Add lint: Plugin not ending in "Plugin" (#111)

Closes #55.

This adds a lint that verifies that all plugins have the "Plugin"
suffix, as that pattern is pretty-much standardized across the
ecosystem. This is our first "Style" lint, meaning it tries to suggest
idiomatic code.

## Testing

Paste the following into `bevy_lint/examples/lint_test.rs`, run `cd
bevy_lint`, then run `cargo build && cargo run -- --example lint_test`:

```rust
#![feature(register_tool)]
#![register_tool(bevy)]

use bevy::prelude::*;

mod module {
    pub mod submodule {
        use std::marker::PhantomData;

        // Verify generics are not included in name.
        pub struct Foo<T>(PhantomData<T>);
    }
}

// Ensure you cannot spoof the name of the plugin.
use module::submodule::Foo as FooPlugin;

impl<T: Send + Sync + 'static> Plugin for FooPlugin<T> {
    fn build(&self, _: &mut App) {}
}

fn main() {}
```

bevy_lint/src/lints/mod.rs

@@ -4,12 +4,14 @@ use rustc_lint::{Lint, LintStore};
 pub mod insert_event_resource;
 pub mod main_return_without_appexit;
 pub mod panicking_methods;
+pub mod plugin_not_ending_in_plugin;
 
 pub(crate) static LINTS: &[&BevyLint] = &[
     insert_event_resource::INSERT_EVENT_RESOURCE,
     main_return_without_appexit::MAIN_RETURN_WITHOUT_APPEXIT,
     panicking_methods::PANICKING_QUERY_METHODS,
     panicking_methods::PANICKING_WORLD_METHODS,
+    plugin_not_ending_in_plugin::PLUGIN_NOT_ENDING_IN_PLUGIN,
 ];
 
 pub(crate) fn register_lints(store: &mut LintStore) {
@@ -21,4 +23,5 @@ pub(crate) fn register_passes(store: &mut LintStore) {
     store.register_late_pass(|_| Box::new(insert_event_resource::InsertEventResource));
     store.register_late_pass(|_| Box::new(main_return_without_appexit::MainReturnWithoutAppExit));
     store.register_late_pass(|_| Box::new(panicking_methods::PanickingMethods));
+    store.register_late_pass(|_| Box::new(plugin_not_ending_in_plugin::PluginNotEndingInPlugin));
 }

bevy_lint/src/lints/plugin_not_ending_in_plugin.rs

@@ -0,0 +1,127 @@
+//! Checks for types who implement `Plugin` but whose names does not end in "Plugin".
+//!
+//! This does _not_ check function-style plugins (`fn plugin(app: &mut App)`), only structures with
+//! `Plugin` explicitly implemented with `impl Plugin for T`.
+//!
+//! # Motivation
+//!
+//! Unlike traits like [`Clone`] or [`Debug`], the primary purpose of a type that implements
+//! `Plugin` is to be a Bevy plugin. As such, it is common practice to suffix plugin names with
+//! "Plugin" to signal how they should be used.
+//!
+//! # Known issues
+//!
+//! Due to technical reasons, if you wish to silence this lint you need to annotate the
+//! `impl Plugin for T` line with `#[allow(bevy::plugin_not_ending_in_plugin)]`, not the `struct T`
+//! line.
+//!
+//! # Example
+//!
+//! ```
+//! # use bevy::prelude::*;
+//! #
+//! struct Physics;
+//!
+//! impl Plugin for Physics {
+//!     fn build(&self, app: &mut App) {
+//!         // ...
+//!     }
+//! }
+//! ```
+//!
+//! Use instead:
+//!
+//! ```
+//! # use bevy::prelude::*;
+//! #
+//! struct PhysicsPlugin;
+//!
+//! impl Plugin for PhysicsPlugin {
+//!     fn build(&self, app: &mut App) {
+//!         // ...
+//!     }
+//! }
+//! ```
+
+use crate::declare_bevy_lint;
+use clippy_utils::{diagnostics::span_lint_and_then, match_def_path, path_res};
+use rustc_errors::Applicability;
+use rustc_hir::{def::Res, Item, ItemKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::symbol::Ident;
+
+declare_bevy_lint! {
+    pub PLUGIN_NOT_ENDING_IN_PLUGIN,
+    STYLE,
+    "implemented `Plugin` for a structure whose name does not end in \"Plugin\"",
+}
+
+declare_lint_pass! {
+    PluginNotEndingInPlugin => [PLUGIN_NOT_ENDING_IN_PLUGIN.lint]
+}
+
+impl<'tcx> LateLintPass<'tcx> for PluginNotEndingInPlugin {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &Item<'tcx>) {
+        // Find `impl` items...
+        if let ItemKind::Impl(impl_) = item.kind
+            // ...that implement a trait...
+            && let Some(of_trait) = impl_.of_trait
+            // ...where the trait is a path to user code... (I don't believe this will ever be
+            // false, since the alternatives are primitives, `Self`, and others that wouldn't make
+            // since in this scenario.)
+            && let Res::Def(_, def_id) = of_trait.path.res
+            // ...where the trait being implemented is Bevy's `Plugin`...
+            && match_def_path(cx, def_id, &crate::paths::PLUGIN)
+        {
+            // Try to resolve where this type was originally defined. This will result in a `DefId`
+            // pointing to the original `struct Foo` definition, or `impl <T>` if it's a generic
+            // parameter.
+            let Some(def_id) = path_res(cx, impl_.self_ty).opt_def_id() else {
+                return;
+            };
+
+            // If this type is a generic parameter, exit. Their names, such as `T`, cannot be
+            // referenced by others.
+            if impl_
+                .generics
+                .params
+                .iter()
+                .any(|param| param.def_id.to_def_id() == def_id)
+            {
+                return;
+            }
+
+            // Find the original name and span of the type. (We don't use the name from the path,
+            // since that can be spoofed through `use Foo as FooPlugin`.)
+            let Some(Ident {
+                name: self_name,
+                span: self_span,
+            }) = cx.tcx.opt_item_ident(def_id)
+            else {
+                return;
+            };
+
+            // If the type's name ends in "Plugin", exit.
+            if self_name.as_str().ends_with("Plugin") {
+                return;
+            }
+
+            span_lint_and_then(
+                cx,
+                PLUGIN_NOT_ENDING_IN_PLUGIN.lint,
+                item.span,
+                PLUGIN_NOT_ENDING_IN_PLUGIN.lint.desc,
+                |diag| {
+                    diag.span_suggestion(
+                        self_span,
+                        "rename the plugin",
+                        format!("{self_name}Plugin"),
+                        // There may be other references that also need to be renamed.
+                        Applicability::MaybeIncorrect,
+                    );
+                },
+            );
+        }
+    }
+}

bevy_lint/src/paths.rs

@@ -8,6 +8,7 @@
 
 pub const APP: [&str; 3] = ["bevy_app", "app", "App"];
 pub const EVENTS: [&str; 3] = ["bevy_ecs", "event", "Events"];
+pub const PLUGIN: [&str; 3] = ["bevy_app", "plugin", "Plugin"];
 pub const QUERY: [&str; 4] = ["bevy_ecs", "system", "query", "Query"];
 pub const QUERY_STATE: [&str; 4] = ["bevy_ecs", "query", "state", "QueryState"];
 pub const WORLD: [&str; 3] = ["bevy_ecs", "world", "World"];