fix: Implement Expactation::Castable and add a test case for it

crates/hir-ty/src/infer.rs

@@ -1024,7 +1024,7 @@ impl<'a> InferenceContext<'a> {
 pub(crate) enum Expectation {
     None,
     HasType(Ty),
-    // Castable(Ty), // rustc has this, we currently just don't propagate an expectation for casts
+    Castable(Ty),
     RValueLikeUnsized(Ty),
 }
 
@@ -1077,6 +1077,7 @@ impl Expectation {
         match self {
             Expectation::None => Expectation::None,
             Expectation::HasType(t) => Expectation::HasType(table.resolve_ty_shallow(t)),
+            Expectation::Castable(t) => Expectation::Castable(table.resolve_ty_shallow(t)),
             Expectation::RValueLikeUnsized(t) => {
                 Expectation::RValueLikeUnsized(table.resolve_ty_shallow(t))
             }
@@ -1086,17 +1087,18 @@ impl Expectation {
     fn to_option(&self, table: &mut unify::InferenceTable<'_>) -> Option<Ty> {
         match self.resolve(table) {
             Expectation::None => None,
-            Expectation::HasType(t) |
+            Expectation::HasType(t)
-            // Expectation::Castable(t) |
+            | Expectation::Castable(t)
-            Expectation::RValueLikeUnsized(t) => Some(t),
+            | Expectation::RValueLikeUnsized(t) => Some(t),
         }
     }
 
     fn only_has_type(&self, table: &mut unify::InferenceTable<'_>) -> Option<Ty> {
         match self {
             Expectation::HasType(t) => Some(table.resolve_ty_shallow(t)),
-            // Expectation::Castable(_) |
+            Expectation::Castable(_) | Expectation::RValueLikeUnsized(_) | Expectation::None => {
-            Expectation::RValueLikeUnsized(_) | Expectation::None => None,
+                None
+            }
         }
     }
 

crates/hir-ty/src/infer/expr.rs

@@ -610,9 +610,9 @@ impl<'a> InferenceContext<'a> {
                 }
             }
             Expr::Cast { expr, type_ref } => {
-                // FIXME: propagate the "castable to" expectation (and find a test case that shows this is necessary)
-                let _inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
                 let cast_ty = self.make_ty(type_ref);
+                let _inner_ty =
+                    self.infer_expr_inner(*expr, &Expectation::Castable(cast_ty.clone()));
                 // FIXME check the cast...
                 cast_ty
             }

crates/hir-ty/src/tests/simple.rs

@@ -3200,3 +3200,34 @@ fn func() {
     "#,
     );
 }
+#[test]
+fn castable_to() {
+    check_infer(
+        r#"
+//- minicore: sized
+#[lang = "owned_box"]
+pub struct Box<T: ?Sized> {
+    inner: *mut T,
+}
+impl<T> Box<T> {
+    fn new(t: T) -> Self { loop {} }
+}
+
+fn func() {
+    let x = Box::new([]) as Box<[i32; 0]>;
+}
+"#,
+        expect![[r#"
+            99..100 't': T
+            113..124 '{ loop {} }': Box<T>
+            115..122 'loop {}': !
+            120..122 '{}': ()
+            138..184 '{     ...0]>; }': ()
+            148..149 'x': Box<[i32; 0]>
+            152..160 'Box::new': fn new<[i32; 0]>([i32; 0]) -> Box<[i32; 0]>
+            152..164 'Box::new([])': Box<[i32; 0]>
+            152..181 'Box::n...2; 0]>': Box<[i32; 0]>
+            161..163 '[]': [i32; 0]
+        "#]],
+    );
+}