Auto merge of rust-lang#141759 - 1c3t3a:discriminants-query, r=saethlin

Insert checks for enum discriminants when debug assertions are enabled

Similar to the existing null-pointer 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
rust-lang#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@.

r? `@saethlin`

Author: bors

core/src/panicking.rs

@@ -314,6 +314,22 @@ fn panic_null_pointer_dereference() -> ! {
     )
 }
 
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold, optimize(size))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
+#[track_caller]
+#[lang = "panic_invalid_enum_construction"] // needed by codegen for panic on invalid enum construction.
+#[rustc_nounwind] // `CheckEnums` MIR pass requires this function to never unwind
+fn panic_invalid_enum_construction(source: u128) -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        super::intrinsics::abort()
+    }
+
+    panic_nounwind_fmt(
+        format_args!("trying to construct an enum from an invalid value {source:#x}"),
+        /* force_no_backtrace */ false,
+    )
+}
+
 /// Panics because we cannot unwind out of a function.
 ///
 /// This is a separate function to avoid the codesize impact of each crate containing the string to