rustdoc-json: Document and Test that args can be patterns.

src/rustdoc-json-types/lib.rs

@@ -618,6 +618,10 @@ pub struct FunctionPointer {
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
 pub struct FnDecl {
+    /// List of argument names and their type.
+    ///
+    /// Note that not all names will be valid identifiers, as some of
+    /// them may be patterns.
     pub inputs: Vec<(String, Type)>,
     pub output: Option<Type>,
     pub c_variadic: bool,

src/test/rustdoc-json/fns/pattern_arg.rs

@@ -0,0 +1,7 @@
+// @is "$.index[*][?(@.name=='fst')].inner.decl.inputs[0][0]" '"(x, _)"'
+pub fn fst<X, Y>((x, _): (X, Y)) -> X {
+    x
+}
+
+// @is "$.index[*][?(@.name=='drop_int')].inner.decl.inputs[0][0]" '"_"'
+pub fn drop_int(_: i32) {}