mir: unused_generic_params query

This commit implements the `unused_generic_params` query, an initial
version of polymorphization which detects when an item does not use
generic parameters and is being needlessly monomorphized as a result.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/polymorphization/const_parameters/functions.rs

@@ -0,0 +1,33 @@
+// build-fail
+// compile-flags: -Zpolymorphize-errors
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+
+// This test checks that the polymorphization analysis correctly detects unused const
+// parameters in functions.
+
+// Function doesn't have any generic parameters to be unused.
+pub fn no_parameters() {}
+
+// Function has an unused generic parameter.
+pub fn unused<const T: usize>() {
+//~^ ERROR item has unused generic parameters
+}
+
+// Function uses generic parameter in value of a binding.
+pub fn used_binding<const T: usize>() -> usize {
+    let x: usize = T;
+    x
+}
+
+// Function uses generic parameter in substitutions to another function.
+pub fn used_substs<const T: usize>() {
+    unused::<T>()
+}
+
+fn main() {
+    no_parameters();
+    unused::<1>();
+    used_binding::<1>();
+    used_substs::<1>();
+}