Auto merge of #13045 - J-ZhengLi:missing_const_for_fn_FP, r=blyxyas

[`missing_const_for_fn`]: fix FP when arg ty is impl trait alias ty

closes: #13009

---

changelog: [`missing_const_for_fn`]: fix FP when arg ty is impl trait alias ty

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -7,6 +7,7 @@
 
 #![warn(clippy::missing_const_for_fn)]
 #![feature(start)]
+#![feature(type_alias_impl_trait)]
 
 extern crate helper;
 extern crate proc_macros;
@@ -190,3 +191,11 @@ mod with_extern {
     extern "system" fn system() {}
     extern "system-unwind" fn system_unwind() {}
 }
+
+mod with_ty_alias {
+    type Foo = impl std::fmt::Debug;
+
+    fn foo(_: Foo) {
+        let _: Foo = 1;
+    }
+}

tests/ui/missing_const_for_fn/could_be_const.fixed

@@ -186,3 +186,18 @@ mod issue12677 {
         }
     }
 }
+
+mod with_ty_alias {
+    trait FooTrait {
+        type Foo: std::fmt::Debug;
+        fn bar(_: Self::Foo) {}
+    }
+    impl FooTrait for () {
+        type Foo = i32;
+    }
+    // NOTE: When checking the type of a function param, make sure it is not an alias with
+    // `AliasTyKind::Projection` before calling `TyCtxt::type_of` to find out what the actual type
+    // is. Because the associate ty could have no default, therefore would cause ICE, as demostrated
+    // in this test.
+    const fn alias_ty_is_projection(bar: <() as FooTrait>::Foo) {}
+}

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -186,3 +186,18 @@ mod issue12677 {
         }
     }
 }
+
+mod with_ty_alias {
+    trait FooTrait {
+        type Foo: std::fmt::Debug;
+        fn bar(_: Self::Foo) {}
+    }
+    impl FooTrait for () {
+        type Foo = i32;
+    }
+    // NOTE: When checking the type of a function param, make sure it is not an alias with
+    // `AliasTyKind::Projection` before calling `TyCtxt::type_of` to find out what the actual type
+    // is. Because the associate ty could have no default, therefore would cause ICE, as demostrated
+    // in this test.
+    fn alias_ty_is_projection(bar: <() as FooTrait>::Foo) {}
+}

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -261,5 +261,16 @@ help: make the function `const`
 LL |         pub const fn new(text: String) -> Self {
    |             +++++
 
-error: aborting due to 19 previous errors
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:202:5
+   |
+LL |     fn alias_ty_is_projection(bar: <() as FooTrait>::Foo) {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: make the function `const`
+   |
+LL |     const fn alias_ty_is_projection(bar: <() as FooTrait>::Foo) {}
+   |     +++++
+
+error: aborting due to 20 previous errors