Add a test for const args that cannot be inferred

Co-Authored-By: Gabriel Smith <[email protected]>

Author: varkor

src/test/ui/const-generics/cannot-infer-const-args.rs

@@ -0,0 +1,10 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+fn foo<const X: usize>() -> usize {
+    0
+}
+
+fn main() {
+    foo(); //~ ERROR type annotations needed
+}

src/test/ui/const-generics/cannot-infer-const-args.stderr

@@ -0,0 +1,15 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/cannot-infer-const-args.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+error[E0282]: type annotations needed
+  --> $DIR/cannot-infer-const-args.rs:9:5
+   |
+LL |     foo();
+   |     ^^^ cannot infer type for `fn() -> usize {foo::<_>}`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.