Auto merge of #645 - LeSeulArtichaut:visitor-controlflow, r=jackh726

Use `ControlFlow` in `Visitor`

This PR (almost) resolves one of the differences between rustc's `TypeVisitor` and chalk's `Visitor` by adopting rustc's design with `std::ops::ControlFlow`.

However as `std::ops::ControlFlow` is still unstable, this PR defines a duplicate of `ControlFlow` in `chalk_ir` along with a `try_break!` macro which behaves like `try!`.

chalk-derive/src/lib.rs

@@ -162,10 +162,7 @@ fn derive_any_visit(
 
     let body = s.each(|bi| {
         quote! {
-            result = result.combine(::chalk_ir::visit::Visit::visit_with(#bi, visitor, outer_binder));
-            if result.return_early() {
-                return result;
-            }
+            ::chalk_ir::try_break!(::chalk_ir::visit::Visit::visit_with(#bi, visitor, outer_binder));
         }
     });
 
@@ -178,19 +175,18 @@ fn derive_any_visit(
     s.bound_impl(
         quote!(::chalk_ir::visit:: #trait_name <#interner>),
         quote! {
-            fn #method_name <'i, R: ::chalk_ir::visit::VisitResult>(
+            fn #method_name <'i>(
                 &self,
-                visitor: &mut dyn ::chalk_ir::visit::Visitor < 'i, #interner, Result = R >,
+                visitor: &mut dyn ::chalk_ir::visit::Visitor < 'i, #interner >,
                 outer_binder: ::chalk_ir::DebruijnIndex,
-            ) -> R
+            ) -> ::chalk_ir::visit::ControlFlow<()>
             where
                 #interner: 'i
             {
-                let mut result = R::new();
                 match *self {
                     #body
                 }
-                return result;
+                ::chalk_ir::visit::ControlFlow::CONTINUE
             }
         },
     )

chalk-engine/src/lib.rs

@@ -58,7 +58,7 @@ use std::usize;
 
 use chalk_derive::{Fold, HasInterner, Visit};
 use chalk_ir::interner::Interner;
-use chalk_ir::visit::VisitResult;
+use chalk_ir::visit::ControlFlow;
 use chalk_ir::{
     AnswerSubst, Canonical, ConstrainedSubst, Constraint, DebruijnIndex, Goal, InEnvironment,
     Substitution,

chalk-ir/src/lib.rs

@@ -9,7 +9,7 @@ extern crate self as chalk_ir;
 use crate::cast::{Cast, CastTo, Caster};
 use crate::fold::shift::Shift;
 use crate::fold::{Fold, Folder, Subst, SuperFold};
-use crate::visit::{SuperVisit, Visit, VisitExt, VisitResult, Visitor};
+use crate::visit::{ControlFlow, SuperVisit, Visit, VisitExt, Visitor};
 use chalk_derive::{Fold, HasInterner, SuperVisit, Visit, Zip};
 use std::marker::PhantomData;