Rollup merge of #143273 - 1c3t3a:enum-check-negative, r=SparrowLii

Make the enum check work for negative discriminants

The discriminant check was not working correctly for negative numbers. This change fixes that by masking out the relevant bits correctly.

Fixes rust-lang/rust#143218.

tests/ui/mir/enum/negative_discr_break.rs

@@ -0,0 +1,14 @@
+//@ run-fail
+//@ compile-flags: -C debug-assertions
+//@ error-pattern: trying to construct an enum from an invalid value 0xfd
+
+#[allow(dead_code)]
+enum Foo {
+    A = -2,
+    B = -1,
+    C = 1,
+}
+
+fn main() {
+    let _val: Foo = unsafe { std::mem::transmute::<i8, Foo>(-3) };
+}

tests/ui/mir/enum/negative_discr_ok.rs

@@ -0,0 +1,53 @@
+//@ run-pass
+//@ compile-flags: -C debug-assertions
+
+#[allow(dead_code)]
+#[derive(Debug, PartialEq)]
+enum Foo {
+    A = -12121,
+    B = -2,
+    C = -1,
+    D = 1,
+    E = 2,
+    F = 12121,
+}
+
+#[allow(dead_code)]
+#[repr(i64)]
+#[derive(Debug, PartialEq)]
+enum Bar {
+    A = i64::MIN,
+    B = -2,
+    C = -1,
+    D = 1,
+    E = 2,
+    F = i64::MAX,
+}
+
+fn main() {
+    let val: Foo = unsafe { std::mem::transmute::<i16, Foo>(-12121) };
+    assert_eq!(val, Foo::A);
+    let val: Foo = unsafe { std::mem::transmute::<i16, Foo>(-2) };
+    assert_eq!(val, Foo::B);
+    let val: Foo = unsafe { std::mem::transmute::<i16, Foo>(-1) };
+    assert_eq!(val, Foo::C);
+    let val: Foo = unsafe { std::mem::transmute::<i16, Foo>(1) };
+    assert_eq!(val, Foo::D);
+    let val: Foo = unsafe { std::mem::transmute::<i16, Foo>(2) };
+    assert_eq!(val, Foo::E);
+    let val: Foo = unsafe { std::mem::transmute::<i16, Foo>(12121) };
+    assert_eq!(val, Foo::F);
+
+    let val: Bar = unsafe { std::mem::transmute::<i64, Bar>(i64::MIN) };
+    assert_eq!(val, Bar::A);
+    let val: Bar = unsafe { std::mem::transmute::<i64, Bar>(-2) };
+    assert_eq!(val, Bar::B);
+    let val: Bar = unsafe { std::mem::transmute::<i64, Bar>(-1) };
+    assert_eq!(val, Bar::C);
+    let val: Bar = unsafe { std::mem::transmute::<i64, Bar>(1) };
+    assert_eq!(val, Bar::D);
+    let val: Bar = unsafe { std::mem::transmute::<i64, Bar>(2) };
+    assert_eq!(val, Bar::E);
+    let val: Bar = unsafe { std::mem::transmute::<i64, Bar>(i64::MAX) };
+    assert_eq!(val, Bar::F);
+}