add tests for using a single impl for two EIIs

Author: jdonszelmann

tests/ui/eii/multiple_decls.rs

@@ -0,0 +1,17 @@
+// Tests whether only one EII attribute cane be applied to a signature.
+#![feature(eii)]
+
+#[eii(a)]
+#[eii(b)]
+//~^ ERROR `#[eii]` can only be specified once
+fn a(x: u64);
+
+#[a]
+fn implementation(x: u64) {
+    println!("{x:?}")
+}
+
+// what you would write:
+fn main() {
+    a(42);
+}

tests/ui/eii/multiple_decls.stderr

@@ -0,0 +1,14 @@
+error: `#[eii]` can only be specified once
+  --> $DIR/multiple_decls.rs:5:1
+   |
+LL | #[eii(b)]
+   | ^^^^^^^^^
+   |
+note: specified again here
+  --> $DIR/multiple_decls.rs:4:1
+   |
+LL | #[eii(a)]
+   | ^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/eii/multiple_impls.rs

@@ -0,0 +1,23 @@
+//@ run-pass
+//@ check-run-results
+//@ ignore-backends: gcc
+// Tests whether one function could implement two EIIs.
+#![feature(eii)]
+
+#[eii]
+fn a(x: u64);
+
+#[eii]
+fn b(x: u64);
+
+#[a]
+#[b]
+fn implementation(x: u64) {
+    println!("{x:?}")
+}
+
+// what you would write:
+fn main() {
+    a(42);
+    b(42);
+}

tests/ui/eii/multiple_impls.run.stdout

@@ -0,0 +1,2 @@
+42
+42