make shim_arg_size ptr-width-independent

Author: RalfJung

tests/fail/shims/shim_arg_size.64bit.stderr

@@ -1,17 +1,10 @@
-//@stderr-per-bitwidth
-
 fn main() {
     extern "C" {
-        // Use the wrong type(ie. not the pointer width) for the `size`
-        // argument.
-        #[cfg(target_pointer_width = "64")]
-        fn malloc(size: u32) -> *mut std::ffi::c_void;
-
-        #[cfg(target_pointer_width = "32")]
-        fn malloc(size: u16) -> *mut std::ffi::c_void;
+        // Use the wrong type (ie. not `i32`) for the `c` argument.
+        fn memchr(s: *const std::ffi::c_void, c: u8, n: usize) -> *mut std::ffi::c_void;
     }
 
     unsafe {
-        let _p1 = malloc(42); //~ ERROR: Undefined Behavior: scalar size mismatch
+        memchr(std::ptr::null(), 0, 0); //~ ERROR: Undefined Behavior: scalar size mismatch
     };
 }

tests/fail/shims/shim_arg_size.rs

@@ -1,12 +1,12 @@
-error: Undefined Behavior: scalar size mismatch: expected 4 bytes but got 2 bytes instead
+error: Undefined Behavior: scalar size mismatch: expected 4 bytes but got 1 bytes instead
   --> $DIR/shim_arg_size.rs:LL:CC
    |
-LL |         let _p1 = malloc(42);
-   |                   ^^^^^^^^^^ scalar size mismatch: expected 4 bytes but got 2 bytes instead
+LL |         memchr(std::ptr::null(), 0, 0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ scalar size mismatch: expected 4 bytes but got 1 bytes instead
    |
    = 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
-   = note: backtrace:
+   = note: BACKTRACE:
    = note: inside `main` at $DIR/shim_arg_size.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace