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 ~rust-lang/rust#102256 ~rust-lang/rust#104844

Fixes rust-lang/rust#99180 rust-lang/rust#53114

Author: bors

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");
+}
+

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
+

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
+}

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
+

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 {
+            _ => {}
+        }
+    }
+}

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!"),
+        }
+    }
+}

tests/pass/union-uninhabited-match-underscore.stdout

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