Insert checks for enum discriminants when debug assertions are enabled

Similar to the existing nullpointer and alignment checks, this checks
for valid enum discriminants on creation of enums through unsafe
transmutes. Essentially this sanitizes patterns like the following:
```rust
let val: MyEnum = unsafe { std::mem::transmute<u32, MyEnum>(42) };
```
This is the only way to create an invalid enum in pure Rust.

An extension of this check will be done in a follow-up that explicitly
sanitizes for extern enum values that come into Rust from e.g. C/C++.

This check is similar to Miri's capabilities of checking for valid
construction of enum values.

This PR is inspired by @saethlin's PR
#104862. Thank you so much for
keeping this code up and the detailed comments!

I also pair-programmed large parts of this together with @vabr-g.

Author: 1c3t3a

compiler/rustc_codegen_cranelift/src/base.rs

@@ -392,6 +392,16 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                             Some(source_info.span),
                         )
                     }
+                    AssertKind::InvalidEnumConstruction(source) => {
+                        let location = fx.get_caller_location(source_info).load_scalar(fx);
+
+                        codegen_panic_inner(
+                            fx,
+                            rustc_hir::LangItem::PanicNullPointerDereference,
+                            &[source, location],
+                            Some(source_info.span),
+                        )
+                    }
                     _ => {
                         let location = fx.get_caller_location(source_info).load_scalar(fx);
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -777,6 +777,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 // `#[track_caller]` adds an implicit argument.
                 (LangItem::PanicNullPointerDereference, vec![location])
             }
+            AssertKind::InvalidEnumConstruction(source) => {
+                let source = self.codegen_operand(bx, source).immediate();
+                // It's `fn panic_invalid_enum_construction()`,
+                // `#[track_caller]` adds an implicit argument.
+                (LangItem::PanicInvalidEnumConstruction, vec![source, location])
+            }
             _ => {
                 // It's `pub fn panic_...()` and `#[track_caller]` adds an implicit argument.
                 (msg.panic_function(), vec![location])

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -508,6 +508,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                 found: eval_to_int(found)?,
             },
             NullPointerDereference => NullPointerDereference,
+            InvalidEnumConstruction(source) => InvalidEnumConstruction(eval_to_int(source)?),
         };
         Err(ConstEvalErrKind::AssertFailure(err)).into()
     }

compiler/rustc_hir/src/lang_items.rs

@@ -310,6 +310,7 @@ language_item_table! {
     PanicAsyncGenFnResumedPanic, sym::panic_const_async_gen_fn_resumed_panic, panic_const_async_gen_fn_resumed_panic, Target::Fn, GenericRequirement::None;
     PanicGenFnNonePanic, sym::panic_const_gen_fn_none_panic, panic_const_gen_fn_none_panic, Target::Fn, GenericRequirement::None;
     PanicNullPointerDereference, sym::panic_null_pointer_dereference, panic_null_pointer_dereference, Target::Fn, GenericRequirement::None;
+    PanicInvalidEnumConstruction, sym::panic_invalid_enum_construction, panic_invalid_enum_construction, Target::Fn, GenericRequirement::None;
     PanicCoroutineResumedDrop, sym::panic_const_coroutine_resumed_drop, panic_const_coroutine_resumed_drop, Target::Fn, GenericRequirement::None;
     PanicAsyncFnResumedDrop, sym::panic_const_async_fn_resumed_drop, panic_const_async_fn_resumed_drop, Target::Fn, GenericRequirement::None;
     PanicAsyncGenFnResumedDrop, sym::panic_const_async_gen_fn_resumed_drop, panic_const_async_gen_fn_resumed_drop, Target::Fn, GenericRequirement::None;

compiler/rustc_middle/messages.ftl

@@ -17,6 +17,9 @@ middle_assert_gen_resume_after_drop = `gen` fn or block cannot be further iterat
 
 middle_assert_gen_resume_after_panic = `gen` fn or block cannot be further iterated on after it panicked
 
+middle_assert_invalid_enum_construction =
+    trying to construct an enum from an invalid value `{$source}`
+
 middle_assert_misaligned_ptr_deref =
     misaligned pointer dereference: address must be a multiple of {$required} but is {$found}
 

compiler/rustc_middle/src/mir/syntax.rs

@@ -1075,6 +1075,7 @@ pub enum AssertKind<O> {
     ResumedAfterDrop(CoroutineKind),
     MisalignedPointerDereference { required: O, found: O },
     NullPointerDereference,
+    InvalidEnumConstruction(O),
 }
 
 #[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]

compiler/rustc_middle/src/mir/terminator.rs

@@ -208,6 +208,7 @@ impl<O> AssertKind<O> {
                 LangItem::PanicGenFnNonePanic
             }
             NullPointerDereference => LangItem::PanicNullPointerDereference,
+            InvalidEnumConstruction(_) => LangItem::PanicInvalidEnumConstruction,
             ResumedAfterDrop(CoroutineKind::Coroutine(_)) => LangItem::PanicCoroutineResumedDrop,
             ResumedAfterDrop(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) => {
                 LangItem::PanicAsyncFnResumedDrop
@@ -284,6 +285,9 @@ impl<O> AssertKind<O> {
                 )
             }
             NullPointerDereference => write!(f, "\"null pointer dereference occurred\""),
+            InvalidEnumConstruction(source) => {
+                write!(f, "\"trying to construct an enum from an invalid value {{}}\", {source:?}")
+            }
             ResumedAfterReturn(CoroutineKind::Coroutine(_)) => {
                 write!(f, "\"coroutine resumed after completion\"")
             }
@@ -367,6 +371,7 @@ impl<O> AssertKind<O> {
                 middle_assert_coroutine_resume_after_panic
             }
             NullPointerDereference => middle_assert_null_ptr_deref,
+            InvalidEnumConstruction(_) => middle_assert_invalid_enum_construction,
             ResumedAfterDrop(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) => {
                 middle_assert_async_resume_after_drop
             }
@@ -420,6 +425,9 @@ impl<O> AssertKind<O> {
                 add!("required", format!("{required:#?}"));
                 add!("found", format!("{found:#?}"));
             }
+            InvalidEnumConstruction(source) => {
+                add!("source", format!("{source:#?}"));
+            }
         }
     }
 }

compiler/rustc_middle/src/mir/visit.rs

@@ -642,7 +642,7 @@ macro_rules! make_mir_visitor {
                         self.visit_operand(l, location);
                         self.visit_operand(r, location);
                     }
-                    OverflowNeg(op) | DivisionByZero(op) | RemainderByZero(op) => {
+                    OverflowNeg(op) | DivisionByZero(op) | RemainderByZero(op) | InvalidEnumConstruction(op) => {
                         self.visit_operand(op, location);
                     }
                     ResumedAfterReturn(_) | ResumedAfterPanic(_) | NullPointerDereference | ResumedAfterDrop(_) => {