don't ICE when callee has the wrong number of arguments

src/helpers.rs

@@ -181,11 +181,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let mut callee_args = this.frame().body.args_iter();
         for arg in args {
             let callee_arg = this.local_place(
-                callee_args.next().expect("callee has fewer arguments than expected"),
+                callee_args.next().ok_or_else(||
+                    err_ub_format!("callee has fewer arguments than expected")
+                )?
             )?;
             this.write_immediate(*arg, &callee_arg)?;
         }
-        assert_eq!(callee_args.next(), None, "callee has more arguments than expected");
+        if callee_args.next().is_some() {
+            throw_ub_format!("callee has more arguments than expected");
+        }
 
         Ok(())
     }

tests/compile-fail/concurrency/too_few_args.rs

@@ -0,0 +1,26 @@
+// ignore-windows: Concurrency on Windows is not supported yet.
+// error-pattern: callee has fewer arguments than expected
+
+//! The thread function must have exactly one argument.
+
+#![feature(rustc_private)]
+
+extern crate libc;
+
+use std::{mem, ptr};
+
+extern "C" fn thread_start() -> *mut libc::c_void {
+    panic!()
+}
+
+fn main() {
+    unsafe {
+        let mut native: libc::pthread_t = mem::zeroed();
+        let attr: libc::pthread_attr_t = mem::zeroed();
+        // assert_eq!(libc::pthread_attr_init(&mut attr), 0); FIXME: this function is not yet implemented.
+        let thread_start: extern "C" fn() -> *mut libc::c_void = thread_start;
+        let thread_start: extern "C" fn(*mut libc::c_void) -> *mut libc::c_void = mem::transmute(thread_start);
+        assert_eq!(libc::pthread_create(&mut native, &attr, thread_start, ptr::null_mut()), 0);
+        assert_eq!(libc::pthread_join(native, ptr::null_mut()), 0);
+    }
+}

tests/compile-fail/concurrency/too_many_args.rs

@@ -0,0 +1,26 @@
+// ignore-windows: Concurrency on Windows is not supported yet.
+// error-pattern: callee has more arguments than expected
+
+//! The thread function must have exactly one argument.
+
+#![feature(rustc_private)]
+
+extern crate libc;
+
+use std::{mem, ptr};
+
+extern "C" fn thread_start(_null: *mut libc::c_void, _x: i32) -> *mut libc::c_void {
+    panic!()
+}
+
+fn main() {
+    unsafe {
+        let mut native: libc::pthread_t = mem::zeroed();
+        let attr: libc::pthread_attr_t = mem::zeroed();
+        // assert_eq!(libc::pthread_attr_init(&mut attr), 0); FIXME: this function is not yet implemented.
+        let thread_start: extern "C" fn(*mut libc::c_void, i32) -> *mut libc::c_void = thread_start;
+        let thread_start: extern "C" fn(*mut libc::c_void) -> *mut libc::c_void = mem::transmute(thread_start);
+        assert_eq!(libc::pthread_create(&mut native, &attr, thread_start, ptr::null_mut()), 0);
+        assert_eq!(libc::pthread_join(native, ptr::null_mut()), 0);
+    }
+}