Disallow Self in type param defaults of ADTs

src/librustc_resolve/diagnostics.rs

@@ -354,6 +354,17 @@ impl<'a> Resolver<'a> {
                     span, "defaulted type parameters cannot be forward declared".to_string());
                 err
             }
+            ResolutionError::SelfInTyParamDefault => {
+                let mut err = struct_span_err!(
+                    self.session,
+                    span,
+                    E0735,
+                    "type parameters cannot use `Self` in their defaults"
+                );
+                err.span_label(
+                    span, "`Self` in type parameter default".to_string());
+                err
+            }
             ResolutionError::ConstParamDependentOnTypeParam => {
                 let mut err = struct_span_err!(
                     self.session,

src/librustc_resolve/error_codes.rs

@@ -8,9 +8,9 @@ Type parameter defaults can only use parameters that occur before them.
 Erroneous code example:
 
 ```compile_fail,E0128
-struct Foo<T=U, U=()> {
+struct Foo<T = U, U = ()> {
     field1: T,
-    filed2: U,
+    field2: U,
 }
 // error: type parameters with a default cannot use forward declared
 // identifiers
@@ -20,9 +20,9 @@ Since type parameters are evaluated in-order, you may be able to fix this issue
 by doing:
 
 ```
-struct Foo<U=(), T=U> {
+struct Foo<U = (), T = U> {
     field1: T,
-    filed2: U,
+    field2: U,
 }
 ```
 
@@ -1705,6 +1705,21 @@ fn const_id<T, const N: T>() -> T { // error: const parameter
 }
 ```
 "##,
+
+E0735: r##"
+Type parameter defaults cannot use `Self` on structs, enums, or unions.
+
+Erroneous code example:
+
+```compile_fail,E0735
+struct Foo<X = Box<Self>> {
+    field1: Option<X>,
+    field2: Option<X>,
+}
+// error: type parameters cannot use `Self` in their defaults.
+```
+"##,
+
 ;
 //  E0153, unused error code
 //  E0157, unused error code

src/librustc_resolve/late.rs

@@ -468,6 +468,19 @@ impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
                 }
             }));
 
+        // rust-lang/rust#61631: The type `Self` is essentially
+        // another type parameter. For ADTs, we consider it
+        // well-defined only after all of the ADT type parameters have
+        // been provided. Therefore, we do not allow use of `Self`
+        // anywhere in ADT type parameter defaults.
+        //
+        // (We however cannot ban `Self` for defaults on *all* generic
+        // lists; e.g. trait generics can usefully refer to `Self`,
+        // such as in the case of `trait Add<Rhs = Self>`.)
+        if self.current_self_item.is_some() { // (`Some` if + only if we are in ADT's generics.)
+            default_ban_rib.bindings.insert(Ident::with_dummy_span(kw::SelfUpper), Res::Err);
+        }
+
         // We also ban access to type parameters for use as the types of const parameters.
         let mut const_ty_param_ban_rib = Rib::new(TyParamAsConstParamTy);
         const_ty_param_ban_rib.bindings.extend(generics.params.iter()

src/librustc_resolve/lib.rs

@@ -214,6 +214,8 @@ enum ResolutionError<'a> {
     BindingShadowsSomethingUnacceptable(&'a str, Name, &'a NameBinding<'a>),
     /// Error E0128: type parameters with a default cannot use forward-declared identifiers.
     ForwardDeclaredTyParam, // FIXME(const_generics:defaults)
+    /// Error E0735: type parameters with a default cannot use `Self`
+    SelfInTyParamDefault,
     /// Error E0671: const parameter cannot depend on type parameter.
     ConstParamDependentOnTypeParam,
 }
@@ -1536,7 +1538,7 @@ impl<'a> Resolver<'a> {
             if let Some(res) = ribs[i].bindings.get(&rib_ident).cloned() {
                 // The ident resolves to a type parameter or local variable.
                 return Some(LexicalScopeBinding::Res(
-                    self.validate_res_from_ribs(i, res, record_used, path_span, ribs),
+                    self.validate_res_from_ribs(i, rib_ident, res, record_used, path_span, ribs),
                 ));
             }
 
@@ -2122,6 +2124,7 @@ impl<'a> Resolver<'a> {
     fn validate_res_from_ribs(
         &mut self,
         rib_index: usize,
+        rib_ident: Ident,
         res: Res,
         record_used: bool,
         span: Span,
@@ -2133,7 +2136,12 @@ impl<'a> Resolver<'a> {
         // An invalid forward use of a type parameter from a previous default.
         if let ForwardTyParamBanRibKind = all_ribs[rib_index].kind {
             if record_used {
-                self.report_error(span, ResolutionError::ForwardDeclaredTyParam);
+                let res_error = if rib_ident.name == kw::SelfUpper {
+                    ResolutionError::SelfInTyParamDefault
+                } else {
+                    ResolutionError::ForwardDeclaredTyParam
+                };
+                self.report_error(span, res_error);
             }
             assert_eq!(res, Res::Err);
             return Res::Err;

src/test/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.rs

@@ -0,0 +1,20 @@
+#![crate_type="lib"]
+
+// rust-lang/rust#61631: The use of `Self` in the defaults of generic
+// types in a *trait* definition are allowed.
+//
+// It *must* be accepted; we have used this pattern extensively since
+// Rust 1.0 (see e.g. `trait Add<Rhs=Self>`).
+trait Tnobound<P = Self> {}
+
+impl Tnobound for () { }
+
+// This variant is accepted at the definition site; but it will be
+// rejected at every possible usage site (such as the one immediately
+// below). Maybe one day we will attempt to catch it at the definition
+// site, but today this is accepted due to compiler implementation
+// limitations.
+trait Tsized<P: Sized = [Self]> {}
+
+impl Tsized for () {}
+//~^ ERROR the size for values of type `[()]` cannot be known at compilation time [E0277]

src/test/ui/generics/issue-61631-default-type-param-can-reference-self-in-trait.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `[()]` cannot be known at compilation time
+  --> $DIR/issue-61631-default-type-param-can-reference-self-in-trait.rs:19:6
+   |
+LL | impl Tsized for () {}
+   |      ^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `[()]`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/generics/issue-61631-default-type-param-cannot-reference-self.rs

@@ -0,0 +1,45 @@
+#![crate_type="lib"]
+
+// rust-lang/rust#61631: Uses of `Self` in the defaults of generic
+// types for ADT's are not allowed. We justify this because the `Self`
+// type could be considered the "final" type parameter, that is only
+// well-defined after all of the other type parameters on the ADT have
+// been instantiated.
+//
+// These were previously were ICE'ing at the usage point anyway (see
+// `demo_usages` below), so there should not be any backwards
+// compatibility concern.
+
+struct Snobound<'a, P = Self> { x: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+enum Enobound<'a, P = Self> { A, B(Option<&'a P>) }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+union Unobound<'a, P = Self> { x: i32, y: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+// Disallowing `Self` in defaults sidesteps need to check the bounds
+// on the defaults in cases like these.
+
+struct Ssized<'a, P: Sized = [Self]> { x: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+enum Esized<'a, P: Sized = [Self]> { A, B(Option<&'a P>) }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+union Usized<'a, P: Sized = [Self]> { x: i32, y: Option<&'a P> }
+//~^ ERROR type parameters cannot use `Self` in their defaults [E0735]
+
+fn demo_usages() {
+    // An ICE means you only get the error from the first line of the
+    // demo; comment each out to observe the other ICEs when trying
+    // this out on older versions of Rust.
+
+    let _ice: Snobound;
+    let _ice: Enobound;
+    let _ice: Unobound;
+    let _ice: Ssized;
+    let _ice: Esized;
+    let _ice: Usized;
+}

src/test/ui/generics/issue-61631-default-type-param-cannot-reference-self.stderr

@@ -0,0 +1,39 @@
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:13:25
+   |
+LL | struct Snobound<'a, P = Self> { x: Option<&'a P> }
+   |                         ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:16:23
+   |
+LL | enum Enobound<'a, P = Self> { A, B(Option<&'a P>) }
+   |                       ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:19:24
+   |
+LL | union Unobound<'a, P = Self> { x: i32, y: Option<&'a P> }
+   |                        ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:25:31
+   |
+LL | struct Ssized<'a, P: Sized = [Self]> { x: Option<&'a P> }
+   |                               ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:28:29
+   |
+LL | enum Esized<'a, P: Sized = [Self]> { A, B(Option<&'a P>) }
+   |                             ^^^^ `Self` in type parameter default
+
+error[E0735]: type parameters cannot use `Self` in their defaults
+  --> $DIR/issue-61631-default-type-param-cannot-reference-self.rs:31:30
+   |
+LL | union Usized<'a, P: Sized = [Self]> { x: i32, y: Option<&'a P> }
+   |                              ^^^^ `Self` in type parameter default
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0735`.