Make ProjectJsonData more backwards-compatible

The structure has been slowly accumulating new, required fields, which
has been breaking clients and requiring them to add `"sysroot_project": null`
just to continue functioning. No more.

Also makes clear to future editors of this code that they should add #[serde(default)]
to new fields.

Author: cormacrelf

crates/project-model/src/project_json.rs

@@ -375,14 +375,19 @@ pub enum RunnableKind {
 
 #[derive(Serialize, Deserialize, Debug, Clone, Eq, PartialEq)]
 pub struct ProjectJsonData {
+    #[serde(default)]
     sysroot: Option<Utf8PathBuf>,
+    #[serde(default)]
     sysroot_src: Option<Utf8PathBuf>,
+    #[serde(default)]
     sysroot_project: Option<Box<ProjectJsonData>>,
     #[serde(default)]
     cfg_groups: FxHashMap<String, CfgList>,
     crates: Vec<CrateData>,
     #[serde(default)]
     runnables: Vec<RunnableData>,
+    //
+    // New fields should be #[serde(default)]. This applies to most of this datastructure.
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, Eq, PartialEq, Default)]
@@ -417,6 +422,8 @@ struct CrateData {
     build: Option<BuildData>,
     #[serde(default)]
     proc_macro_cwd: Option<Utf8PathBuf>,
+    //
+    // New fields should be #[serde(default)].
 }
 
 mod cfg_ {
@@ -464,14 +471,18 @@ pub struct BuildData {
     label: String,
     build_file: Utf8PathBuf,
     target_kind: TargetKindData,
+    //
+    // New fields should be #[serde(default)].
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct RunnableData {
-    pub program: String,
-    pub args: Vec<String>,
-    pub cwd: Utf8PathBuf,
-    pub kind: RunnableKindData,
+    program: String,
+    args: Vec<String>,
+    cwd: Utf8PathBuf,
+    kind: RunnableKindData,
+    //
+    // New fields should be #[serde(default)].
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]