rustdoc-json: Keep empty generic args if parenthesized

Because in the case of for example

    pub fn my_fn3(f: impl FnMut()) {}

we want to keep `()` even if it is empty since that matches e.g. Rust
syntax requirements.

Author: Enselic

src/librustdoc/json/conversions.rs

@@ -194,22 +194,25 @@ impl FromClean<attrs::Deprecation> for Deprecation {
 }
 
 impl FromClean<clean::GenericArgs> for Option<Box<GenericArgs>> {
-    fn from_clean(args: &clean::GenericArgs, renderer: &JsonRenderer<'_>) -> Self {
+    fn from_clean(generic_args: &clean::GenericArgs, renderer: &JsonRenderer<'_>) -> Self {
         use clean::GenericArgs::*;
-        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),
-            },
-            Parenthesized { inputs, output } => GenericArgs::Parenthesized {
+        match generic_args {
+            AngleBracketed { args, constraints } => {
+                if generic_args.is_empty() {
+                    None
+                } else {
+                    Some(Box::new(GenericArgs::AngleBracketed {
+                        args: args.into_json(renderer),
+                        constraints: constraints.into_json(renderer),
+                    }))
+                }
+            }
+            Parenthesized { inputs, output } => Some(Box::new(GenericArgs::Parenthesized {
                 inputs: inputs.into_json(renderer),
                 output: output.into_json(renderer),
-            },
-            ReturnTypeNotation => GenericArgs::ReturnTypeNotation,
-        }))
+            })),
+            ReturnTypeNotation => Some(Box::new(GenericArgs::ReturnTypeNotation)),
+        }
     }
 }
 

tests/rustdoc-json/generic-args.rs

@@ -17,4 +17,7 @@ pub fn my_fn1(_: <MyStruct as MyTrait>::MyType) {}
 //@ is "$.index[?(@.name=='my_fn2')].inner.function.sig.inputs[0][1].dyn_trait.traits[0].trait.args.angle_bracketed.constraints[0].args" null
 pub fn my_fn2(_: IntoIterator<Item = MyStruct, IntoIter = impl Clone>) {}
 
+//@ is "$.index[?(@.name=='my_fn3')].inner.function.sig.inputs[0][1].impl_trait[0].trait_bound.trait.args.parenthesized.inputs" []
+pub fn my_fn3(f: impl FnMut()) {}
+
 fn main() {}