Detect inherent associated types not having CamelCase

Fixes #105341.

compiler/rustc_lint/src/nonstandard_style.rs

@@ -175,13 +175,23 @@ impl EarlyLintPass for NonCamelCaseTypes {
             return;
         }
 
-        match it.kind {
+        match &it.kind {
             ast::ItemKind::TyAlias(..)
             | ast::ItemKind::Enum(..)
             | ast::ItemKind::Struct(..)
             | ast::ItemKind::Union(..) => self.check_case(cx, "type", &it.ident),
             ast::ItemKind::Trait(..) => self.check_case(cx, "trait", &it.ident),
             ast::ItemKind::TraitAlias(..) => self.check_case(cx, "trait alias", &it.ident),
+
+            // N.B. This check is only for inherent associated types, so that we don't lint against
+            // trait impls where we should have warned for the trait definition already.
+            ast::ItemKind::Impl(box ast::Impl { of_trait: None, items, .. }) => {
+                for it in items {
+                    if let ast::AssocItemKind::Type(..) = it.kind {
+                        self.check_case(cx, "associated type", &it.ident);
+                    }
+                }
+            }
             _ => (),
         }
     }

src/test/ui/associated-inherent-types/style.rs

@@ -0,0 +1,12 @@
+#![feature(inherent_associated_types)]
+#![allow(incomplete_features, dead_code)]
+#![deny(non_camel_case_types)]
+
+struct S;
+
+impl S {
+    type typ = ();
+    //~^ ERROR associated type `typ` should have an upper camel case name
+}
+
+fn main() {}

src/test/ui/associated-inherent-types/style.stderr

@@ -0,0 +1,14 @@
+error: associated type `typ` should have an upper camel case name
+  --> $DIR/style.rs:8:10
+   |
+LL |     type typ = ();
+   |          ^^^ help: convert the identifier to upper camel case: `Typ`
+   |
+note: the lint level is defined here
+  --> $DIR/style.rs:3:9
+   |
+LL | #![deny(non_camel_case_types)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+