miri: fix out-of-bounds error for ptrs with negative offsets

Author: RalfJung

compiler/rustc_const_eval/messages.ftl

@@ -294,19 +294,23 @@ const_eval_pointer_arithmetic_overflow =
     overflowing pointer arithmetic: the total offset in bytes does not fit in an `isize`
 
 const_eval_pointer_out_of_bounds =
-    {const_eval_bad_pointer_op_attempting}, but got {$pointer} which {$inbounds_size_is_neg ->
-        [false] {$alloc_size_minus_ptr_offset ->
-                [0] is at or beyond the end of the allocation of size {$alloc_size ->
-                    [1] 1 byte
-                    *[x] {$alloc_size} bytes
+    {const_eval_bad_pointer_op_attempting}, but got {$pointer} which {$ptr_offset_is_neg ->
+        [true] points to before the beginning of the allocation
+        *[false] {$inbounds_size_is_neg ->
+            [false] {$alloc_size_minus_ptr_offset ->
+                        [0] is at or beyond the end of the allocation of size {$alloc_size ->
+                            [1] 1 byte
+                            *[x] {$alloc_size} bytes
+                        }
+                        [1] is only 1 byte from the end of the allocation
+                        *[x] is only {$alloc_size_minus_ptr_offset} bytes from the end of the allocation
+                    }
+                
+            *[true] {$ptr_offset_abs ->
+                    [0] is at the beginning of the allocation
+                    *[other] is only {$ptr_offset_abs} bytes from the beginning of the allocation
                 }
-                [1] is only 1 byte from the end of the allocation
-                *[x] is only {$alloc_size_minus_ptr_offset} bytes from the end of the allocation
-            }
-        *[true] {$ptr_offset_abs ->
-                [0] is at the beginning of the allocation
-                *[other] is only {$ptr_offset_abs} bytes from the beginning of the allocation
-            }
+        }
     }
 
 const_eval_pointer_use_after_free =

src/tools/miri/tests/fail/dangling_pointers/out_of_bounds_read_neg_offset.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let v: Vec<u16> = vec![1, 2];
+    // This read is also misaligned. We make sure that the OOB message has priority.
+    let x = unsafe { *v.as_ptr().wrapping_byte_sub(5) }; //~ ERROR: before the beginning of the allocation
+    panic!("this should never print: {}", x);
+}

src/tools/miri/tests/fail/dangling_pointers/out_of_bounds_read_neg_offset.stderr

@@ -0,0 +1,21 @@
+error: Undefined Behavior: memory access failed: attempting to access 2 bytes, but got ALLOC-0x5 which points to before the beginning of the allocation
+  --> tests/fail/dangling_pointers/out_of_bounds_read_neg_offset.rs:LL:CC
+   |
+LL |     let x = unsafe { *v.as_ptr().wrapping_byte_sub(5) };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+help: ALLOC was allocated here:
+  --> tests/fail/dangling_pointers/out_of_bounds_read_neg_offset.rs:LL:CC
+   |
+LL |     let v: Vec<u16> = vec![1, 2];
+   |                       ^^^^^^^^^^
+   = note: BACKTRACE (of the first span):
+   = note: inside `main` at tests/fail/dangling_pointers/out_of_bounds_read_neg_offset.rs:LL:CC
+   = note: this error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/fail/intrinsics/ptr_offset_out_of_bounds_neg2.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let v = [0i8; 4];
+    let x = &v as *const i8;
+    let x = unsafe { x.wrapping_offset(-1).offset(-1) }; //~ERROR: before the beginning of the allocation
+    panic!("this should never print: {:?}", x);
+}

src/tools/miri/tests/fail/intrinsics/ptr_offset_out_of_bounds_neg2.stderr

@@ -0,0 +1,20 @@
+error: Undefined Behavior: in-bounds pointer arithmetic failed: attempting to offset pointer by -1 bytes, but got ALLOC-0x1 which points to before the beginning of the allocation
+  --> tests/fail/intrinsics/ptr_offset_out_of_bounds_neg2.rs:LL:CC
+   |
+LL |     let x = unsafe { x.wrapping_offset(-1).offset(-1) };
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+help: ALLOC was allocated here:
+  --> tests/fail/intrinsics/ptr_offset_out_of_bounds_neg2.rs:LL:CC
+   |
+LL |     let v = [0i8; 4];
+   |         ^
+   = note: BACKTRACE (of the first span):
+   = note: inside `main` at tests/fail/intrinsics/ptr_offset_out_of_bounds_neg2.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+