Pretty-print `PatKind::Missing` as `_`.

Printing "no pattern" as `_` isn't ideal, but better than crashing, and
HIR pretty-printing already has plenty of imperfections. The added `f2`
and `f6` examples are ones that triggered the crash.

Note that some of the added examples are printed badly, e.g.
`fn(, ...)`. The next commit will fix those.

Fixes #139633.

3 files changed, 56 insertions, 3 deletions

diff --git a/compiler/rustc_hir_pretty/src/lib.rs b/compiler/rustc_hir_pretty/src/lib.rs
index e5ab317685f..9cae080aeb3 100644
--- a/compiler/rustc_hir_pretty/src/lib.rs
+++ b/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) => {
diff --git a/tests/pretty/hir-fn-variadic.pp b/tests/pretty/hir-fn-variadic.pp
index dfbaff69644..136e8b44ec2 100644
--- a/tests/pretty/hir-fn-variadic.pp
+++ b/tests/pretty/hir-fn-variadic.pp
@@ -13,3 +13,39 @@ extern "C" {
 }
 
 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(, _: ...) { }
+            };
+    };
+}
diff --git a/tests/pretty/hir-fn-variadic.rs b/tests/pretty/hir-fn-variadic.rs
index 3d3f7ee1831..99aa402c480 100644
--- a/tests/pretty/hir-fn-variadic.rs
+++ b/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(...) {} };
+}