Rollup merge of #84903 - hyd-dev:dead-check-in-alloc-msg, r=RalfJung

Remove `rustc_middle::mir::interpret::CheckInAllocMsg::NullPointerTest`

Removing it per #84842 (comment): it's a dead enum variant.

Note that `PointerArithmeticTest` also seems dead:
```
$ rg -F PointerArithmeticTest -C5
compiler/rustc_middle/src/mir/interpret/error.rs
169-
170-/// Details of why a pointer had to be in-bounds.
171-#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
172-pub enum CheckInAllocMsg {
173-    MemoryAccessTest,
174:    PointerArithmeticTest,
175-    InboundsTest,
176-}
177-
178-impl fmt::Display for CheckInAllocMsg {
179-    /// When this is printed as an error the context looks like this
--
182-        write!(
183-            f,
184-            "{}",
185-            match *self {
186-                CheckInAllocMsg::MemoryAccessTest => "memory access",
187:                CheckInAllocMsg::PointerArithmeticTest => "pointer arithmetic",
188-                CheckInAllocMsg::InboundsTest => "inbounds test",
189-            }
190-        )
191-    }
192-}
```
Not sure if that is also desirable to be removed, however.

Author: RalfJung

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -171,7 +171,6 @@ impl fmt::Display for InvalidProgramInfo<'_> {
 #[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
 pub enum CheckInAllocMsg {
     MemoryAccessTest,
-    NullPointerTest,
     PointerArithmeticTest,
     InboundsTest,
 }
@@ -185,7 +184,6 @@ impl fmt::Display for CheckInAllocMsg {
             "{}",
             match *self {
                 CheckInAllocMsg::MemoryAccessTest => "memory access",
-                CheckInAllocMsg::NullPointerTest => "null pointer test",
                 CheckInAllocMsg::PointerArithmeticTest => "pointer arithmetic",
                 CheckInAllocMsg::InboundsTest => "inbounds test",
             }
@@ -308,9 +306,6 @@ impl fmt::Display for UndefinedBehaviorInfo<'_> {
                 ptr.alloc_id,
                 allocation_size.bytes()
             ),
-            DanglingIntPointer(_, CheckInAllocMsg::NullPointerTest) => {
-                write!(f, "null pointer is not allowed for this operation")
-            }
             DanglingIntPointer(i, msg) => {
                 write!(f, "{} failed: 0x{:x} is not a valid pointer", msg, i)
             }