put fake reads for guards' fake borrows on the guard's failure path

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -2462,8 +2462,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             debug!("Exiting guard building context with locals: {:?}", guard_frame);
 
             for &(_, temp, _) in fake_borrows {
+                // We put fake reads of fake borrows on the guard's failure path to make sure
+                // they're reachable if we continue matching (#161578) and we put them on the
+                // success path to make sure we can create by-value bindings. This won't keep fake
+                // borrows live on paths where the guard diverges unconditionally, but that should
+                // be sound since we can't continue matching or make bindings after diverging.
                 let cause = FakeReadCause::ForMatchGuard;
                 self.cfg.push_fake_read(guard_true_block, guard_end, cause, Place::from(temp));
+                self.cfg.push_fake_read(guard_false_block, guard_end, cause, Place::from(temp));
             }
 
             self.cfg.goto(guard_false_block, source_info, sub_branch.otherwise_block);

tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir

@@ -106,6 +106,7 @@ fn full_tested_match() -> () {
     bb13: {
         StorageDead(_7);
         StorageDead(_6);
+        FakeRead(ForMatchGuard, _3);
         goto -> bb9;
     }
 

tests/mir-opt/building/match/match_false_edges.full_tested_match2.built.after.mir

@@ -106,6 +106,7 @@ fn full_tested_match2() -> () {
     bb13: {
         StorageDead(_7);
         StorageDead(_6);
+        FakeRead(ForMatchGuard, _3);
         goto -> bb9;
     }
 

tests/mir-opt/building/match/match_false_edges.main.built.after.mir

@@ -137,6 +137,7 @@ fn main() -> () {
     bb17: {
         StorageDead(_8);
         StorageDead(_7);
+        FakeRead(ForMatchGuard, _3);
         goto -> bb13;
     }
 
@@ -165,6 +166,7 @@ fn main() -> () {
         StorageDead(_13);
         StorageDead(_12);
         StorageDead(_11);
+        FakeRead(ForMatchGuard, _3);
         goto -> bb10;
     }
 

tests/mir-opt/building/match/sort_candidates.constant_eq.SimplifyCfg-initial.after.mir

@@ -111,6 +111,9 @@ fn constant_eq(_1: &str, _2: bool) -> u32 {
 
     bb17: {
         StorageDead(_13);
+        FakeRead(ForMatchGuard, _6);
+        FakeRead(ForMatchGuard, _7);
+        FakeRead(ForMatchGuard, _8);
         falseEdge -> [real: bb3, imaginary: bb5];
     }
 

tests/mir-opt/building/match/sort_candidates.disjoint_ranges.SimplifyCfg-initial.after.mir

@@ -79,6 +79,7 @@ fn disjoint_ranges(_1: i32, _2: bool) -> u32 {
 
     bb13: {
         StorageDead(_8);
+        FakeRead(ForMatchGuard, _3);
         falseEdge -> [real: bb1, imaginary: bb3];
     }
 

tests/mir-opt/coroutine/async_closure_fake_read_for_by_move.foo-{closure#0}-{closure#0}.built.after.mir

@@ -59,6 +59,8 @@ yields ()
 
     bb9: {
         StorageDead(_6);
+        FakeRead(ForMatchGuard, _3);
+        FakeRead(ForMatchGuard, _4);
         goto -> bb6;
     }
 

tests/mir-opt/coroutine/async_closure_fake_read_for_by_move.foo-{closure#0}-{synthetic#0}.built.after.mir

@@ -43,6 +43,8 @@ yields ()
 
     bb5: {
         StorageDead(_6);
+        FakeRead(ForMatchGuard, _3);
+        FakeRead(ForMatchGuard, _4);
         falseEdge -> [real: bb1, imaginary: bb1];
     }
 

tests/mir-opt/match_arm_scopes.complicated_match.panic-abort.SimplifyCfg-initial.after-ElaborateDrops.after.diff

@@ -152,6 +152,8 @@
           StorageDead(_9);
           StorageDead(_8);
           StorageDead(_6);
+-         FakeRead(ForMatchGuard, _3);
+-         FakeRead(ForMatchGuard, _4);
 -         falseEdge -> [real: bb3, imaginary: bb3];
 +         goto -> bb2;
       }
@@ -194,6 +196,8 @@
           StorageDead(_12);
           StorageDead(_8);
           StorageDead(_6);
+-         FakeRead(ForMatchGuard, _3);
+-         FakeRead(ForMatchGuard, _4);
 -         falseEdge -> [real: bb1, imaginary: bb1];
 +         goto -> bb1;
       }

tests/mir-opt/match_arm_scopes.complicated_match.panic-unwind.SimplifyCfg-initial.after-ElaborateDrops.after.diff

@@ -152,6 +152,8 @@
           StorageDead(_9);
           StorageDead(_8);
           StorageDead(_6);
+-         FakeRead(ForMatchGuard, _3);
+-         FakeRead(ForMatchGuard, _4);
 -         falseEdge -> [real: bb3, imaginary: bb3];
 +         goto -> bb2;
       }
@@ -194,6 +196,8 @@
           StorageDead(_12);
           StorageDead(_8);
           StorageDead(_6);
+-         FakeRead(ForMatchGuard, _3);
+-         FakeRead(ForMatchGuard, _4);
 -         falseEdge -> [real: bb1, imaginary: bb1];
 +         goto -> bb1;
       }

tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-abort.diff

@@ -62,7 +62,15 @@
 
       bb6: {
           StorageDead(_8);
+-         FakeRead(ForMatchGuard, _3);
+-         FakeRead(ForMatchGuard, _4);
+-         FakeRead(ForMatchGuard, _5);
+-         FakeRead(ForMatchGuard, _6);
 -         falseEdge -> [real: bb1, imaginary: bb1];
++         nop;
++         nop;
++         nop;
++         nop;
 +         goto -> bb1;
       }
 

tests/mir-opt/remove_fake_borrows.match_guard.CleanupPostBorrowck.panic-unwind.diff

@@ -62,7 +62,15 @@
 
       bb6: {
           StorageDead(_8);
+-         FakeRead(ForMatchGuard, _3);
+-         FakeRead(ForMatchGuard, _4);
+-         FakeRead(ForMatchGuard, _5);
+-         FakeRead(ForMatchGuard, _6);
 -         falseEdge -> [real: bb1, imaginary: bb1];
++         nop;
++         nop;
++         nop;
++         nop;
 +         goto -> bb1;
       }
 

tests/ui/match/divergent-guard-soundness-failure.rs

@@ -0,0 +1,16 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/161578>: the fake borrow on `x`
+//! below was ignored previously because the fake read keeping it live was unreachable.
+
+fn main() {
+    let mut x: Option<Box<u64>> = Some(Box::new(7));
+    match x {
+        Some(_) if { x = None; false } && return => {}
+        //~^ ERROR: cannot assign `x` in match guard
+        Some(_) if { x = None; false } || return => {}
+        //~^ ERROR: cannot assign `x` in match guard
+        Some(_) if false && ({ x = None; false } || return) => {}
+        //~^ ERROR: cannot assign `x` in match guard
+        Some(b) => println!("{b}"),
+        None => println!("none"),
+    }
+}

tests/ui/match/divergent-guard-soundness-failure.stderr

@@ -0,0 +1,29 @@
+error[E0510]: cannot assign `x` in match guard
+  --> $DIR/divergent-guard-soundness-failure.rs:7:22
+   |
+LL |     match x {
+   |           - value is immutable in match guard
+LL |         Some(_) if { x = None; false } && return => {}
+   |                      ^ cannot assign
+
+error[E0510]: cannot assign `x` in match guard
+  --> $DIR/divergent-guard-soundness-failure.rs:9:22
+   |
+LL |     match x {
+   |           - value is immutable in match guard
+...
+LL |         Some(_) if { x = None; false } || return => {}
+   |                      ^ cannot assign
+
+error[E0510]: cannot assign `x` in match guard
+  --> $DIR/divergent-guard-soundness-failure.rs:11:32
+   |
+LL |     match x {
+   |           - value is immutable in match guard
+...
+LL |         Some(_) if false && ({ x = None; false } || return) => {}
+   |                                ^ cannot assign
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0510`.

tests/ui/match/divergent-guard-soundness-success.rs

@@ -0,0 +1,15 @@
+//! Test to document strange behavior related to <https://github.com/rust-lang/rust/issues/161578>:
+//! in these gaurds, the guard's success and failures are both unreachable after mutating `x`. This
+//! means the fake reads on the fake borrow on `x` are unreachable when assigning to `x`, so `x` can
+//! be overwritten. This is fine since we don't create bindings and don't continue matching.
+//@ check-pass
+
+fn main() {
+    let mut x: Option<&u64> = Some(&7);
+    match x {
+        Some(&y) if { x = None; return } => {}
+        Some(&y) if true || { x = None; return } => {}
+        Some(b) => println!("{b}"),
+        None => println!("none"),
+    }
+}