Auto merge of #103208 - cjgillot:match-fake-read, r=oli-obk,RalfJung

Allow partially moved values in match

This PR attempts to unify the behaviour between `let _ = PLACE`, `let _: TY = PLACE;` and `match PLACE { _ => {} }`.
The logical conclusion is that the `match` version should not check for uninitialised places nor check that borrows are still live.

The `match PLACE {}` case is handled by keeping a `FakeRead` in the unreachable fallback case to verify that `PLACE` has a legal value.

Schematically, `match PLACE { arms }` in surface rust becomes in MIR:
```rust
PlaceMention(PLACE)
match PLACE {
  // Decision tree for the explicit arms
  arms,
  // An extra fallback arm
  _ => {
    FakeRead(ForMatchedPlace, PLACE);
    unreachable
  }
}
```

`match *borrow { _ => {} }` continues to check that `*borrow` is live, but does not read the value.
`match *borrow {}` both checks that `*borrow` is live, and fake-reads the value.

Continuation of ~#102256 ~#104844

Fixes #99180 #53114

Author: bors

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

@@ -157,7 +157,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// [ 0. Pre-match ]
     ///        |
     /// [ 1. Evaluate Scrutinee (expression being matched on) ]
-    /// [ (fake read of scrutinee) ]
+    /// [ (PlaceMention of scrutinee) ]
     ///        |
     /// [ 2. Decision tree -- check discriminants ] <--------+
     ///        |                                             |
@@ -184,7 +184,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ///
     /// We generate MIR in the following steps:
     ///
-    /// 1. Evaluate the scrutinee and add the fake read of it ([Builder::lower_scrutinee]).
+    /// 1. Evaluate the scrutinee and add the PlaceMention of it ([Builder::lower_scrutinee]).
     /// 2. Create the decision tree ([Builder::lower_match_tree]).
     /// 3. Determine the fake borrows that are needed from the places that were
     ///    matched against and create the required temporaries for them
@@ -223,6 +223,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let fake_borrow_temps = self.lower_match_tree(
             block,
             scrutinee_span,
+            &scrutinee_place,
             match_start_span,
             match_has_guard,
             &mut candidates,
@@ -238,34 +239,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         )
     }
 
-    /// Evaluate the scrutinee and add the fake read of it.
+    /// Evaluate the scrutinee and add the PlaceMention for it.
     fn lower_scrutinee(
         &mut self,
         mut block: BasicBlock,
         scrutinee: &Expr<'tcx>,
         scrutinee_span: Span,
     ) -> BlockAnd<PlaceBuilder<'tcx>> {
         let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee));
-        // Matching on a `scrutinee_place` with an uninhabited type doesn't
-        // generate any memory reads by itself, and so if the place "expression"
-        // contains unsafe operations like raw pointer dereferences or union
-        // field projections, we wouldn't know to require an `unsafe` block
-        // around a `match` equivalent to `std::intrinsics::unreachable()`.
-        // See issue #47412 for this hole being discovered in the wild.
-        //
-        // HACK(eddyb) Work around the above issue by adding a dummy inspection
-        // of `scrutinee_place`, specifically by applying `ReadForMatch`.
-        //
-        // NOTE: ReadForMatch also checks that the scrutinee is initialized.
-        // This is currently needed to not allow matching on an uninitialized,
-        // uninhabited value. If we get never patterns, those will check that
-        // the place is initialized, and so this read would only be used to
-        // check safety.
-        let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
-        let source_info = self.source_info(scrutinee_span);
-
         if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) {
-            self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place);
+            let source_info = self.source_info(scrutinee_span);
+            self.cfg.push_place_mention(block, source_info, scrutinee_place);
         }
 
         block.and(scrutinee_place_builder)
@@ -304,6 +288,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         &mut self,
         block: BasicBlock,
         scrutinee_span: Span,
+        scrutinee_place_builder: &PlaceBuilder<'tcx>,
         match_start_span: Span,
         match_has_guard: bool,
         candidates: &mut [&mut Candidate<'pat, 'tcx>],
@@ -331,6 +316,33 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             // otherwise block. Match checking will ensure this is actually
             // unreachable.
             let source_info = self.source_info(scrutinee_span);
+
+            // Matching on a `scrutinee_place` with an uninhabited type doesn't
+            // generate any memory reads by itself, and so if the place "expression"
+            // contains unsafe operations like raw pointer dereferences or union
+            // field projections, we wouldn't know to require an `unsafe` block
+            // around a `match` equivalent to `std::intrinsics::unreachable()`.
+            // See issue #47412 for this hole being discovered in the wild.
+            //
+            // HACK(eddyb) Work around the above issue by adding a dummy inspection
+            // of `scrutinee_place`, specifically by applying `ReadForMatch`.
+            //
+            // NOTE: ReadForMatch also checks that the scrutinee is initialized.
+            // This is currently needed to not allow matching on an uninitialized,
+            // uninhabited value. If we get never patterns, those will check that
+            // the place is initialized, and so this read would only be used to
+            // check safety.
+            let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
+
+            if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) {
+                self.cfg.push_fake_read(
+                    otherwise_block,
+                    source_info,
+                    cause_matched_place,
+                    scrutinee_place,
+                );
+            }
+
             self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
         }
 
@@ -599,13 +611,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             _ => {
-                let place_builder = unpack!(block = self.as_place_builder(block, initializer));
-
-                if let Some(place) = place_builder.try_to_place(self) {
-                    let source_info = self.source_info(initializer.span);
-                    self.cfg.push_place_mention(block, source_info, place);
-                }
-
+                let place_builder =
+                    unpack!(block = self.lower_scrutinee(block, initializer, initializer.span));
                 self.place_into_pattern(block, &irrefutable_pat, place_builder, true)
             }
         }
@@ -622,6 +629,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let fake_borrow_temps = self.lower_match_tree(
             block,
             irrefutable_pat.span,
+            &initializer,
             irrefutable_pat.span,
             false,
             &mut [&mut candidate],
@@ -1841,6 +1849,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let fake_borrow_temps = self.lower_match_tree(
             block,
             pat.span,
+            &expr_place_builder,
             pat.span,
             false,
             &mut [&mut guard_candidate, &mut otherwise_candidate],
@@ -2342,6 +2351,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             let fake_borrow_temps = this.lower_match_tree(
                 block,
                 initializer_span,
+                &scrutinee,
                 pattern.span,
                 false,
                 &mut [&mut candidate, &mut wildcard],

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_match_never.rs

@@ -0,0 +1,17 @@
+// Make sure we find these even with many checks disabled.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+#![allow(unreachable_code)]
+#![feature(never_type)]
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32 as *const !
+    };
+    unsafe {
+        match *p {} //~ ERROR: entering unreachable code
+    }
+    panic!("this should never print");
+}
+

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_match_never.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: entering unreachable code
+  --> $DIR/dangling_pointer_deref_match_never.rs:LL:CC
+   |
+LL |         match *p {}
+   |               ^^ entering unreachable code
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/dangling_pointer_deref_match_never.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

src/tools/miri/tests/fail/never_match_never.rs

@@ -0,0 +1,10 @@
+// This should fail even without validation
+//@compile-flags: -Zmiri-disable-validation
+
+#![feature(never_type)]
+#![allow(unreachable_code)]
+
+fn main() {
+    let ptr: *const (i32, !) = &0i32 as *const i32 as *const _;
+    unsafe { match (*ptr).1 {} } //~ ERROR: entering unreachable code
+}

src/tools/miri/tests/fail/never_match_never.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: entering unreachable code
+  --> $DIR/never_match_never.rs:LL:CC
+   |
+LL |     unsafe { match (*ptr).1 {} }
+   |                    ^^^^^^^^ entering unreachable code
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/never_match_never.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

src/tools/miri/tests/pass/dangling_pointer_deref_match_underscore.rs

@@ -0,0 +1,14 @@
+// A `_` binding in a match is a nop, so we do not detect that the pointer is dangling.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32
+    };
+    unsafe {
+        match *p {
+            _ => {}
+        }
+    }
+}

src/tools/miri/tests/pass/union-uninhabited-match-underscore.rs

@@ -0,0 +1,17 @@
+fn main() {
+    #[derive(Copy, Clone)]
+    enum Void {}
+    union Uninit<T: Copy> {
+        value: T,
+        uninit: (),
+    }
+    unsafe {
+        let x: Uninit<Void> = Uninit { uninit: () };
+        match x.value {
+            // rustc warns about un unreachable pattern,
+            // but is wrong in unsafe code.
+            #[allow(unreachable_patterns)]
+            _ => println!("hi from the void!"),
+        }
+    }
+}

src/tools/miri/tests/pass/union-uninhabited-match-underscore.stdout

@@ -0,0 +1 @@
+hi from the void!

tests/mir-opt/building/async_await.b-{closure#0}.coroutine_resume.0.mir

@@ -127,6 +127,7 @@ fn b::{closure#0}(_1: Pin<&mut {async fn body@$DIR/async_await.rs:15:18: 18:2}>,
 
     bb3: {
         StorageDead(_5);
+        PlaceMention(_4);
         nop;
         (((*(_1.0: &mut {async fn body@$DIR/async_await.rs:15:18: 18:2})) as variant#3).0: {async fn body@$DIR/async_await.rs:12:14: 12:16}) = move _4;
         goto -> bb4;
@@ -162,6 +163,7 @@ fn b::{closure#0}(_1: Pin<&mut {async fn body@$DIR/async_await.rs:15:18: 18:2}>,
     bb7: {
         StorageDead(_13);
         StorageDead(_10);
+        PlaceMention(_9);
         _16 = discriminant(_9);
         switchInt(move _16) -> [0: bb10, 1: bb8, otherwise: bb9];
     }
@@ -223,6 +225,7 @@ fn b::{closure#0}(_1: Pin<&mut {async fn body@$DIR/async_await.rs:15:18: 18:2}>,
 
     bb15: {
         StorageDead(_22);
+        PlaceMention(_21);
         nop;
         (((*(_1.0: &mut {async fn body@$DIR/async_await.rs:15:18: 18:2})) as variant#4).0: {async fn body@$DIR/async_await.rs:12:14: 12:16}) = move _21;
         goto -> bb16;
@@ -258,6 +261,7 @@ fn b::{closure#0}(_1: Pin<&mut {async fn body@$DIR/async_await.rs:15:18: 18:2}>,
     bb19: {
         StorageDead(_29);
         StorageDead(_26);
+        PlaceMention(_25);
         _32 = discriminant(_25);
         switchInt(move _32) -> [0: bb21, 1: bb20, otherwise: bb9];
     }

tests/mir-opt/building/issue_101867.main.built.after.mir

@@ -25,7 +25,7 @@ fn main() -> () {
         FakeRead(ForLet(None), _1);
         AscribeUserType(_1, o, UserTypeProjection { base: UserType(1), projs: [] });
         StorageLive(_5);
-        FakeRead(ForMatchedPlace(None), _1);
+        PlaceMention(_1);
         _6 = discriminant(_1);
         switchInt(move _6) -> [1: bb4, otherwise: bb3];
     }