KCFI: Use legal charset in shim encoding

To separate `ReifyReason::FnPtr` from `ReifyReason::VTable`, we
hyphenated the shims. Hyphens are not actually legal, but underscores
are, so use those instead.

compiler/rustc_symbol_mangling/src/v0.rs

@@ -45,8 +45,8 @@ pub(super) fn mangle<'tcx>(
         ty::InstanceDef::ThreadLocalShim(_) => Some("tls"),
         ty::InstanceDef::VTableShim(_) => Some("vtable"),
         ty::InstanceDef::ReifyShim(_, None) => Some("reify"),
-        ty::InstanceDef::ReifyShim(_, Some(ReifyReason::FnPtr)) => Some("reify-fnptr"),
-        ty::InstanceDef::ReifyShim(_, Some(ReifyReason::Vtable)) => Some("reify-vtable"),
+        ty::InstanceDef::ReifyShim(_, Some(ReifyReason::FnPtr)) => Some("reify_fnptr"),
+        ty::InstanceDef::ReifyShim(_, Some(ReifyReason::Vtable)) => Some("reify_vtable"),
 
         ty::InstanceDef::ConstructCoroutineInClosureShim { .. }
         | ty::InstanceDef::CoroutineKindShim { .. } => Some("fn_once"),

tests/ui/sanitizer/kcfi-mangling.rs

@@ -0,0 +1,30 @@
+// Check KCFI extra mangling works correctly on v0
+
+//@ needs-sanitizer-kcfi
+//@ no-prefer-dynamic
+//@ compile-flags: -C panic=abort -Zsanitizer=kcfi -C symbol-mangling-version=v0
+//@ build-pass
+
+trait Foo {
+    fn foo(&self);
+}
+
+struct Bar;
+impl Foo for Bar {
+    fn foo(&self) {}
+}
+
+struct Baz;
+impl Foo for Baz {
+    #[track_caller]
+    fn foo(&self) {}
+}
+
+fn main() {
+    // Produces `ReifyShim(_, ReifyReason::FnPtr)`
+    let f: fn(&Bar) = Bar::foo;
+    f(&Bar);
+    // Produces `ReifyShim(_, ReifyReason::Vtable)`
+    let v: &dyn Foo = &Baz as _;
+    v.foo();
+}