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matthewjaspermatthewjasper
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Add tests for new match borrows
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‎src/test/ui/issues/issue-27282-mutate-before-diverging-arm-3.rs

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// This is testing an attempt to corrupt the discriminant of the match
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// arm in a guard, followed by an attempt to continue matching on that
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// corrupted discriminant in the remaining match arms.
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//
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// Basically this is testing that our new NLL feature of emitting a
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// fake read on each match arm is catching cases like this.
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//
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// This case is interesting because a borrow of **x is untracked, because **x is
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// immutable. However, for matches we care that **x refers to the same value
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// until we have chosen a match arm.
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#![feature(nll)]
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struct ForceFnOnce;
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fn main() {
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let mut x = &mut &Some(&2);
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let force_fn_once = ForceFnOnce;
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match **x {
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None => panic!("unreachable"),
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Some(&_) if {
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// ForceFnOnce needed to exploit #27282
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(|| { *x = &None; drop(force_fn_once); })();
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//~^ ERROR cannot mutably borrow `x` in match guard [E0510]
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false
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} => {}
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Some(&a) if { // this binds to garbage if we've corrupted discriminant
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println!("{}", a);
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panic!()
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} => {}
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_ => panic!("unreachable"),
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}
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}

‎src/test/ui/issues/issue-27282-mutate-before-diverging-arm-3.stderr

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error[E0510]: cannot mutably borrow `x` in match guard
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--> $DIR/issue-27282-mutate-before-diverging-arm-3.rs:20:14
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|
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LL | match **x {
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| --- value is immutable in match guard
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...
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LL | (|| { *x = &None; drop(force_fn_once); })();
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| ^^ - borrow occurs due to use of `x` in closure
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| |
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| cannot mutably borrow
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error: aborting due to previous error
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For more information about this error, try `rustc --explain E0510`.

‎src/test/ui/nll/match-guards-partially-borrow.rs

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// Test that a (partially) mutably borrowed place can be matched on, so long as
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// we don't have to read any values that are mutably borrowed to determine
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// which arm to take.
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//
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// Test that we don't allow mutating the value being matched on in a way that
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// changes which patterns it matches, until we have chosen an arm.
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// compile-flags: -Zdisable-ast-check-for-mutation-in-guard
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#![feature(nll)]
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fn ok_mutation_in_guard(mut q: i32) {
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match q {
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// OK, mutation doesn't change which patterns g matches
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_ if { q = 1; false } => (),
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_ => (),
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}
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}
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fn ok_indirect_mutation_in_guard(mut p: &bool) {
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match *p {
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// OK, mutation doesn't change which patterns s matches
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_ if {
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p = &true;
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false
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} => (),
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_ => (),
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}
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}
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fn mutation_invalidates_pattern_in_guard(mut q: bool) {
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match q {
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// s doesn't match the pattern with the guard by the end of the guard.
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false if {
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q = true; //~ ERROR
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true
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} => (),
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_ => (),
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}
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}
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fn mutation_invalidates_previous_pattern_in_guard(mut r: bool) {
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match r {
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// s matches a previous pattern by the end of the guard.
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true => (),
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_ if {
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r = true; //~ ERROR
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true
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} => (),
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_ => (),
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}
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}
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fn match_on_borrowed_early_end(mut s: bool) {
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let h = &mut s;
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match s { //~ ERROR
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// s changes value between the start of the match and when its value is checked.
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_ if {
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*h = !*h;
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false
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} => (),
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true => (),
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false => (),
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}
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}
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fn bad_mutation_in_guard(mut t: bool) {
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match t {
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true => (),
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false if {
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t = true; //~ ERROR
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false
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} => (),
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false => (),
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}
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}
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fn bad_mutation_in_guard2(mut u: bool) {
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match u {
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// Guard changes the value bound in the last pattern.
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_ => (),
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_ if {
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u = true; //~ ERROR
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false
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} => (),
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x => (),
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}
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}
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pub fn bad_mutation_in_guard3(mut x: Option<Option<&i32>>) {
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// Check that nested patterns are checked.
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match x {
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None => (),
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Some(None) => (),
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_ if {
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match x {
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Some(ref mut r) => *r = None, //~ ERROR
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_ => return,
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};
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false
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} => (),
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Some(Some(r)) => println!("{}", r),
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}
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}
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fn bad_mutation_in_guard4(mut w: (&mut bool,)) {
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match w {
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// Guard changes the value bound in the last pattern.
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_ => (),
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_ if {
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*w.0 = true; //~ ERROR
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false
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} => (),
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x => (),
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}
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}
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fn bad_indirect_mutation_in_guard(mut y: &bool) {
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match *y {
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true => (),
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false if {
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y = &true; //~ ERROR
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false
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} => (),
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false => (),
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}
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}
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fn bad_indirect_mutation_in_guard2(mut z: &bool) {
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match z {
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&true => (),
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&false if {
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z = &true; //~ ERROR
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false
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} => (),
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&false => (),
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}
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}
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fn bad_indirect_mutation_in_guard3(mut a: &bool) {
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// Same as bad_indirect_mutation_in_guard2, but using match ergonomics
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match a {
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true => (),
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false if {
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a = &true; //~ ERROR
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false
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} => (),
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false => (),
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}
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}
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fn bad_indirect_mutation_in_guard4(mut b: &bool) {
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match b {
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&_ => (),
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&_ if {
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b = &true; //~ ERROR
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false
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} => (),
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&b => (),
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}
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}
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fn main() {}

‎src/test/ui/nll/match-guards-partially-borrow.stderr

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error[E0510]: cannot assign `q` in match guard
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--> $DIR/match-guards-partially-borrow.rs:35:13
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|
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LL | match q {
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| - value is immutable in match guard
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...
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LL | q = true; //~ ERROR
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| ^^^^^^^^ cannot assign
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...
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LL | _ => (),
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| - borrow later used here
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error[E0510]: cannot assign `r` in match guard
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--> $DIR/match-guards-partially-borrow.rs:47:13
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|
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LL | match r {
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| - value is immutable in match guard
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...
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LL | r = true; //~ ERROR
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| ^^^^^^^^ cannot assign
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...
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LL | _ => (),
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| - borrow later used here
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error[E0503]: cannot use `s` because it was mutably borrowed
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--> $DIR/match-guards-partially-borrow.rs:56:11
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|
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LL | let h = &mut s;
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| ------ borrow of `s` occurs here
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LL | match s { //~ ERROR
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| ^ use of borrowed `s`
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...
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LL | *h = !*h;
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| -- borrow later used here
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error[E0510]: cannot assign `t` in match guard
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--> $DIR/match-guards-partially-borrow.rs:71:13
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|
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LL | match t {
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| - value is immutable in match guard
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...
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LL | t = true; //~ ERROR
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| ^^^^^^^^ cannot assign
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...
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LL | false => (),
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| ----- borrow later used here
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error[E0506]: cannot assign to `u` because it is borrowed
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--> $DIR/match-guards-partially-borrow.rs:83:13
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|
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LL | match u {
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| - borrow of `u` occurs here
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...
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LL | u = true; //~ ERROR
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| ^^^^^^^^ assignment to borrowed `u` occurs here
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...
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LL | x => (),
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| - borrow later used here
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error[E0510]: cannot mutably borrow `x.0` in match guard
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--> $DIR/match-guards-partially-borrow.rs:97:22
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|
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LL | match x {
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| - value is immutable in match guard
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...
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LL | Some(ref mut r) => *r = None, //~ ERROR
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| ^^^^^^^^^ cannot mutably borrow
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error[E0506]: cannot assign to `*w.0` because it is borrowed
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--> $DIR/match-guards-partially-borrow.rs:112:13
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|
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LL | match w {
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| - borrow of `*w.0` occurs here
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...
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LL | *w.0 = true; //~ ERROR
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| ^^^^^^^^^^^ assignment to borrowed `*w.0` occurs here
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...
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LL | x => (),
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| - borrow later used here
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error[E0510]: cannot assign `y` in match guard
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--> $DIR/match-guards-partially-borrow.rs:123:13
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|
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LL | match *y {
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| -- value is immutable in match guard
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...
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LL | y = &true; //~ ERROR
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| ^^^^^^^^^ cannot assign
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...
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LL | false => (),
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| ----- borrow later used here
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error[E0510]: cannot assign `z` in match guard
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--> $DIR/match-guards-partially-borrow.rs:134:13
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|
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LL | match z {
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| - value is immutable in match guard
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...
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LL | z = &true; //~ ERROR
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| ^^^^^^^^^ cannot assign
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...
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LL | &false => (),
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| ------ borrow later used here
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error[E0510]: cannot assign `a` in match guard
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--> $DIR/match-guards-partially-borrow.rs:146:13
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|
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LL | match a {
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| - value is immutable in match guard
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...
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LL | a = &true; //~ ERROR
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| ^^^^^^^^^ cannot assign
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...
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LL | false => (),
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| ----- borrow later used here
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error[E0510]: cannot assign `b` in match guard
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--> $DIR/match-guards-partially-borrow.rs:157:13
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|
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LL | match b {
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| - value is immutable in match guard
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...
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LL | b = &true; //~ ERROR
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| ^^^^^^^^^ cannot assign
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...
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LL | &b => (),
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| -- borrow later used here
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error: aborting due to 11 previous errors
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Some errors occurred: E0503, E0506, E0510.
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For more information about an error, try `rustc --explain E0503`.

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