Add a pretty printing test for fn params.

Note that multiple parts of the output are incorrect. The following
commits will fix this.

tests/pretty/hir-fn-params.pp

@@ -0,0 +1,38 @@
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+//@ pretty-compare-only
+//@ pretty-mode:hir
+//@ pp-exact:hir-fn-params.pp
+
+// This tests the pretty-printing of various kinds of function parameters.
+
+//---------------------------------------------------------------------------
+// Normal functions and methods.
+
+fn normal_fn(_: u32, a: u32) { }
+
+struct S;
+impl S {
+    fn method(_: u32, a: u32) { }
+}
+
+//---------------------------------------------------------------------------
+// More exotic forms, which get a different pretty-printing path. In the past,
+// anonymous params and `_` params printed incorrectly, e.g. `fn(u32, _: u32)`
+// was printed as `fn(: u32, : u32)`.
+//
+// Ideally we would also test invalid patterns, e.g. `fn(1: u32, &a: u32)`,
+// because they had similar problems. But the pretty-printing tests currently
+// can't contain compile errors.
+
+fn bare_fn(x: fn(: u32, : u32, a: u32)) { }
+
+extern "C" {
+    unsafe fn foreign_fn(: u32, a: u32);
+}
+
+trait T {
+    fn trait_fn(: u32, : u32, a: u32);
+}

tests/pretty/hir-fn-params.rs

@@ -0,0 +1,34 @@
+//@ pretty-compare-only
+//@ pretty-mode:hir
+//@ pp-exact:hir-fn-params.pp
+
+// This tests the pretty-printing of various kinds of function parameters.
+
+//---------------------------------------------------------------------------
+// Normal functions and methods.
+
+fn normal_fn(_: u32, a: u32) {}
+
+struct S;
+impl S {
+    fn method(_: u32, a: u32) {}
+}
+
+//---------------------------------------------------------------------------
+// More exotic forms, which get a different pretty-printing path. In the past,
+// anonymous params and `_` params printed incorrectly, e.g. `fn(u32, _: u32)`
+// was printed as `fn(: u32, : u32)`.
+//
+// Ideally we would also test invalid patterns, e.g. `fn(1: u32, &a: u32)`,
+// because they had similar problems. But the pretty-printing tests currently
+// can't contain compile errors.
+
+fn bare_fn(x: fn(u32, _: u32, a: u32)) {}
+
+extern "C" {
+    fn foreign_fn(_: u32, a: u32);
+}
+
+trait T {
+    fn trait_fn(u32, _: u32, a: u32);
+}