Rollup merge of #95221 - RalfJung:check_and_deref_ptr, r=oli-obk

interpret/memory: simplify check_and_deref_ptr

*Finally* I saw a way to make this code simpler. The odd preprocessing in `let ptr_or_addr =` has bothered me since forever, but it actually became unnecessary in the last provenance refactoring. :)

This also leads to slightly more explicit error messages as a nice side-effect. 🎉

r? `@oli-obk`

Author: matthiaskrgr

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -427,22 +427,12 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             }
         }
 
-        // Extract from the pointer an `Option<AllocId>` and an offset, which is relative to the
-        // allocation or (if that is `None`) an absolute address.
-        let ptr_or_addr = if size.bytes() == 0 {
-            // Let's see what we can do, but don't throw errors if there's nothing there.
-            self.ptr_try_get_alloc(ptr)
-        } else {
-            // A "real" access, we insist on getting an `AllocId`.
-            Ok(self.ptr_get_alloc(ptr)?)
-        };
-        Ok(match ptr_or_addr {
+        Ok(match self.ptr_try_get_alloc(ptr) {
             Err(addr) => {
-                // No memory is actually being accessed.
-                debug_assert!(size.bytes() == 0);
-                // Must be non-null.
-                if addr == 0 {
-                    throw_ub!(DanglingIntPointer(0, msg))
+                // We couldn't get a proper allocation. This is only okay if the access size is 0,
+                // and the address is not null.
+                if size.bytes() > 0 || addr == 0 {
+                    throw_ub!(DanglingIntPointer(addr, msg));
                 }
                 // Must be aligned.
                 if let Some(align) = align {

src/test/ui/consts/const-deref-ptr.stderr

@@ -2,7 +2,7 @@ error[E0080]: could not evaluate static initializer
   --> $DIR/const-deref-ptr.rs:4:29
    |
 LL |     static C: u64 = unsafe {*(0xdeadbeef as *const u64)};
-   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ 0xdeadbeef is not a valid pointer
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: 0xdeadbeef is not a valid pointer
 
 error: aborting due to previous error
 

src/test/ui/consts/const-eval/const_raw_ptr_ops2.stderr

@@ -2,13 +2,13 @@ error[E0080]: evaluation of constant value failed
   --> $DIR/const_raw_ptr_ops2.rs:7:26
    |
 LL | const Z2: i32 = unsafe { *(42 as *const i32) };
-   |                          ^^^^^^^^^^^^^^^^^^^ 0x2a is not a valid pointer
+   |                          ^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: 0x2a is not a valid pointer
 
 error[E0080]: evaluation of constant value failed
   --> $DIR/const_raw_ptr_ops2.rs:9:26
    |
 LL | const Z3: i32 = unsafe { *(44 as *const i32) };
-   |                          ^^^^^^^^^^^^^^^^^^^ 0x2c is not a valid pointer
+   |                          ^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: 0x2c is not a valid pointer
 
 error: aborting due to 2 previous errors
 

src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr

@@ -296,7 +296,7 @@ error[E0080]: could not evaluate static initializer
   --> $DIR/ub-wide-ptr.rs:135:5
    |
 LL |     mem::transmute::<_, &dyn Trait>((&92u8, 0usize))
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ null pointer is not a valid pointer for this operation
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ memory access failed: null pointer is not a valid pointer
 
 error[E0080]: could not evaluate static initializer
   --> $DIR/ub-wide-ptr.rs:139:5

src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr

@@ -296,7 +296,7 @@ error[E0080]: could not evaluate static initializer
   --> $DIR/ub-wide-ptr.rs:135:5
    |
 LL |     mem::transmute::<_, &dyn Trait>((&92u8, 0usize))
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ null pointer is not a valid pointer for this operation
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ memory access failed: null pointer is not a valid pointer
 
 error[E0080]: could not evaluate static initializer
   --> $DIR/ub-wide-ptr.rs:139:5

src/test/ui/consts/const-mut-refs/mut_ref_in_final_dynamic_check.stderr

@@ -4,7 +4,7 @@ error[E0080]: evaluation of constant value failed
 LL |     Some(&mut *(42 as *mut i32))
    |          ^^^^^^^^^^^^^^^^^^^^^^
    |          |
-   |          0x2a is not a valid pointer
+   |          dereferencing pointer failed: 0x2a is not a valid pointer
    |          inside `helper` at $DIR/mut_ref_in_final_dynamic_check.rs:13:10
 ...
 LL | const A: Option<&mut i32> = helper();

src/test/ui/consts/offset_ub.stderr

@@ -130,7 +130,7 @@ error[E0080]: evaluation of constant value failed
 LL |         unsafe { intrinsics::offset(self, count) as *mut T }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                  |
-   |                  0x1 is not a valid pointer
+   |                  pointer arithmetic failed: 0x1 is not a valid pointer
    |                  inside `ptr::mut_ptr::<impl *mut u8>::offset` at $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
    |
   ::: $DIR/offset_ub.rs:19:42
@@ -158,7 +158,7 @@ error[E0080]: evaluation of constant value failed
 LL |         unsafe { intrinsics::offset(self, count) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                  |
-   |                  0x7f..f is not a valid pointer
+   |                  pointer arithmetic failed: 0x7f..f is not a valid pointer
    |                  inside `ptr::const_ptr::<impl *const u8>::offset` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
    |
   ::: $DIR/offset_ub.rs:25:47

src/test/ui/error-codes/E0396-fixed.stderr

@@ -2,7 +2,7 @@ error[E0080]: evaluation of constant value failed
   --> $DIR/E0396-fixed.rs:5:28
    |
 LL | const VALUE: u8 = unsafe { *REG_ADDR };
-   |                            ^^^^^^^^^ 0x5f3759df is not a valid pointer
+   |                            ^^^^^^^^^ dereferencing pointer failed: 0x5f3759df is not a valid pointer
 
 error: aborting due to previous error