Make SimpleTypeName support types defined inside unnamed constants

[davidpdrsn]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

I just discovered that if you have a type defined inside an unnamed constant like

trait A {
    type T;
}

struct Foo;

const _: () = {
    struct Bar<T>(T);

    impl A for Foo {
        type T = Bar<String>;
    }
};

then type_name::<<Foo as A>::T>() will be crate::_::Bar. Such code is common with macros because you can import stuff without polluting the surrounding namespace.

However syn does not support parsing such type names 😕 Stripping the ::_ part is probably the easiest solution.

[bnjbvr]

Arguably that's an error in syn that doesn't handle this specific syntax, right? Wonder if we should report this error upstream...

[davidpdrsn]

Arguably that's an error in syn that doesn't handle this specific syntax, right? Wonder if we should report this error upstream...

Yeah I agree. I'll file an issue tomorrow.

[davidpdrsn]

syn issue dtolnay/syn#1378