Replace some often empty Vecs with boxed slices

crates/hir-def/src/item_tree/lower.rs

@@ -659,15 +659,16 @@ fn desugar_future_path(orig: TypeRef) -> Path {
     let path = path![core::future::Future];
     let mut generic_args: Vec<_> =
         std::iter::repeat(None).take(path.segments().len() - 1).collect();
-    let mut last = GenericArgs::empty();
     let binding = AssociatedTypeBinding {
         name: name![Output],
         args: None,
         type_ref: Some(orig),
-        bounds: Vec::new(),
+        bounds: Box::default(),
     };
-    last.bindings.push(binding);
-    generic_args.push(Some(Interned::new(last)));
+    generic_args.push(Some(Interned::new(GenericArgs {
+        bindings: Box::new([binding]),
+        ..GenericArgs::empty()
+    })));
 
     Path::from_known_path(path, generic_args)
 }

crates/hir-def/src/path.rs

@@ -49,15 +49,15 @@ pub struct Path {
 /// also includes bindings of associated types, like in `Iterator<Item = Foo>`.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct GenericArgs {
-    pub args: Vec<GenericArg>,
+    pub args: Box<[GenericArg]>,
     /// This specifies whether the args contain a Self type as the first
     /// element. This is the case for path segments like `<T as Trait>`, where
     /// `T` is actually a type parameter for the path `Trait` specifying the
     /// Self type. Otherwise, when we have a path `Trait<X, Y>`, the Self type
     /// is left out.
     pub has_self_type: bool,
     /// Associated type bindings like in `Iterator<Item = T>`.
-    pub bindings: Vec<AssociatedTypeBinding>,
+    pub bindings: Box<[AssociatedTypeBinding]>,
     /// Whether these generic args were desugared from `Trait(Arg) -> Output`
     /// parenthesis notation typically used for the `Fn` traits.
     pub desugared_from_fn: bool,
@@ -77,7 +77,7 @@ pub struct AssociatedTypeBinding {
     /// Bounds for the associated type, like in `Iterator<Item:
     /// SomeOtherTrait>`. (This is the unstable `associated_type_bounds`
     /// feature.)
-    pub bounds: Vec<Interned<TypeBound>>,
+    pub bounds: Box<[Interned<TypeBound>]>,
 }
 
 /// A single generic argument.
@@ -212,9 +212,9 @@ impl GenericArgs {
 
     pub(crate) fn empty() -> GenericArgs {
         GenericArgs {
-            args: Vec::new(),
+            args: Box::default(),
             has_self_type: false,
-            bindings: Vec::new(),
+            bindings: Box::default(),
             desugared_from_fn: false,
         }
     }

crates/hir-def/src/path/lower.rs

@@ -1,5 +1,7 @@
 //! Transforms syntax into `Path` objects, ideally with accounting for hygiene
 
+use std::iter;
+
 use crate::type_ref::ConstScalarOrPath;
 
 use either::Either;
@@ -86,15 +88,26 @@ pub(super) fn lower_path(mut path: ast::Path, ctx: &LowerCtx<'_>) -> Option<Path
                             generic_args.resize(segments.len(), None);
                         }
 
+                        let self_type = GenericArg::Type(self_type);
+
                         // Insert the type reference (T in the above example) as Self parameter for the trait
                         let last_segment = generic_args.get_mut(segments.len() - num_segments)?;
-                        let mut args_inner = match last_segment {
-                            Some(it) => it.as_ref().clone(),
-                            None => GenericArgs::empty(),
-                        };
-                        args_inner.has_self_type = true;
-                        args_inner.args.insert(0, GenericArg::Type(self_type));
-                        *last_segment = Some(Interned::new(args_inner));
+                        *last_segment = Some(Interned::new(match last_segment.take() {
+                            Some(it) => GenericArgs {
+                                args: iter::once(self_type)
+                                    .chain(it.args.iter().cloned())
+                                    .collect(),
+
+                                has_self_type: true,
+                                bindings: it.bindings.clone(),
+                                desugared_from_fn: it.desugared_from_fn,
+                            },
+                            None => GenericArgs {
+                                args: Box::new([self_type]),
+                                has_self_type: true,
+                                ..GenericArgs::empty()
+                            },
+                        }));
                     }
                 }
             }
@@ -187,7 +200,7 @@ pub(super) fn lower_generic_args(
                             .map(|it| Interned::new(TypeBound::from_ast(lower_ctx, it)))
                             .collect()
                     } else {
-                        Vec::new()
+                        Box::default()
                     };
                     bindings.push(AssociatedTypeBinding { name, args, type_ref, bounds });
                 }
@@ -208,7 +221,12 @@ pub(super) fn lower_generic_args(
     if args.is_empty() && bindings.is_empty() {
         return None;
     }
-    Some(GenericArgs { args, has_self_type: false, bindings, desugared_from_fn: false })
+    Some(GenericArgs {
+        args: args.into_boxed_slice(),
+        has_self_type: false,
+        bindings: bindings.into_boxed_slice(),
+        desugared_from_fn: false,
+    })
 }
 
 /// Collect `GenericArgs` from the parts of a fn-like path, i.e. `Fn(X, Y)
@@ -218,33 +236,30 @@ fn lower_generic_args_from_fn_path(
     params: Option<ast::ParamList>,
     ret_type: Option<ast::RetType>,
 ) -> Option<GenericArgs> {
-    let mut args = Vec::new();
-    let mut bindings = Vec::new();
     let params = params?;
     let mut param_types = Vec::new();
     for param in params.params() {
         let type_ref = TypeRef::from_ast_opt(ctx, param.ty());
         param_types.push(type_ref);
     }
-    let arg = GenericArg::Type(TypeRef::Tuple(param_types));
-    args.push(arg);
-    if let Some(ret_type) = ret_type {
+    let args = Box::new([GenericArg::Type(TypeRef::Tuple(param_types))]);
+    let bindings = if let Some(ret_type) = ret_type {
         let type_ref = TypeRef::from_ast_opt(ctx, ret_type.ty());
-        bindings.push(AssociatedTypeBinding {
+        Box::new([AssociatedTypeBinding {
             name: name![Output],
             args: None,
             type_ref: Some(type_ref),
-            bounds: Vec::new(),
-        });
+            bounds: Box::default(),
+        }])
     } else {
         // -> ()
         let type_ref = TypeRef::Tuple(Vec::new());
-        bindings.push(AssociatedTypeBinding {
+        Box::new([AssociatedTypeBinding {
             name: name![Output],
             args: None,
             type_ref: Some(type_ref),
-            bounds: Vec::new(),
-        });
-    }
+            bounds: Box::default(),
+        }])
+    };
     Some(GenericArgs { args, has_self_type: false, bindings, desugared_from_fn: true })
 }

crates/hir-def/src/pretty.rs

@@ -71,7 +71,7 @@ pub(crate) fn print_generic_args(generics: &GenericArgs, buf: &mut dyn Write) ->
         first = false;
         print_generic_arg(arg, buf)?;
     }
-    for binding in &generics.bindings {
+    for binding in generics.bindings.iter() {
         if !first {
             write!(buf, ", ")?;
         }

crates/hir-def/src/type_ref.rs

@@ -292,7 +292,7 @@ impl TypeRef {
             }
             for segment in path.segments().iter() {
                 if let Some(args_and_bindings) = segment.args_and_bindings {
-                    for arg in &args_and_bindings.args {
+                    for arg in args_and_bindings.args.iter() {
                         match arg {
                             crate::path::GenericArg::Type(type_ref) => {
                                 go(type_ref, f);
@@ -301,11 +301,11 @@ impl TypeRef {
                             | crate::path::GenericArg::Lifetime(_) => {}
                         }
                     }
-                    for binding in &args_and_bindings.bindings {
+                    for binding in args_and_bindings.bindings.iter() {
                         if let Some(type_ref) = &binding.type_ref {
                             go(type_ref, f);
                         }
-                        for bound in &binding.bounds {
+                        for bound in binding.bounds.iter() {
                             match bound.as_ref() {
                                 TypeBound::Path(path, _) | TypeBound::ForLifetime(_, path) => {
                                     go_path(path, f)

crates/hir-ty/src/display.rs

@@ -1419,7 +1419,7 @@ impl HirDisplay for Path {
 
                 write!(f, "<")?;
                 let mut first = true;
-                for arg in &generic_args.args {
+                for arg in generic_args.args.iter() {
                     if first {
                         first = false;
                         if generic_args.has_self_type {
@@ -1431,7 +1431,7 @@ impl HirDisplay for Path {
                     }
                     arg.hir_fmt(f)?;
                 }
-                for binding in &generic_args.bindings {
+                for binding in generic_args.bindings.iter() {
                     if first {
                         first = false;
                     } else {
@@ -1445,7 +1445,7 @@ impl HirDisplay for Path {
                         }
                         None => {
                             write!(f, ": ")?;
-                            f.write_joined(&binding.bounds, " + ")?;
+                            f.write_joined(binding.bounds.iter(), " + ")?;
                         }
                     }
                 }

crates/hir-ty/src/lower.rs

@@ -1025,7 +1025,7 @@ impl<'a> TyLoweringContext<'a> {
         last_segment
             .into_iter()
             .filter_map(|segment| segment.args_and_bindings)
-            .flat_map(|args_and_bindings| &args_and_bindings.bindings)
+            .flat_map(|args_and_bindings| args_and_bindings.bindings.iter())
             .flat_map(move |binding| {
                 let found = associated_type_by_name_including_super_traits(
                     self.db,
@@ -1068,7 +1068,7 @@ impl<'a> TyLoweringContext<'a> {
                         AliasEq { alias: AliasTy::Projection(projection_ty.clone()), ty };
                     preds.push(crate::wrap_empty_binders(WhereClause::AliasEq(alias_eq)));
                 }
-                for bound in &binding.bounds {
+                for bound in binding.bounds.iter() {
                     preds.extend(self.lower_type_bound(
                         bound,
                         TyKind::Alias(AliasTy::Projection(projection_ty.clone())).intern(Interner),