Rollup merge of rust-lang#71284 - JOE1994:serialize_id, r=petrochenkov

fix -Zast-json to output correct JSON form

fixes rust-lang#71086 (Reverts some of the changes made in rust-lang#70215)

JSON output (from an empty library) after fix has something for **"id"** field
```shell
lonelyjoe@lonelyjoe-desktop:~/workspace/empty_lib/src$ rustc lib.rs -Zast-json
```
```json
{
   "module":{
      "inner":{
         "lo":0,
         "hi":94
      },
      "items":[
         {
            "attrs":[
               {
                  "kind":{
                     "variant":"Normal",
                     "fields":[
                        {
                           "path":{
                              "span":{
                                 "lo":0,
                                 "hi":0
                              },
                              "segments":[
                                 {
                                    "ident":{
                                       "name":"prelude_import",
                                       "span":{
                                          "lo":0,
                                          "hi":0
                                       }
                                    },
                                    "id":3,
                                    "args":null
                                 }
                              ]
                           },
                           "args":"Empty"
                        }
                     ]
                  },
                  "id":null,
                  "style":"Outer",
                  "span":{
                     "lo":0,
                     "hi":0
                  }
               }
            ],
            "id":4,
            "span":{
               "lo":0,
               "hi":0
            },
            "vis":{
               "node":"Inherited",
               "span":{
                  "lo":0,
                  "hi":0
               }
            },
            "ident":{
               "name":"",
               "span":{
                  "lo":0,
                  "hi":0
               }
            },
            "kind":{
               "variant":"Use",
               "fields":[
                  {
                     "prefix":{
                        "span":{
                           "lo":0,
                           "hi":0
                        },
                        "segments":[
                           {
                              "ident":{
                                 "name":"{{root}}",
                                 "span":{
                                    "lo":0,
                                    "hi":0
                                 }
                              },
                              "id":5,
                              "args":null
                           },
                           {
                              "ident":{
                                 "name":"std",
                                 "span":{
                                    "lo":0,
                                    "hi":0
                                 }
                              },
                              "id":6,
                              "args":null
                           },
                           {
                              "ident":{
                                 "name":"prelude",
                                 "span":{
                                    "lo":0,
                                    "hi":0
                                 }
                              },
                              "id":7,
                              "args":null
                           },
                           {
                              "ident":{
                                 "name":"v1",
                                 "span":{
                                    "lo":0,
                                    "hi":0
                                 }
                              },
                              "id":8,
                              "args":null
                           }
                        ]
                     },
                     "kind":"Glob",
                     "span":{
                        "lo":0,
                        "hi":0
                     }
                  }
               ]
            },
            "tokens":null
         },
         {
            "attrs":[
               {
                  "kind":{
                     "variant":"Normal",
                     "fields":[
                        {
                           "path":{
                              "span":{
                                 "lo":0,
                                 "hi":0
                              },
                              "segments":[
                                 {
                                    "ident":{
                                       "name":"macro_use",
                                       "span":{
                                          "lo":0,
                                          "hi":0
                                       }
                                    },
                                    "id":9,
                                    "args":null
                                 }
                              ]
                           },
                           "args":"Empty"
                        }
                     ]
                  },
                  "id":null,
                  "style":"Outer",
                  "span":{
                     "lo":0,
                     "hi":0
                  }
               }
            ],
            "id":10,
            "span":{
               "lo":0,
               "hi":0
            },
            "vis":{
               "node":"Inherited",
               "span":{
                  "lo":0,
                  "hi":0
               }
            },
            "ident":{
               "name":"std",
               "span":{
                  "lo":0,
                  "hi":0
               }
            },
            "kind":{
               "variant":"ExternCrate",
               "fields":[
                  null
               ]
            },
            "tokens":null
         }
      ],
      "inline":true
   },
   "attrs":[

   ],
   "span":{
      "lo":0,
      "hi":94
   },
   "proc_macros":[

   ]
}
```

src/librustc_ast/ast.rs

@@ -2211,14 +2211,14 @@ rustc_index::newtype_index! {
 }
 
 impl rustc_serialize::Encodable for AttrId {
-    fn encode<S: Encoder>(&self, _: &mut S) -> Result<(), S::Error> {
-        Ok(())
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        s.emit_unit()
     }
 }
 
 impl rustc_serialize::Decodable for AttrId {
-    fn decode<D: Decoder>(_: &mut D) -> Result<AttrId, D::Error> {
-        Ok(crate::attr::mk_attr_id())
+    fn decode<D: Decoder>(d: &mut D) -> Result<AttrId, D::Error> {
+        d.read_nil().map(|_| crate::attr::mk_attr_id())
     }
 }
 

src/test/ui/ast-json/ast-json-noexpand-output.rs

@@ -0,0 +1,10 @@
+// Check that AST json printing works.
+#![crate_type = "lib"]
+
+// check-pass
+// compile-flags: -Zast-json-noexpand
+// normalize-stdout-test ":\d+" -> ":0"
+
+// Only include a single item to reduce how often the test output needs
+// updating.
+extern crate core;

src/test/ui/ast-json/ast-json-noexpand-output.stdout

@@ -0,0 +1 @@
+{"module":{"inner":{"lo":0,"hi":0},"items":[{"attrs":[],"id":0,"span":{"lo":0,"hi":0},"vis":{"node":"Inherited","span":{"lo":0,"hi":0}},"ident":{"name":"core","span":{"lo":0,"hi":0}},"kind":{"variant":"ExternCrate","fields":[null]},"tokens":{"_field0":[[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["extern",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["crate",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["core",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":"Semi","span":{"lo":0,"hi":0}}]},"NonJoint"]]}}],"inline":true},"attrs":[{"kind":{"variant":"Normal","fields":[{"path":{"span":{"lo":0,"hi":0},"segments":[{"ident":{"name":"crate_type","span":{"lo":0,"hi":0}},"id":0,"args":null}]},"args":{"variant":"Eq","fields":[{"lo":0,"hi":0},{"_field0":[[{"variant":"Token","fields":[{"kind":{"variant":"Literal","fields":[{"kind":"Str","symbol":"lib","suffix":null}]},"span":{"lo":0,"hi":0}}]},"NonJoint"]]}]}}]},"id":null,"style":"Inner","span":{"lo":0,"hi":0}}],"span":{"lo":0,"hi":0},"proc_macros":[]}

src/test/ui/ast-json/ast-json-output.rs

@@ -1,7 +1,8 @@
 // Check that AST json printing works.
+#![crate_type = "lib"]
 
 // check-pass
-// compile-flags: -Zast-json-noexpand
+// compile-flags: -Zast-json
 // normalize-stdout-test ":\d+" -> ":0"
 
 // Only include a single item to reduce how often the test output needs

src/test/ui/ast-json/ast-json-output.stdout

@@ -1 +1 @@
-{"module":{"inner":{"lo":0,"hi":0},"items":[{"attrs":[],"id":0,"span":{"lo":0,"hi":0},"vis":{"node":"Inherited","span":{"lo":0,"hi":0}},"ident":{"name":"core","span":{"lo":0,"hi":0}},"kind":{"variant":"ExternCrate","fields":[null]},"tokens":{"_field0":[[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["extern",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["crate",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["core",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":"Semi","span":{"lo":0,"hi":0}}]},"NonJoint"]]}}],"inline":true},"attrs":[],"span":{"lo":0,"hi":0},"proc_macros":[]}
+{"module":{"inner":{"lo":0,"hi":0},"items":[{"attrs":[{"kind":{"variant":"Normal","fields":[{"path":{"span":{"lo":0,"hi":0},"segments":[{"ident":{"name":"prelude_import","span":{"lo":0,"hi":0}},"id":0,"args":null}]},"args":"Empty"}]},"id":null,"style":"Outer","span":{"lo":0,"hi":0}}],"id":0,"span":{"lo":0,"hi":0},"vis":{"node":"Inherited","span":{"lo":0,"hi":0}},"ident":{"name":"","span":{"lo":0,"hi":0}},"kind":{"variant":"Use","fields":[{"prefix":{"span":{"lo":0,"hi":0},"segments":[{"ident":{"name":"{{root}}","span":{"lo":0,"hi":0}},"id":0,"args":null},{"ident":{"name":"std","span":{"lo":0,"hi":0}},"id":0,"args":null},{"ident":{"name":"prelude","span":{"lo":0,"hi":0}},"id":0,"args":null},{"ident":{"name":"v1","span":{"lo":0,"hi":0}},"id":0,"args":null}]},"kind":"Glob","span":{"lo":0,"hi":0}}]},"tokens":null},{"attrs":[{"kind":{"variant":"Normal","fields":[{"path":{"span":{"lo":0,"hi":0},"segments":[{"ident":{"name":"macro_use","span":{"lo":0,"hi":0}},"id":0,"args":null}]},"args":"Empty"}]},"id":null,"style":"Outer","span":{"lo":0,"hi":0}}],"id":0,"span":{"lo":0,"hi":0},"vis":{"node":"Inherited","span":{"lo":0,"hi":0}},"ident":{"name":"std","span":{"lo":0,"hi":0}},"kind":{"variant":"ExternCrate","fields":[null]},"tokens":null},{"attrs":[],"id":0,"span":{"lo":0,"hi":0},"vis":{"node":"Inherited","span":{"lo":0,"hi":0}},"ident":{"name":"core","span":{"lo":0,"hi":0}},"kind":{"variant":"ExternCrate","fields":[null]},"tokens":{"_field0":[[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["extern",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["crate",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":{"variant":"Ident","fields":["core",false]},"span":{"lo":0,"hi":0}}]},"NonJoint"],[{"variant":"Token","fields":[{"kind":"Semi","span":{"lo":0,"hi":0}}]},"NonJoint"]]}}],"inline":true},"attrs":[{"kind":{"variant":"Normal","fields":[{"path":{"span":{"lo":0,"hi":0},"segments":[{"ident":{"name":"crate_type","span":{"lo":0,"hi":0}},"id":0,"args":null}]},"args":{"variant":"Eq","fields":[{"lo":0,"hi":0},{"_field0":[[{"variant":"Token","fields":[{"kind":{"variant":"Literal","fields":[{"kind":"Str","symbol":"lib","suffix":null}]},"span":{"lo":0,"hi":0}}]},"NonJoint"]]}]}}]},"id":null,"style":"Inner","span":{"lo":0,"hi":0}}],"span":{"lo":0,"hi":0},"proc_macros":[]}