Use one BevyManifest instance in proc macros (#16766)

# Objective

- Minor consistency improvement in proc macro code.
- Remove `get_path_direct` since it was only used once anyways and
doesn't add much.

## Solution
- Possibly a minor performance improvement since the `Cargo.toml` wont
be parsed as often.

## Testing

- I don't think it breaks anything.
- This is my first time working on bevy itself. Is there a script to do
a quick verify of my pr?

## Other PR

Similar to #7536 but has no extra dependencies.

Co-authored-by: François Mockers <mockersf@gmail.com>

crates/bevy_asset/macros/src/lib.rs

@@ -8,7 +8,7 @@ use quote::{format_ident, quote};
 use syn::{parse_macro_input, Data, DeriveInput, Path};
 
 pub(crate) fn bevy_asset_path() -> Path {
-    BevyManifest::default().get_path("bevy_asset")
+    BevyManifest::shared().get_path("bevy_asset")
 }
 
 const DEPENDENCY_ATTRIBUTE: &str = "dependency";

crates/bevy_derive/src/lib.rs

@@ -205,7 +205,7 @@ pub fn derive_enum_variant_meta(input: TokenStream) -> TokenStream {
 #[proc_macro_derive(AppLabel)]
 pub fn derive_app_label(input: TokenStream) -> TokenStream {
     let input = syn::parse_macro_input!(input as syn::DeriveInput);
-    let mut trait_path = BevyManifest::default().get_path("bevy_app");
+    let mut trait_path = BevyManifest::shared().get_path("bevy_app");
     let mut dyn_eq_path = trait_path.clone();
     trait_path.segments.push(format_ident!("AppLabel").into());
     dyn_eq_path.segments.push(format_ident!("DynEq").into());

crates/bevy_ecs/macros/src/lib.rs

@@ -708,7 +708,7 @@ pub fn derive_system_set(input: TokenStream) -> TokenStream {
 }
 
 pub(crate) fn bevy_ecs_path() -> syn::Path {
-    BevyManifest::default().get_path("bevy_ecs")
+    BevyManifest::shared().get_path("bevy_ecs")
 }
 
 #[proc_macro_derive(Event)]

crates/bevy_encase_derive/src/lib.rs

@@ -13,7 +13,7 @@ use encase_derive_impl::{implement, syn};
 const ENCASE: &str = "encase";
 
 fn bevy_encase_path() -> syn::Path {
-    let bevy_manifest = BevyManifest::default();
+    let bevy_manifest = BevyManifest::shared();
     bevy_manifest
         .get_subcrate("render")
         .map(|bevy_render_path| {

crates/bevy_gizmos/macros/src/lib.rs

@@ -11,8 +11,8 @@ use syn::{parse_macro_input, parse_quote, DeriveInput, Path};
 #[proc_macro_derive(GizmoConfigGroup)]
 pub fn derive_gizmo_config_group(input: TokenStream) -> TokenStream {
     let mut ast = parse_macro_input!(input as DeriveInput);
-    let bevy_gizmos_path: Path = BevyManifest::default().get_path("bevy_gizmos");
-    let bevy_reflect_path: Path = BevyManifest::default().get_path("bevy_reflect");
+    let bevy_gizmos_path: Path = BevyManifest::shared().get_path("bevy_gizmos");
+    let bevy_reflect_path: Path = BevyManifest::shared().get_path("bevy_reflect");
 
     ast.generics.make_where_clause().predicates.push(
         parse_quote! { Self: #bevy_reflect_path::Reflect + #bevy_reflect_path::TypePath + Default},

crates/bevy_macro_utils/src/bevy_manifest.rs

@@ -1,7 +1,7 @@
 extern crate proc_macro;
 
 use proc_macro::TokenStream;
-use std::{env, path::PathBuf};
+use std::{env, path::PathBuf, sync::LazyLock};
 use toml_edit::{DocumentMut, Item};
 
 /// The path to the `Cargo.toml` file for the Bevy project.
@@ -9,9 +9,13 @@ pub struct BevyManifest {
     manifest: DocumentMut,
 }
 
-impl Default for BevyManifest {
-    fn default() -> Self {
-        Self {
+const BEVY: &str = "bevy";
+const BEVY_INTERNAL: &str = "bevy_internal";
+
+impl BevyManifest {
+    /// Returns a global shared instance of the [`BevyManifest`] struct.
+    pub fn shared() -> &'static LazyLock<Self> {
+        static LAZY_SELF: LazyLock<BevyManifest> = LazyLock::new(|| BevyManifest {
             manifest: env::var_os("CARGO_MANIFEST_DIR")
                 .map(PathBuf::from)
                 .map(|mut path| {
@@ -30,13 +34,10 @@ impl Default for BevyManifest {
                     })
                 })
                 .expect("CARGO_MANIFEST_DIR is not defined."),
-        }
+        });
+        &LAZY_SELF
     }
-}
-const BEVY: &str = "bevy";
-const BEVY_INTERNAL: &str = "bevy_internal";
 
-impl BevyManifest {
     /// Attempt to retrieve the [path](syn::Path) of a particular package in
     /// the [manifest](BevyManifest) by [name](str).
     pub fn maybe_get_path(&self, name: &str) -> Option<syn::Path> {
@@ -73,21 +74,6 @@ impl BevyManifest {
             .or_else(|| deps_dev.and_then(find_in_deps))
     }
 
-    /// Returns the path for the crate with the given name.
-    ///
-    /// This is a convenience method for constructing a [manifest] and
-    /// calling the [`get_path`] method.
-    ///
-    /// This method should only be used where you just need the path and can't
-    /// cache the [manifest]. If caching is possible, it's recommended to create
-    /// the [manifest] yourself and use the [`get_path`] method.
-    ///
-    /// [`get_path`]: Self::get_path
-    /// [manifest]: Self
-    pub fn get_path_direct(name: &str) -> syn::Path {
-        Self::default().get_path(name)
-    }
-
     /// Returns the path for the crate with the given name.
     pub fn get_path(&self, name: &str) -> syn::Path {
         self.maybe_get_path(name)

crates/bevy_reflect/derive/src/meta.rs

@@ -3,5 +3,5 @@ use syn::Path;
 
 /// Returns the correct path for `bevy_reflect`.
 pub(crate) fn get_bevy_reflect_path() -> Path {
-    BevyManifest::get_path_direct("bevy_reflect")
+    BevyManifest::shared().get_path("bevy_reflect")
 }

crates/bevy_reflect/derive/src/trait_reflection.rs

@@ -34,7 +34,7 @@ pub(crate) fn reflect_trait(_args: &TokenStream, input: TokenStream) -> TokenStr
     let trait_ident = &item_trait.ident;
     let trait_vis = &item_trait.vis;
     let reflect_trait_ident = crate::ident::get_reflect_ident(&item_trait.ident.to_string());
-    let bevy_reflect_path = BevyManifest::default().get_path("bevy_reflect");
+    let bevy_reflect_path = BevyManifest::shared().get_path("bevy_reflect");
 
     let struct_doc = format!(
         " A type generated by the #[reflect_trait] macro for the `{trait_ident}` trait.\n\n This allows casting from `dyn Reflect` to `dyn {trait_ident}`.",

crates/bevy_render/macros/src/as_bind_group.rs

@@ -40,7 +40,7 @@ enum BindingState<'a> {
 }
 
 pub fn derive_as_bind_group(ast: syn::DeriveInput) -> Result<TokenStream> {
-    let manifest = BevyManifest::default();
+    let manifest = BevyManifest::shared();
     let render_path = manifest.get_path("bevy_render");
     let image_path = manifest.get_path("bevy_image");
     let asset_path = manifest.get_path("bevy_asset");

crates/bevy_render/macros/src/extract_component.rs

@@ -5,7 +5,7 @@ use syn::{parse_macro_input, parse_quote, DeriveInput, Path};
 pub fn derive_extract_component(input: TokenStream) -> TokenStream {
     let mut ast = parse_macro_input!(input as DeriveInput);
     let bevy_render_path: Path = crate::bevy_render_path();
-    let bevy_ecs_path: Path = bevy_macro_utils::BevyManifest::default()
+    let bevy_ecs_path: Path = bevy_macro_utils::BevyManifest::shared()
         .maybe_get_path("bevy_ecs")
         .expect("bevy_ecs should be found in manifest");
 

crates/bevy_render/macros/src/lib.rs

@@ -12,7 +12,7 @@ use quote::format_ident;
 use syn::{parse_macro_input, DeriveInput};
 
 pub(crate) fn bevy_render_path() -> syn::Path {
-    BevyManifest::default()
+    BevyManifest::shared()
         .maybe_get_path("bevy_render")
         // NOTE: If the derivation is within bevy_render, then we need to return 'crate'
         .unwrap_or_else(|| BevyManifest::parse_str("crate"))

crates/bevy_state/macros/src/lib.rs

@@ -20,5 +20,5 @@ pub fn derive_substates(input: TokenStream) -> TokenStream {
 }
 
 pub(crate) fn bevy_state_path() -> syn::Path {
-    BevyManifest::default().get_path("bevy_state")
+    BevyManifest::shared().get_path("bevy_state")
 }