------ immutable borrow occurs here LL |         if let [ref mut s2.., _, _, _] = *s { //~ERROR |                 ^^^^^^^^^^ mutable borrow occurs here LL |             nop_subslice(s1); |                          -- borrow later used here error: aborting due to 11 previous errors For more information about this error, try `rustc --explain E0502`. </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