rustdoc_json: represent generic args consistently.

They show up in three places: once as `Option<Box<GenericArgs>>`, once
as `Box<GenericArgs>`, and once as `GenericArgs`. The first option is
best. It is more compact because generic args are often missing. This
commit changes the latter two to the former.

Example output, before and after, for the `AssocItemConstraint` change:
```
{"name":"Offset","args":{"angle_bracketed":{"args":[],"constraints":[]}},"binding":{...}}
{"name":"Offset","args":null,"binding":{...}}
```
Example output, before and after, for the `Type::QualifiedPath` change:
```
{"qualified_path":{"name":"Offset","args":{"angle_bracketed":{"args":[],"constraints":[]}}, ...}}
{"qualified_path":{"name":"Offset","args":null, ...}}
```
This reduces JSON output size, but not by much (e.g. 0.5%), because
`AssocItemConstraint` and `Type::QualifiedPath` are uncommon.

Author: nnethercote

src/librustdoc/json/conversions.rs

@@ -170,10 +170,13 @@ pub(crate) fn from_deprecation(deprecation: attrs::Deprecation) -> Deprecation {
     Deprecation { since, note: note.map(|s| s.to_string()) }
 }
 
-impl FromClean<clean::GenericArgs> for GenericArgs {
+impl FromClean<clean::GenericArgs> for Option<Box<GenericArgs>> {
     fn from_clean(args: &clean::GenericArgs, renderer: &JsonRenderer<'_>) -> Self {
         use clean::GenericArgs::*;
-        match args {
+        if args.is_empty() {
+            return None;
+        }
+        Some(Box::new(match args {
             AngleBracketed { args, constraints } => GenericArgs::AngleBracketed {
                 args: args.into_json(renderer),
                 constraints: constraints.into_json(renderer),
@@ -183,7 +186,7 @@ impl FromClean<clean::GenericArgs> for GenericArgs {
                 output: output.as_ref().map(|a| a.as_ref().into_json(renderer)),
             },
             ReturnTypeNotation => GenericArgs::ReturnTypeNotation,
-        }
+        }))
     }
 }
 
@@ -589,11 +592,7 @@ impl FromClean<clean::Path> for Path {
                     // used in the final segment, e.g. `std::vec::Vec<u32>`. So
                     // check that the non-final segments have no generic args.
                     assert!(rest_segs.iter().all(|seg| seg.args.is_empty()));
-                    if final_seg.args.is_empty() {
-                        None
-                    } else {
-                        Some(Box::new(final_seg.args.into_json(renderer)))
-                    }
+                    final_seg.args.into_json(renderer)
                 } else {
                     None // no generics on any segments because there are no segments
                 }
@@ -608,7 +607,7 @@ impl FromClean<clean::QPathData> for Type {
 
         Self::QualifiedPath {
             name: assoc.name.to_string(),
-            args: Box::new(assoc.args.into_json(renderer)),
+            args: assoc.args.into_json(renderer),
             self_type: Box::new(self_type.into_json(renderer)),
             trait_: trait_.as_ref().map(|trait_| trait_.into_json(renderer)),
         }

src/rustdoc-json-types/lib.rs

@@ -30,7 +30,7 @@ pub type FxHashMap<K, V> = HashMap<K, V>; // re-export for use in src/librustdoc
 /// This integer is incremented with every breaking change to the API,
 /// and is returned along with the JSON blob as [`Crate::format_version`].
 /// Consuming code should assert that this value matches the format version(s) that it supports.
-pub const FORMAT_VERSION: u32 = 46;
+pub const FORMAT_VERSION: u32 = 47;
 
 /// The root of the emitted JSON blob.
 ///
@@ -354,7 +354,7 @@ pub struct AssocItemConstraint {
     /// The name of the associated type/constant.
     pub name: String,
     /// Arguments provided to the associated type/constant.
-    pub args: GenericArgs,
+    pub args: Option<Box<GenericArgs>>,
     /// The kind of bound applied to the associated type/constant.
     pub binding: AssocItemConstraintKind,
 }
@@ -1110,7 +1110,7 @@ pub enum Type {
         /// <core::slice::IterMut<'static, u32> as BetterIterator>::Item<'static>
         /// //                                                          ^^^^^^^^^
         /// ```
-        args: Box<GenericArgs>,
+        args: Option<Box<GenericArgs>>,
         /// The type with which this type is associated.
         ///
         /// ```ignore (incomplete expression)

src/rustdoc-json-types/tests.rs

@@ -45,7 +45,7 @@ fn test_union_info_roundtrip() {
 #[cfg(target_pointer_width = "64")]
 fn test_type_sizes() {
     // tidy-alphabetical-start
-    assert_eq!(size_of::<AssocItemConstraint>(), 208);
+    assert_eq!(size_of::<AssocItemConstraint>(), 112);
     assert_eq!(size_of::<Crate>(), 184);
     assert_eq!(size_of::<ExternalCrate>(), 48);
     assert_eq!(size_of::<FunctionPointer>(), 168);

src/tools/jsondoclint/src/validator.rs

@@ -271,7 +271,7 @@ impl<'a> Validator<'a> {
             Type::RawPointer { is_mutable: _, type_ } => self.check_type(&**type_),
             Type::BorrowedRef { lifetime: _, is_mutable: _, type_ } => self.check_type(&**type_),
             Type::QualifiedPath { name: _, args, self_type, trait_ } => {
-                self.check_generic_args(&**args);
+                self.check_opt_generic_args(&args);
                 self.check_type(&**self_type);
                 if let Some(trait_) = trait_ {
                     self.check_path(trait_, PathKind::Trait);
@@ -309,13 +309,12 @@ impl<'a> Validator<'a> {
             self.fail(&x.id, ErrorKind::Custom(format!("No entry in '$.paths' for {x:?}")));
         }
 
-        if let Some(args) = &x.args {
-            self.check_generic_args(&**args);
-        }
+        self.check_opt_generic_args(&x.args);
     }
 
-    fn check_generic_args(&mut self, x: &'a GenericArgs) {
-        match x {
+    fn check_opt_generic_args(&mut self, x: &'a Option<Box<GenericArgs>>) {
+        let Some(x) = x else { return };
+        match &**x {
             GenericArgs::AngleBracketed { args, constraints } => {
                 args.iter().for_each(|arg| self.check_generic_arg(arg));
                 constraints.iter().for_each(|bind| self.check_assoc_item_constraint(bind));
@@ -355,7 +354,7 @@ impl<'a> Validator<'a> {
     }
 
     fn check_assoc_item_constraint(&mut self, bind: &'a AssocItemConstraint) {
-        self.check_generic_args(&bind.args);
+        self.check_opt_generic_args(&bind.args);
         match &bind.binding {
             AssocItemConstraintKind::Equality(term) => self.check_term(term),
             AssocItemConstraintKind::Constraint(bounds) => {