feat: add failing test for zero-sized memset

Signed-off-by: vishruth-thimmaiah <[email protected]>

Author: vishruth-thimmaiah

src/tools/miri/src/shims/foreign_items.rs

@@ -837,6 +837,7 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 #[expect(clippy::as_conversions)]
                 let val = val as u8;
 
+                // C requires that this must always be a valid pointer, even if `n` is zero, so we better check that.
                 this.ptr_get_alloc_id(ptr_dest, 0)?;
 
                 let bytes = std::iter::repeat_n(val, n.try_into().unwrap());

src/tools/miri/tests/fail-dep/libc/memset_null.rs

@@ -0,0 +1,8 @@
+use std::ptr;
+
+// null is explicitly called out as UB in the C docs for `memset`.
+fn main() {
+    unsafe {
+        libc::memset(ptr::null_mut(), 0, 0); //~ERROR: null pointer
+    }
+}

src/tools/miri/tests/fail-dep/libc/memset_null.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: pointer not dereferenceable: pointer must point to some allocation, but got null pointer
+  --> tests/fail-dep/libc/memset_null.rs:LL:CC
+   |
+LL |         libc::memset(ptr::null_mut(), 0, 0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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
+   = note: BACKTRACE:
+   = note: inside `main` at tests/fail-dep/libc/memset_null.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
+