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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
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17 changes: 17 additions & 0 deletions
17
tests/fail/dangling_pointers/dangling_pointer_deref_match_never.rs
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// Make sure we find these even with many checks disabled. | ||
//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation | ||
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#![allow(unreachable_code)] | ||
#![feature(never_type)] | ||
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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"); | ||
} | ||
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tests/fail/dangling_pointers/dangling_pointer_deref_match_never.stderr
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error: Undefined Behavior: entering unreachable code | ||
--> $DIR/dangling_pointer_deref_match_never.rs:LL:CC | ||
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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 | ||
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note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace | ||
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error: aborting due to previous error | ||
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// This should fail even without validation | ||
//@compile-flags: -Zmiri-disable-validation | ||
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#![feature(never_type)] | ||
#![allow(unreachable_code)] | ||
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fn main() { | ||
let ptr: *const (i32, !) = &0i32 as *const i32 as *const _; | ||
unsafe { match (*ptr).1 {} } //~ ERROR: entering unreachable code | ||
} |
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error: Undefined Behavior: entering unreachable code | ||
--> $DIR/never_match_never.rs:LL:CC | ||
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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 | ||
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note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace | ||
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error: aborting due to previous error | ||
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// 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 | ||
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fn main() { | ||
let p = { | ||
let b = Box::new(42); | ||
&*b as *const i32 | ||
}; | ||
unsafe { | ||
match *p { | ||
_ => {} | ||
} | ||
} | ||
} |
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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!"), | ||
} | ||
} | ||
} |
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hi from the void! |