Try to only suggest traits that can be implemented for method calls.

src/librustc_typeck/check/method/suggest.rs

@@ -34,6 +34,11 @@ pub fn report_error<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                               method_name: ast::Name,
                               error: MethodError)
 {
+    // avoid suggestions when we don't know what's going on.
+    if ty::type_is_error(rcvr_ty) {
+        return
+    }
+
     match error {
         MethodError::NoMatch(static_sources, out_of_scope_traits) => {
             let cx = fcx.tcx();
@@ -135,7 +140,7 @@ pub type AllTraitsVec = Vec<TraitInfo>;
 
 fn suggest_traits_to_import<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                       span: Span,
-                                      _rcvr_ty: Ty<'tcx>,
+                                      rcvr_ty: Ty<'tcx>,
                                       method_name: ast::Name,
                                       valid_out_of_scope_traits: Vec<ast::DefId>)
 {
@@ -165,16 +170,32 @@ fn suggest_traits_to_import<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
         return
     }
 
-    // there's no implemented traits, so lets suggest some traits to implement
+    let type_is_local = type_derefs_to_local(fcx, span, rcvr_ty);
+
+    // there's no implemented traits, so lets suggest some traits to
+    // implement, by finding ones that have the method name, and are
+    // legal to implement.
     let mut candidates = all_traits(fcx.ccx)
-        .filter(|info| trait_method(tcx, info.def_id, method_name).is_some())
+        .filter(|info| {
+            // we approximate the coherence rules to only suggest
+            // traits that are legal to implement by requiring that
+            // either the type or trait is local. Multidispatch means
+            // this isn't perfect (that is, there are cases when
+            // implementing a trait would be legal but is rejected
+            // here).
+            (type_is_local || ast_util::is_local(info.def_id))
+                && trait_method(tcx, info.def_id, method_name).is_some()
+        })
         .collect::<Vec<_>>();
 
     if candidates.len() > 0 {
         // sort from most relevant to least relevant
         candidates.sort_by(|a, b| a.cmp(b).reverse());
         candidates.dedup();
 
+        // FIXME #21673 this help message could be tuned to the case
+        // of a type parameter: suggest adding a trait bound rather
+        // than implementing.
         let msg = format!(
             "methods from traits can only be called if the trait is implemented and in scope; \
              the following {traits_define} a method `{name}`, \
@@ -194,6 +215,39 @@ fn suggest_traits_to_import<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
     }
 }
 
+/// Checks whether there is a local type somewhere in the chain of
+/// autoderefs of `rcvr_ty`.
+fn type_derefs_to_local<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
+                                  span: Span,
+                                  rcvr_ty: Ty<'tcx>) -> bool {
+    check::autoderef(fcx, span, rcvr_ty, None,
+                     check::UnresolvedTypeAction::Ignore, check::NoPreference,
+                     |&: ty, _| {
+        let is_local = match ty.sty {
+            ty::ty_enum(did, _) | ty::ty_struct(did, _) => ast_util::is_local(did),
+
+            ty::ty_trait(ref tr) => ast_util::is_local(tr.principal_def_id()),
+
+            ty::ty_param(_) => true,
+
+            // the user cannot implement traits for unboxed closures, so
+            // there's no point suggesting anything at all, local or not.
+            ty::ty_closure(..) => return Some(false),
+
+            // everything else (primitive types etc.) is effectively
+            // non-local (there are "edge" cases, e.g. (LocalType,), but
+            // the noise from these sort of types is usually just really
+            // annoying, rather than any sort of help).
+            _ => false
+        };
+        if is_local {
+            Some(true)
+        } else {
+            None
+        }
+    }).2.unwrap_or(false)
+}
+
 #[derive(Copy)]
 pub struct TraitInfo {
     pub def_id: ast::DefId,

src/test/auxiliary/no_method_suggested_traits.rs

@@ -10,6 +10,9 @@
 
 pub use reexport::Reexported;
 
+pub struct Foo;
+pub enum Bar { X }
+
 pub mod foo {
     pub trait PubPub {
         fn method(&self) {}

src/test/compile-fail/method-suggestion-no-duplication.rs

@@ -10,7 +10,9 @@
 
 // issue #21405
 
-fn foo<F>(f: F) where F: FnMut(usize) {}
+struct Foo;
+
+fn foo<F>(f: F) where F: FnMut(Foo) {}
 
 fn main() {
     foo(|s| s.is_empty());

src/test/compile-fail/no-method-suggested-traits.rs

@@ -12,6 +12,9 @@
 
 extern crate no_method_suggested_traits;
 
+struct Foo;
+enum Bar { X }
+
 mod foo {
     trait Bar {
         fn method(&self) {}
@@ -25,23 +28,48 @@ mod foo {
 }
 
 fn main() {
+    // test the values themselves, and autoderef.
+
+
     1u32.method();
     //~^ HELP following traits are implemented but not in scope, perhaps add a `use` for one of them
     //~^^ ERROR does not implement
     //~^^^ HELP `foo::Bar`
     //~^^^^ HELP `no_method_suggested_traits::foo::PubPub`
+    std::rc::Rc::new(&mut Box::new(&1u32)).method();
+    //~^ HELP following traits are implemented but not in scope, perhaps add a `use` for one of them
+    //~^^ ERROR does not implement
+    //~^^^ HELP `foo::Bar`
+    //~^^^^ HELP `no_method_suggested_traits::foo::PubPub`
 
     'a'.method();
     //~^ ERROR does not implement
     //~^^ HELP the following trait is implemented but not in scope, perhaps add a `use` for it:
     //~^^^ HELP `foo::Bar`
+    std::rc::Rc::new(&mut Box::new(&'a')).method();
+    //~^ ERROR does not implement
+    //~^^ HELP the following trait is implemented but not in scope, perhaps add a `use` for it:
+    //~^^^ HELP `foo::Bar`
 
     1i32.method();
     //~^ ERROR does not implement
     //~^^ HELP the following trait is implemented but not in scope, perhaps add a `use` for it:
     //~^^^ HELP `no_method_suggested_traits::foo::PubPub`
+    std::rc::Rc::new(&mut Box::new(&1i32)).method();
+    //~^ ERROR does not implement
+    //~^^ HELP the following trait is implemented but not in scope, perhaps add a `use` for it:
+    //~^^^ HELP `no_method_suggested_traits::foo::PubPub`
 
-    1u64.method();
+    Foo.method();
+    //~^ ERROR does not implement
+    //~^^ HELP following traits define a method `method`, perhaps you need to implement one of them
+    //~^^^ HELP `foo::Bar`
+    //~^^^^ HELP `no_method_suggested_traits::foo::PubPub`
+    //~^^^^^ HELP `no_method_suggested_traits::reexport::Reexported`
+    //~^^^^^^ HELP `no_method_suggested_traits::bar::PubPriv`
+    //~^^^^^^^ HELP `no_method_suggested_traits::qux::PrivPub`
+    //~^^^^^^^^ HELP `no_method_suggested_traits::quz::PrivPriv`
+    std::rc::Rc::new(&mut Box::new(&Foo)).method();
     //~^ ERROR does not implement
     //~^^ HELP following traits define a method `method`, perhaps you need to implement one of them
     //~^^^ HELP `foo::Bar`
@@ -55,8 +83,52 @@ fn main() {
     //~^ ERROR does not implement
     //~^^ HELP the following trait defines a method `method2`, perhaps you need to implement it
     //~^^^ HELP `foo::Bar`
-    1u64.method3();
+    std::rc::Rc::new(&mut Box::new(&1u64)).method2();
     //~^ ERROR does not implement
-    //~^^ HELP the following trait defines a method `method3`, perhaps you need to implement it
+    //~^^ HELP the following trait defines a method `method2`, perhaps you need to implement it
+    //~^^^ HELP `foo::Bar`
+
+    no_method_suggested_traits::Foo.method2();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method2`, perhaps you need to implement it
+    //~^^^ HELP `foo::Bar`
+    std::rc::Rc::new(&mut Box::new(&no_method_suggested_traits::Foo)).method2();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method2`, perhaps you need to implement it
+    //~^^^ HELP `foo::Bar`
+    no_method_suggested_traits::Bar::X.method2();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method2`, perhaps you need to implement it
+    //~^^^ HELP `foo::Bar`
+    std::rc::Rc::new(&mut Box::new(&no_method_suggested_traits::Bar::X)).method2();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method2`, perhaps you need to implement it
+    //~^^^ HELP `foo::Bar`
+
+    Foo.method3();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method3`, perhaps you need to implement it
+    //~^^^ HELP `no_method_suggested_traits::foo::PubPub`
+    std::rc::Rc::new(&mut Box::new(&Foo)).method3();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method3`, perhaps you need to implement it
+    //~^^^ HELP `no_method_suggested_traits::foo::PubPub`
+    Bar::X.method3();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method3`, perhaps you need to implement it
     //~^^^ HELP `no_method_suggested_traits::foo::PubPub`
+    std::rc::Rc::new(&mut Box::new(&Bar::X)).method3();
+    //~^ ERROR does not implement
+    //~^^ HELP following trait defines a method `method3`, perhaps you need to implement it
+    //~^^^ HELP `no_method_suggested_traits::foo::PubPub`
+
+    // should have no help:
+    1us.method3(); //~ ERROR does not implement
+    std::rc::Rc::new(&mut Box::new(&1us)).method3(); //~ ERROR does not implement
+    no_method_suggested_traits::Foo.method3();  //~ ERROR does not implement
+    std::rc::Rc::new(&mut Box::new(&no_method_suggested_traits::Foo)).method3();
+    //~^ ERROR does not implement
+    no_method_suggested_traits::Bar::X.method3();  //~ ERROR does not implement
+    std::rc::Rc::new(&mut Box::new(&no_method_suggested_traits::Bar::X)).method3();
+    //~^ ERROR does not implement
 }