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

compiler/rustc_const_eval/messages.ftl

@@ -294,19 +294,22 @@ 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
+

tests/ui/consts/offset_ub.rs

@@ -5,17 +5,17 @@ use std::ptr;
 //@ normalize-stderr: "\d+ bytes" -> "$$BYTES bytes"
 
 
-pub const BEFORE_START: *const u8 = unsafe { (&0u8 as *const u8).offset(-1) }; //~ ERROR
-pub const AFTER_END: *const u8 = unsafe { (&0u8 as *const u8).offset(2) }; //~ ERROR
-pub const AFTER_ARRAY: *const u8 = unsafe { [0u8; 100].as_ptr().offset(101) }; //~ ERROR
+pub const BEFORE_START: *const u8 = unsafe { (&0u8 as *const u8).offset(-1) }; //~ ERROR: is at the beginning of the allocation
+pub const AFTER_END: *const u8 = unsafe { (&0u8 as *const u8).offset(2) }; //~ ERROR: only 1 byte from the end of the allocation
+pub const AFTER_ARRAY: *const u8 = unsafe { [0u8; 100].as_ptr().offset(101) }; //~ ERROR: only 100 bytes from the end of the allocation
 
-pub const OVERFLOW: *const u16 = unsafe { [0u16; 1].as_ptr().offset(isize::MAX) }; //~ ERROR
-pub const UNDERFLOW: *const u16 = unsafe { [0u16; 1].as_ptr().offset(isize::MIN) }; //~ ERROR
+pub const OVERFLOW: *const u16 = unsafe { [0u16; 1].as_ptr().offset(isize::MAX) }; //~ ERROR: does not fit in an `isize`
+pub const UNDERFLOW: *const u16 = unsafe { [0u16; 1].as_ptr().offset(isize::MIN) }; //~ ERROR: does not fit in an `isize`
 pub const OVERFLOW_ADDRESS_SPACE: *const u8 = unsafe { (usize::MAX as *const u8).offset(2) }; //~ ERROR
 pub const UNDERFLOW_ADDRESS_SPACE: *const u8 = unsafe { (1 as *const u8).offset(-2) }; //~ ERROR
-pub const NEGATIVE_OFFSET: *const u8 = unsafe { [0u8; 1].as_ptr().wrapping_offset(-2).offset(-2) }; //~ ERROR
+pub const NEGATIVE_OFFSET: *const u8 = unsafe { [0u8; 1].as_ptr().wrapping_offset(-2).offset(-2) }; //~ ERROR: before the beginning of the allocation
 
-pub const ZERO_SIZED_ALLOC: *const u8 = unsafe { [0u8; 0].as_ptr().offset(1) }; //~ ERROR
+pub const ZERO_SIZED_ALLOC: *const u8 = unsafe { [0u8; 0].as_ptr().offset(1) }; //~ ERROR: at or beyond the end of the allocation
 pub const DANGLING: *const u8 = unsafe { ptr::NonNull::<u8>::dangling().as_ptr().offset(4) }; //~ ERROR
 
 // Make sure that we don't panic when computing abs(offset*size_of::<T>())

tests/ui/consts/offset_ub.stderr

@@ -40,11 +40,11 @@ error[E0080]: in-bounds pointer arithmetic failed: attempting to offset pointer
 LL | pub const UNDERFLOW_ADDRESS_SPACE: *const u8 = unsafe { (1 as *const u8).offset(-2) };
    |                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `UNDERFLOW_ADDRESS_SPACE` failed here
 
-error[E0080]: in-bounds pointer arithmetic failed: attempting to offset pointer by -$BYTES bytes, but got ALLOC3-0x2 which is only $BYTES bytes from the beginning of the allocation
+error[E0080]: in-bounds pointer arithmetic failed: attempting to offset pointer by -$BYTES bytes, but got ALLOC3-0x2 which points to before the beginning of the allocation
   --> $DIR/offset_ub.rs:16:49
    |
-LL | pub const NEGATIVE_OFFSET: *const u8 = unsafe { [0u8; 1].as_ptr().wrapping_offset(-2).offset(-2) };
-   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `NEGATIVE_OFFSET` failed here
+LL | ...*const u8 = unsafe { [0u8; 1].as_ptr().wrapping_offset(-2).offset(-2) };
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `NEGATIVE_OFFSET` failed here
 
 error[E0080]: in-bounds pointer arithmetic failed: attempting to offset pointer by 1 byte, but got ALLOC4 which is at or beyond the end of the allocation of size $BYTES bytes
   --> $DIR/offset_ub.rs:18:50