Fix some HIR pretty-printing problems

compiler/rustc_hir_pretty/src/lib.rs

@@ -1871,10 +1871,11 @@ impl<'a> State<'a> {
     fn print_pat(&mut self, pat: &hir::Pat<'_>) {
         self.maybe_print_comment(pat.span.lo());
         self.ann.pre(self, AnnNode::Pat(pat));
-        // Pat isn't normalized, but the beauty of it
-        // is that it doesn't matter
+        // Pat isn't normalized, but the beauty of it is that it doesn't matter.
         match pat.kind {
-            PatKind::Missing => unreachable!(),
+            // Printing `_` isn't ideal for a missing pattern, but it's easy and good enough.
+            // E.g. `fn(u32)` gets printed as `fn(_: u32)`.
+            PatKind::Missing => self.word("_"),
             PatKind::Wild => self.word("_"),
             PatKind::Never => self.word("!"),
             PatKind::Binding(BindingMode(by_ref, mutbl), _, ident, sub) => {
@@ -2164,7 +2165,9 @@ impl<'a> State<'a> {
             s.end();
         });
         if decl.c_variadic {
-            self.word(", ");
+            if !decl.inputs.is_empty() {
+                self.word(", ");
+            }
             print_arg(self, None);
             self.word("...");
         }

tests/pretty/hir-fn-variadic.pp

@@ -13,3 +13,39 @@
 }
 
 unsafe extern "C" fn bar(_: i32, mut va2: ...) -> usize { va2.arg::<usize>() }
+
+fn main() {
+    fn g1(_: extern "C" fn(_: u8, va: ...)) { }
+    fn g2(_: extern "C" fn(_: u8, ...)) { }
+    fn g3(_: extern "C" fn(u8, va: ...)) { }
+    fn g4(_: extern "C" fn(u8, ...)) { }
+
+    fn g5(_: extern "C" fn(va: ...)) { }
+    fn g6(_: extern "C" fn(...)) { }
+
+    {
+        let _ =
+            {
+                unsafe extern "C" fn f1(_: u8, va: ...) { }
+            };
+    };
+    {
+        let _ =
+            {
+                unsafe extern "C" fn f2(_: u8, _: ...) { }
+            };
+    };
+
+    {
+        let _ =
+            {
+                unsafe extern "C" fn f5(va: ...) { }
+            };
+    };
+    {
+        let _ =
+            {
+                unsafe extern "C" fn f6(_: ...) { }
+            };
+    };
+}

tests/pretty/hir-fn-variadic.rs

@@ -11,3 +11,19 @@ extern "C" {
 pub unsafe extern "C" fn bar(_: i32, mut va2: ...) -> usize {
     va2.arg::<usize>()
 }
+
+fn main() {
+    fn g1(_: extern "C" fn(_: u8, va: ...)) {}
+    fn g2(_: extern "C" fn(_: u8, ...)) {}
+    fn g3(_: extern "C" fn(u8, va: ...)) {}
+    fn g4(_: extern "C" fn(u8, ...)) {}
+
+    fn g5(_: extern "C" fn(va: ...)) {}
+    fn g6(_: extern "C" fn(...)) {}
+
+    _ = { unsafe extern "C" fn f1(_: u8, va: ...) {} };
+    _ = { unsafe extern "C" fn f2(_: u8, ...) {} };
+
+    _ = { unsafe extern "C" fn f5(va: ...) {} };
+    _ = { unsafe extern "C" fn f6(...) {} };
+}