Load missing type of impl associated constant from trait definition

compiler/rustc_hir_typeck/src/lib.rs

@@ -192,10 +192,20 @@ fn typeck_with_fallback<'tcx>(
         check_fn(&mut fcx, fn_sig, None, decl, def_id, body, tcx.features().unsized_fn_params);
     } else {
         let expected_type = if let Some(&hir::Ty { kind: hir::TyKind::Infer, span, .. }) = body_ty {
-            Some(fcx.next_ty_var(TypeVariableOrigin {
-                kind: TypeVariableOriginKind::TypeInference,
-                span,
-            }))
+            if let Some(item) = tcx.opt_associated_item(def_id.into())
+                && let ty::AssocKind::Const = item.kind
+                && let ty::ImplContainer = item.container
+                && let Some(trait_item) = item.trait_item_def_id
+            {
+                let args =
+                    tcx.impl_trait_ref(item.container_id(tcx)).unwrap().instantiate_identity().args;
+                Some(tcx.type_of(trait_item).instantiate(tcx, args))
+            } else {
+                Some(fcx.next_ty_var(TypeVariableOrigin {
+                    kind: TypeVariableOriginKind::TypeInference,
+                    span,
+                }))
+            }
         } else if let Node::AnonConst(_) = node {
             match tcx.parent_hir_node(id) {
                 Node::Ty(&hir::Ty { kind: hir::TyKind::Typeof(ref anon_const), .. })

tests/ui/consts/missing_assoc_const_type.rs

@@ -0,0 +1,28 @@
+//! Test that we compute the right type for associated constants
+//! of impls, even if the type is missing. We know it from the trait
+//! declaration after all.
+
+trait Range {
+    const FIRST: u8;
+    const LAST: u8;
+}
+
+struct TwoDigits;
+impl Range for TwoDigits {
+    const FIRST:  = 10;
+    //~^ ERROR: missing type for `const` item
+    const LAST: u8 = 99;
+}
+
+const fn digits(x: u8) -> usize {
+    match x {
+        TwoDigits::FIRST..=TwoDigits::LAST => 0,
+        0..=9 | 100..=255 => panic!(),
+    }
+}
+
+const FOOMP: [(); {
+    digits(42)
+}] = [];
+
+fn main() {}

tests/ui/consts/missing_assoc_const_type.stderr

@@ -0,0 +1,8 @@
+error: missing type for `const` item
+  --> $DIR/missing_assoc_const_type.rs:12:17
+   |
+LL |     const FIRST:  = 10;
+   |                 ^ help: provide a type for the associated constant: `u8`
+
+error: aborting due to 1 previous error
+

tests/ui/consts/missing_assoc_const_type2.rs

@@ -0,0 +1,21 @@
+//! Test that we compute the right type for associated constants
+//! of impls, even if the type is missing. We know it from the trait
+//! declaration after all.
+
+trait Range {
+    const FIRST: u8;
+    const LAST: u8;
+}
+
+struct TwoDigits;
+impl Range for TwoDigits {
+    const FIRST:  = 10;
+    //~^ ERROR: missing type
+    const LAST: u8 = 99;
+}
+
+const FOOMP: [(); {
+    TwoDigits::FIRST as usize
+}] = [(); 10];
+
+fn main() {}

tests/ui/consts/missing_assoc_const_type2.stderr

@@ -0,0 +1,8 @@
+error: missing type for `const` item
+  --> $DIR/missing_assoc_const_type2.rs:12:17
+   |
+LL |     const FIRST:  = 10;
+   |                 ^ help: provide a type for the associated constant: `u8`
+
+error: aborting due to 1 previous error
+

tests/ui/typeck/typeck_type_placeholder_item.stderr

@@ -546,6 +546,15 @@ help: use type parameters instead
 LL |         fn fn_test10<T>(&self, _x : T) { }
    |                     +++             ~
 
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated constants
+  --> $DIR/typeck_type_placeholder_item.rs:194:14
+   |
+LL |     const D: _ = 42;
+   |              ^
+   |              |
+   |              not allowed in type signatures
+   |              help: replace with the correct type: `i32`
+
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
   --> $DIR/typeck_type_placeholder_item.rs:217:31
    |
@@ -574,19 +583,7 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
   --> $DIR/typeck_type_placeholder_item.rs:209:14
    |
 LL |     const D: _ = 42;
-   |              ^
-   |              |
-   |              not allowed in type signatures
-   |              help: replace with the correct type: `i32`
-
-error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated constants
-  --> $DIR/typeck_type_placeholder_item.rs:194:14
-   |
-LL |     const D: _ = 42;
-   |              ^
-   |              |
-   |              not allowed in type signatures
-   |              help: replace with the correct type: `i32`
+   |              ^ not allowed in type signatures
 
 error[E0046]: not all trait items implemented, missing: `F`
   --> $DIR/typeck_type_placeholder_item.rs:200:1