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>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/test/ui/polymorphization/predicates.rs b/src/test/ui/polymorphization/predicates.rs
new file mode 100644
index 00000000000..49f8668cff8
--- /dev/null
+++ b/src/test/ui/polymorphization/predicates.rs
@@ -0,0 +1,21 @@
+// build-fail
+// compile-flags: -Zpolymorphize-errors
+
+// This test checks that `T` is considered used in `foo`, because it is used in a predicate for
+// `I`, which is used.
+
+fn bar<I>() {
+//~^ ERROR item has unused generic parameters
+}
+
+fn foo<I, T>(_: I)
+where
+    I: Iterator<Item = T>,
+{
+    bar::<I>()
+}
+
+fn main() {
+    let x = &[2u32];
+    foo(x.iter());
+}