tests: Verify unsupported varargs ABIs in fnptrs do not ICE

Author: workingjubilee

tests/ui/abi/unsupported-varargs-fnptr.rs

@@ -0,0 +1,18 @@
+// FIXME(workingjubilee): add revisions and generalize to other platform-specific varargs ABIs,
+// preferably after the only-arch directive is enhanced with an "or pattern" syntax
+//@ only-x86_64
+//@ build-pass
+
+#![feature(extended_varargs_abi_support)]
+
+// sometimes fn ptrs with varargs make layout and ABI computation ICE
+// as found in https://github.com/rust-lang/rust/issues/142107
+
+fn aapcs(f: extern "aapcs" fn(usize, ...)) {
+//~^ warning: the calling convention "aapcs" is not supported on this target
+//~| warning: this was previously accepted by the compiler but is being phased out
+    f(22, 44);
+}
+
+
+fn main() {}

tests/ui/abi/unsupported-varargs-fnptr.stderr

@@ -0,0 +1,23 @@
+warning: the calling convention "aapcs" is not supported on this target
+  --> $DIR/unsupported-varargs-fnptr.rs:7:13
+   |
+LL | fn aapcs(f: extern "aapcs" fn(usize, ...)) {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #130260 <https://github.com/rust-lang/rust/issues/130260>
+   = note: `#[warn(unsupported_fn_ptr_calling_conventions)]` on by default
+
+warning: 1 warning emitted
+
+Future incompatibility report: Future breakage diagnostic:
+warning: the calling convention "aapcs" is not supported on this target
+  --> $DIR/unsupported-varargs-fnptr.rs:7:13
+   |
+LL | fn aapcs(f: extern "aapcs" fn(usize, ...)) {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #130260 <https://github.com/rust-lang/rust/issues/130260>
+   = note: `#[warn(unsupported_fn_ptr_calling_conventions)]` on by default
+