rustdoc: use ThinVec for generic arg parts

This reduces the size of both these args, and of path segments,
so should measurably help with memory use.

Author: notriddle

src/librustdoc/clean/types.rs

@@ -2254,8 +2254,8 @@ impl GenericArg {
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub(crate) enum GenericArgs {
-    AngleBracketed { args: Box<[GenericArg]>, constraints: ThinVec<AssocItemConstraint> },
-    Parenthesized { inputs: Box<[Type]>, output: Option<Box<Type>> },
+    AngleBracketed { args: ThinVec<GenericArg>, constraints: ThinVec<AssocItemConstraint> },
+    Parenthesized { inputs: ThinVec<Type>, output: Option<Box<Type>> },
 }
 
 impl GenericArgs {
@@ -2279,7 +2279,7 @@ impl GenericArgs {
                         assoc: PathSegment {
                             name: sym::Output,
                             args: GenericArgs::AngleBracketed {
-                                args: Vec::new().into_boxed_slice(),
+                                args: ThinVec::new(),
                                 constraints: ThinVec::new(),
                             },
                         },
@@ -2596,12 +2596,12 @@ mod size_asserts {
     static_assert_size!(Crate, 56); // frequently moved by-value
     static_assert_size!(DocFragment, 32);
     static_assert_size!(GenericArg, 32);
-    static_assert_size!(GenericArgs, 32);
+    static_assert_size!(GenericArgs, 24);
     static_assert_size!(GenericParamDef, 40);
     static_assert_size!(Generics, 16);
     static_assert_size!(Item, 48);
     static_assert_size!(ItemKind, 48);
-    static_assert_size!(PathSegment, 40);
+    static_assert_size!(PathSegment, 32);
     static_assert_size!(Type, 32);
     // tidy-alphabetical-end
 }

src/librustdoc/html/render/search_index.rs

@@ -821,7 +821,7 @@ pub(crate) fn get_function_type_for_search(
                     .map(|name| clean::PathSegment {
                         name: *name,
                         args: clean::GenericArgs::AngleBracketed {
-                            args: Vec::new().into_boxed_slice(),
+                            args: ThinVec::new(),
                             constraints: ThinVec::new(),
                         },
                     })

src/librustdoc/json/conversions.rs

@@ -231,11 +231,11 @@ impl FromClean<clean::GenericArgs> for GenericArgs {
         use clean::GenericArgs::*;
         match args {
             AngleBracketed { args, constraints } => GenericArgs::AngleBracketed {
-                args: args.into_vec().into_json(renderer),
+                args: args.into_json(renderer),
                 constraints: constraints.into_json(renderer),
             },
             Parenthesized { inputs, output } => GenericArgs::Parenthesized {
-                inputs: inputs.into_vec().into_json(renderer),
+                inputs: inputs.into_json(renderer),
                 output: output.map(|a| (*a).into_json(renderer)),
             },
         }