Rewrote check_method_receiver and ExplicitSelf, got a borrow checker error

Rewrote ExplicitSelf, adding a new `Other` variant for arbitrary self
types. It’s a bit more sophisticated now, and checks for type equality,
so you have to pass the type context and param env as arguments.
There’s a borrow-checker error here that I have to fix

Rewrote check_method_receiver, so it acts as if arbitrary self types
are allowed, and then checks for ExplicitSelf::Other at the end and
disallows it unless the feature is present.

Author: mikeyhew

src/librustc_typeck/astconv.rs

@@ -1407,7 +1407,8 @@ impl<'a, 'gcx, 'tcx> Bounds<'tcx> {
 pub enum ExplicitSelf<'tcx> {
     ByValue,
     ByReference(ty::Region<'tcx>, hir::Mutability),
-    ByBox
+    ByBox,
+    Other
 }
 
 impl<'tcx> ExplicitSelf<'tcx> {
@@ -1431,36 +1432,27 @@ impl<'tcx> ExplicitSelf<'tcx> {
     /// }
     /// ```
     ///
-    /// To do the check we just count the number of "modifiers"
-    /// on each type and compare them. If they are the same or
-    /// the impl has more, we call it "by value". Otherwise, we
-    /// look at the outermost modifier on the method decl and
-    /// call it by-ref, by-box as appropriate. For method1, for
-    /// example, the impl type has one modifier, but the method
-    /// type has two, so we end up with
-    /// ExplicitSelf::ByReference.
-    pub fn determine(untransformed_self_ty: Ty<'tcx>,
-                     self_arg_ty: Ty<'tcx>)
-                     -> ExplicitSelf<'tcx> {
-        fn count_modifiers(ty: Ty) -> usize {
-            match ty.sty {
-                ty::TyRef(_, mt) => count_modifiers(mt.ty) + 1,
-                ty::TyAdt(def, _) if def.is_box() => count_modifiers(ty.boxed_ty()) + 1,
-                _ => 0,
-            }
-        }
+    pub fn determine<'a, 'gcx>(
+        tcx: TyCtxt<'a, 'gcx, 'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        self_ty: Ty<'a>,
+        self_arg_ty: Ty<'a>
+    ) -> ExplicitSelf<'tcx>
+    {
+        use self::ExplicitSelf::*;
 
-        let impl_modifiers = count_modifiers(untransformed_self_ty);
-        let method_modifiers = count_modifiers(self_arg_ty);
+        tcx.infer_ctxt().enter(|infcx| {
+            let can_eq = |expected, actual| {
+                let cause = traits::ObligationCause::dummy();
+                infcx.at(&cause, param_env).eq(expected, actual).is_ok()
+            };
 
-        if impl_modifiers >= method_modifiers {
-            ExplicitSelf::ByValue
-        } else {
             match self_arg_ty.sty {
-                ty::TyRef(r, mt) => ExplicitSelf::ByReference(r, mt.mutbl),
-                ty::TyAdt(def, _) if def.is_box() => ExplicitSelf::ByBox,
-                _ => ExplicitSelf::ByValue,
+                _ if can_eq(self_arg_ty, self_ty) => ByValue,
+                ty::TyRef(region, ty::TypeAndMut { ty, mutbl}) if can_eq(ty, self_ty) => ByReference(region, mutbl),
+                ty::TyAdt(def, _) if def.is_box() && can_eq(self_arg_ty.boxed_ty(), self_ty) => ByBox,
+                _ => Other
             }
-        }
+        })
     }
 }

src/librustc_typeck/check/compare_method.rs

@@ -503,7 +503,9 @@ fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             ty::TraitContainer(_) => tcx.mk_self_type()
         };
         let self_arg_ty = *tcx.fn_sig(method.def_id).input(0).skip_binder();
-        match ExplicitSelf::determine(untransformed_self_ty, self_arg_ty) {
+        let param_env = ty::ParamEnv::empty(Reveal::All);
+
+        match ExplicitSelf::determine(tcx, param_env, untransformed_self_ty, self_arg_ty) {
             ExplicitSelf::ByValue => "self".to_string(),
             ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_string(),
             ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_string(),

src/librustc_typeck/check/wfcheck.rs

@@ -13,6 +13,7 @@ use check::{Inherited, FnCtxt};
 use constrained_type_params::{identify_constrained_type_params, Parameter};
 
 use hir::def_id::DefId;
+use rustc::infer::InferOk;
 use rustc::traits::{self, ObligationCauseCode};
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::util::nodemap::{FxHashSet, FxHashMap};
@@ -451,8 +452,7 @@ impl<'a, 'gcx> CheckTypeWellFormedVisitor<'a, 'gcx> {
                                          method: &ty::AssociatedItem,
                                          self_ty: Ty<'tcx>)
     {
-        // check that the type of the method's receiver matches the
-        // method's first parameter.
+        // check that the method has a valid receiver type, given the type `Self`
         debug!("check_method_receiver({:?}, self_ty={:?})",
                method, self_ty);
 
@@ -470,47 +470,29 @@ impl<'a, 'gcx> CheckTypeWellFormedVisitor<'a, 'gcx> {
 
         let self_arg_ty = sig.inputs()[0];
 
-        if fcx.tcx.sess.features.borrow().arbitrary_self_types {
-            let cause = fcx.cause(span, ObligationCauseCode::MethodReceiver);
+        let cause = fcx.cause(span, ObligationCauseCode::MethodReceiver);
+        let at = fcx.at(&cause, fcx.param_env);
+        let mut autoderef = fcx.autoderef(span, self_arg_ty);
 
-            let mut autoderef = fcx.autoderef(span, self_arg_ty);
-            while let Some((potential_self_ty, _)) = autoderef.next() {
+        loop {
+            if let Some((potential_self_ty, _)) = autoderef.next() {
                 debug!("check_method_receiver: potential self type `{:?}` to match `{:?}`", potential_self_ty, self_ty);
 
-                // there's gotta be a more idiomatic way of checking if types are equal than this
-                if let Some(mut err) = fcx.demand_eqtype_with_origin(&cause, self_ty, potential_self_ty) {
-                    err.cancel();
-                    continue;
-                } else {
-                    // we found a type that matches `self_ty`
+                if let Ok(InferOk { obligations, value: () }) = at.eq(self_ty, potential_self_ty) {
+                    fcx.register_predicates(obligations);
                     autoderef.finalize();
-                    return;
+                    break;
                 }
-            }
 
-            span_err!(fcx.tcx.sess, span, E0307, "invalid `self` type: {:?}", self_arg_ty);
-            return;
+            } else {
+                span_err!(fcx.tcx.sess, span, E0307, "invalid self type: {:?}", self_arg_ty);
+            }
         }
 
-        let rcvr_ty = match ExplicitSelf::determine(self_ty, self_arg_ty) {
-            ExplicitSelf::ByValue => self_ty,
-            ExplicitSelf::ByReference(region, mutbl) => {
-                fcx.tcx.mk_ref(region, ty::TypeAndMut {
-                    ty: self_ty,
-                    mutbl,
-                })
+        if let ExplicitSelf::Other = ExplicitSelf::determine(fcx.tcx, fcx.param_env, self_ty, self_arg_ty) {
+            if !fcx.tcx.sess.features.borrow().arbitrary_self_types {
+                fcx.tcx.sess.span_err(span, "Arbitrary `self` types are experimental");
             }
-            ExplicitSelf::ByBox => fcx.tcx.mk_box(self_ty)
-        };
-        let rcvr_ty = fcx.normalize_associated_types_in(span, &rcvr_ty);
-        let rcvr_ty = fcx.liberate_late_bound_regions(method.def_id,
-                                                      &ty::Binder(rcvr_ty));
-
-        debug!("check_method_receiver: receiver ty = {:?}", rcvr_ty);
-
-        let cause = fcx.cause(span, ObligationCauseCode::MethodReceiver);
-        if let Some(mut err) = fcx.demand_eqtype_with_origin(&cause, rcvr_ty, self_arg_ty) {
-            err.emit();
         }
     }