Do not ICE on range patterns in function arguments

src/librustdoc/clean/utils.rs

@@ -289,10 +289,7 @@ fn to_src(&self, cx: &DocContext<'_>) -> String {
             );
             return Symbol::intern("()");
         }
-        PatKind::Range(..) => panic!(
-            "tried to get argument name from PatKind::Range, \
-             which is not allowed in function arguments"
-        ),
+        PatKind::Range(..) => return kw::Underscore,
         PatKind::Slice(ref begin, ref mid, ref end) => {
             let begin = begin.iter().map(|p| name_from_pat(&**p).to_string());
             let mid = mid.as_ref().map(|p| format!("..{}", name_from_pat(&**p))).into_iter();

src/test/rustdoc/range-arg-pattern.rs

@@ -1,5 +1,5 @@
 #![crate_name = "foo"]
 
 // @has foo/fn.f.html
-// @has - '//*[@class="rust fn"]' 'pub fn f(0u8 ...255: u8)'
+// @has - '//*[@class="rust fn"]' 'pub fn f(_: u8)'
 pub fn f(0u8...255: u8) {}