Implement suggestions for traits to import.

[huonw]

For a call like foo.bar() where the method bar can't be resolved,
the compiler will search for traits that have methods with name bar to
give a more informative error, providing a list of possibilities.

Closes #7643.

[rust-highfive]

r? @pcwalton

(rust_highfive has picked a reviewer for you, use r? to override)

[huonw]

r? @nikomatsakis

This... isn't quite ready yet. The test's full output is:

no-method-suggested-traits.rs:23:10: 23:18 error: type `u32` does not implement any method in scope named `method`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: methods from traits can only be called if the trait is implemented and in scope; the following traits define a method `method`:
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:17:5: 19:6 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:17     trait Bar { //~ HELP `foo::Bar`
no-method-suggested-traits.rs:18         fn method(&self);
no-method-suggested-traits.rs:19     }
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #2: `no_method_suggested_traits::foo::PubPub`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #3: `no_method_suggested_traits::bar::PubPriv`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #4: `no_method_suggested_traits::qux::PrivPub`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #5: `no_method_suggested_traits::quz::PrivPriv`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #6: `no_method_suggested_traits::reexport::Reexported`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #7: `no_method_suggested_traits::reexport::Reexported`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
error: aborting due to previous error

In particular: it lists private traits too.

[huonw]

(Also, the first help's wording is awkward.)

Would appreciate any guidance for either.

[nagisa]

Repeating the span for each help message sounds redundant.

[flaper87]

you may want to filter out private methods here ( meth.vis )

[huonw]

It's a trait, so there's no distinction.

[flaper87]

ah, you're right.

[huonw]

Improved it slightly, but it has the reexported path backwards (i.e. filtering by visibility removed the pub use'd path, not the actually-private one... meaning the listed path isn't actually a valid import):

no-method-suggested-traits.rs:23:10: 23:18 error: type `u32` does not implement any method in scope named `method`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: methods from traits can only be called if the trait is implemented and in scope; the following traits define a method `method`:
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:17:5: 19:6 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:17     trait Bar { //~ HELP `foo::Bar`
no-method-suggested-traits.rs:18         fn method(&self);
no-method-suggested-traits.rs:19     }
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #2: `no_method_suggested_traits::foo::PubPub`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:23:10: 23:18 help: candidate #3: `no_method_suggested_traits::reexport::Reexported`
no-method-suggested-traits.rs:23     1u32.method();
                                          ^~~~~~~~
error: aborting due to previous error

[nikomatsakis]

goodness but do we need some helpers for these kinds of things...

[nikomatsakis]

Also, let's pull this code into its own file method/suggest.rs or something.

[huonw]

Updated with tweaks and better organisation. The test output now looks like:

no-method-suggested-traits.rs:22:10: 22:18 error: type `u32` does not implement any method in scope named `method`
no-method-suggested-traits.rs:22     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:22:10: 22:18 help: methods from traits can only be called if the trait is implemented and in scope; the following traits define a method `method`:
no-method-suggested-traits.rs:22     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:18:6: 18:6 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:22:18: 22:18 help: candidate #2: `no_method_suggested_traits::foo::PubPub`
no-method-suggested-traits.rs:22:18: 22:18 help: candidate #3: `no_method_suggested_traits::bar::PubPriv`
no-method-suggested-traits.rs:22:18: 22:18 help: candidate #4: `no_method_suggested_traits::qux::PrivPub`
no-method-suggested-traits.rs:22:18: 22:18 help: candidate #5: `no_method_suggested_traits::quz::PrivPriv`
no-method-suggested-traits.rs:22:18: 22:18 help: candidate #6: `no_method_suggested_traits::reexport::Reexported`
no-method-suggested-traits.rs:22:18: 22:18 help: candidate #7: `no_method_suggested_traits::reexport::Reexported`
error: aborting due to previous error

[huonw]

I've made a few attempts to e.g. extract whether a non-local trait can be accessed from the current crate but I have not been able to achieve it in a correct-enough way as yet.

[nikomatsakis]

@huonw do you find this is a big issue in practice? I would have expected we could kind of just suggest everything and the user can sort it out (and/or leave the screening of unreachable traits as a fixme).

[nikomatsakis]

anyway, I think we could land the code as is, but with some relatively simple tweaks we could at least indicate whether the trait is implemented -- so we could do something like this, only print out the traits that are implemented but not imported, and if none are implemented, suggest traits to implement. That's probably good enough.

[huonw]

Updated again, now with is-trait-implemented filtering.

no-method-suggested-traits.rs:28:10: 28:18 error: type `u32` does not implement any method in scope named `method`
no-method-suggested-traits.rs:28     1u32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:28:18: 28:18 help: methods from traits can only be called if the trait is in scope; the following traits are implemented and define a method `method`:
no-method-suggested-traits.rs:28:18: 28:18 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:28:18: 28:18 help: candidate #2: `no_method_suggested_traits::foo::PubPub`
no-method-suggested-traits.rs:34:9: 34:17 error: type `char` does not implement any method in scope named `method`
no-method-suggested-traits.rs:34     'a'.method();
                                         ^~~~~~~~
no-method-suggested-traits.rs:34:17: 34:17 help: methods from traits can only be called if the trait is in scope; the following trait is implemented and defines a method `method`:
no-method-suggested-traits.rs:34:17: 34:17 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:39:10: 39:18 error: type `i32` does not implement any method in scope named `method`
no-method-suggested-traits.rs:39     1i32.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:39:18: 39:18 help: methods from traits can only be called if the trait is in scope; the following trait is implemented and defines a method `method`:
no-method-suggested-traits.rs:39:18: 39:18 help: candidate #1: `no_method_suggested_traits::foo::PubPub`
no-method-suggested-traits.rs:44:10: 44:18 error: type `u64` does not implement any method in scope named `method`
no-method-suggested-traits.rs:44     1u64.method();
                                          ^~~~~~~~
no-method-suggested-traits.rs:44:18: 44:18 help: methods from traits can only be called if the trait is implemented and in scope; no such traits are but the following traits define a method `method`:
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #2: `no_method_suggested_traits::foo::PubPub`
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #3: `no_method_suggested_traits::bar::PubPriv`
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #4: `no_method_suggested_traits::qux::PrivPub`
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #5: `no_method_suggested_traits::quz::PrivPriv`
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #6: `no_method_suggested_traits::reexport::Reexported`
no-method-suggested-traits.rs:44:18: 44:18 help: candidate #7: `no_method_suggested_traits::reexport::Reexported`
no-method-suggested-traits.rs:54:10: 54:19 error: type `u64` does not implement any method in scope named `method2`
no-method-suggested-traits.rs:54     1u64.method2();
                                          ^~~~~~~~~
no-method-suggested-traits.rs:54:19: 54:19 help: methods from traits can only be called if the trait is implemented and in scope; no such traits are but the following trait defines a method `method2`:
no-method-suggested-traits.rs:54:19: 54:19 help: candidate #1: `foo::Bar`
no-method-suggested-traits.rs:58:10: 58:19 error: type `u64` does not implement any method in scope named `method3`
no-method-suggested-traits.rs:58     1u64.method3();
                                          ^~~~~~~~~
no-method-suggested-traits.rs:58:19: 58:19 help: methods from traits can only be called if the trait is implemented and in scope; no such traits are but the following trait defines a method `method3`:
no-method-suggested-traits.rs:58:19: 58:19 help: candidate #1: `no_method_suggested_traits::foo::PubPub`
error: aborting due to 6 previous errors

[nikomatsakis]

Nice, I'll read over the code shortly. I think we should modify the error message to make it clearer what it means to be "in scope" -- like we should say "try adding use foo::bar or something"

[nikomatsakis]

So I had a few nits on huonw@0a55aac but @huonw said he wouldn't be around much over the next two days. So I'm giving r+ anyhow. For the record:

• we could probably factor out the recursive call in a cleaner way than using a mutable boolean
• I think the error messages could be rephrased to be more explicit. For example, "the following traits are implemented but not in scope; perhaps add a use statement" or something like that.

Nonetheless, I think this is a big step up in clarify, so I'd like to land.