feat: trait bounds const opt out (#4056)

Author: calebcartwright

rustfmt-core/rustfmt-lib/src/types.rs

@@ -523,7 +523,12 @@ impl Rewrite for ast::GenericBound {
                     ast::TraitBoundModifier::Maybe => poly_trait_ref
                         .rewrite(context, shape.offset_left(1)?)
                         .map(|s| format!("?{}", s)),
-                    _ => unimplemented!(),
+                    ast::TraitBoundModifier::MaybeConst => poly_trait_ref
+                        .rewrite(context, shape.offset_left(7)?)
+                        .map(|s| format!("?const {}", s)),
+                    ast::TraitBoundModifier::MaybeConstMaybe => poly_trait_ref
+                        .rewrite(context, shape.offset_left(8)?)
+                        .map(|s| format!("?const ?{}", s)),
                 };
                 rewrite.map(|s| if has_paren { format!("({})", s) } else { s })
             }

rustfmt-core/rustfmt-lib/tests/source/type.rs

@@ -139,3 +139,30 @@ fn foo(a: SomeLongComplexType, b: SomeOtherLongComplexType) -> Box<Future<Item =
 }
 
 type MyFn = fn(a: SomeLongComplexType, b: SomeOtherLongComplexType,) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>>;
+
+// Const opt-out
+
+trait T: ?   const  Super {}
+
+const fn maybe_const<S: ?   const    T>() -> i32 { <S as T>::CONST }
+
+struct S<T:?  const   ?  Sized>(std::marker::PhantomData<T>);
+
+impl ?    const T {}
+
+fn trait_object() -> &'static dyn ?  const T { &S }
+
+fn i(_: impl IntoIterator<Item = Box<dyn ?    const    T>>) {}
+
+fn apit(_: impl ?const T) {}
+
+fn rpit() -> impl ?  const T { S }
+
+pub struct Foo<T: Trait>(T);
+impl<T:   ?  const Trait> Foo<T> {
+    fn new(t: T) -> Self {
+        // not calling methods on `t`, so we opt out of requiring
+        // `<T as Trait>` to have const methods via `?const`
+        Self(t)
+    }
+}

rustfmt-core/rustfmt-lib/tests/target/type.rs

@@ -144,3 +144,36 @@ type MyFn = fn(
     a: SomeLongComplexType,
     b: SomeOtherLongComplexType,
 ) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>>;
+
+// Const opt-out
+
+trait T: ?const Super {}
+
+const fn maybe_const<S: ?const T>() -> i32 {
+    <S as T>::CONST
+}
+
+struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+
+impl ?const T {}
+
+fn trait_object() -> &'static dyn ?const T {
+    &S
+}
+
+fn i(_: impl IntoIterator<Item = Box<dyn ?const T>>) {}
+
+fn apit(_: impl ?const T) {}
+
+fn rpit() -> impl ?const T {
+    S
+}
+
+pub struct Foo<T: Trait>(T);
+impl<T: ?const Trait> Foo<T> {
+    fn new(t: T) -> Self {
+        // not calling methods on `t`, so we opt out of requiring
+        // `<T as Trait>` to have const methods via `?const`
+        Self(t)
+    }
+}