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

[ghost]

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.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[RalfJung]

r? @RalfJung

[RalfJung]

Note that PointerArithmeticTest also seems dead:

Hm, this is strange... it should be used by this code:

rust/compiler/rustc_mir/src/interpret/intrinsics.rs

Line 529 in 4e8edfb

CheckInAllocMsg::InboundsTest,

but it looks like at some point we switched to the wrong variant...

[RalfJung]

Ah wait... we still have "inbounds" vs "memory access". So we still have that distinction.
Now I wonder what "pointer arithmetic" was for.

Arguably, "InboundsTest" is a bad name -- "Details of why a pointer had to be in-bounds" should not be answered with "because it has to be inbounds". I will need to check what InboundsTest is used for though before we make further changes here.

[RalfJung]

Meanwhile, this seems like a reasonable step.
@bors r+ rollup

[bors]

📌 Commit ee7a6c6 has been approved by RalfJung