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) };
```
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
https://github.com/rust-lang/rust/pull/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@.

4 files changed, 13 insertions, 1 deletions

diff --git a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
index 42b3e59b73a..85e71ed2c25 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
@@ -506,6 +506,9 @@ impl<'tcx> Stable<'tcx> for mir::AssertMessage<'tcx> {
             AssertKind::NullPointerDereference => {
                 stable_mir::mir::AssertMessage::NullPointerDereference
             }
+            AssertKind::InvalidEnumConstruction(source) => {
+                stable_mir::mir::AssertMessage::InvalidEnumConstruction(source.stable(tables))
+            }
         }
     }
 }
diff --git a/compiler/rustc_smir/src/stable_mir/mir/body.rs b/compiler/rustc_smir/src/stable_mir/mir/body.rs
index 660cd7db080..e4b7659ce7f 100644
--- a/compiler/rustc_smir/src/stable_mir/mir/body.rs
+++ b/compiler/rustc_smir/src/stable_mir/mir/body.rs
@@ -270,6 +270,7 @@ pub enum AssertMessage {
     ResumedAfterDrop(CoroutineKind),
     MisalignedPointerDereference { required: Operand, found: Operand },
     NullPointerDereference,
+    InvalidEnumConstruction(Operand),
 }
 
 impl AssertMessage {
@@ -342,6 +343,9 @@ impl AssertMessage {
                 Ok("misaligned pointer dereference")
             }
             AssertMessage::NullPointerDereference => Ok("null pointer dereference occurred"),
+            AssertMessage::InvalidEnumConstruction(_) => {
+                Ok("trying to construct an enum from an invalid value")
+            }
         }
     }
 }
diff --git a/compiler/rustc_smir/src/stable_mir/mir/pretty.rs b/compiler/rustc_smir/src/stable_mir/mir/pretty.rs
index ba20651f993..b068a9a1081 100644
--- a/compiler/rustc_smir/src/stable_mir/mir/pretty.rs
+++ b/compiler/rustc_smir/src/stable_mir/mir/pretty.rs
@@ -313,6 +313,10 @@ fn pretty_assert_message<W: Write>(writer: &mut W, msg: &AssertMessage) -> io::R
         AssertMessage::NullPointerDereference => {
             write!(writer, "\"null pointer dereference occurred\"")
         }
+        AssertMessage::InvalidEnumConstruction(op) => {
+            let pretty_op = pretty_operand(op);
+            write!(writer, "\"trying to construct an enum from an invalid value {{}}\",{pretty_op}")
+        }
         AssertMessage::ResumedAfterReturn(_)
         | AssertMessage::ResumedAfterPanic(_)
         | AssertMessage::ResumedAfterDrop(_) => {
diff --git a/compiler/rustc_smir/src/stable_mir/mir/visit.rs b/compiler/rustc_smir/src/stable_mir/mir/visit.rs
index e21dc11eea9..b7dd433eb09 100644
--- a/compiler/rustc_smir/src/stable_mir/mir/visit.rs
+++ b/compiler/rustc_smir/src/stable_mir/mir/visit.rs
@@ -367,7 +367,8 @@ macro_rules! make_mir_visitor {
                     }
                     AssertMessage::OverflowNeg(op)
                     | AssertMessage::DivisionByZero(op)
-                    | AssertMessage::RemainderByZero(op) => {
+                    | AssertMessage::RemainderByZero(op)
+                    | AssertMessage::InvalidEnumConstruction(op) => {
                         self.visit_operand(op, location);
                     }
                     AssertMessage::ResumedAfterReturn(_)