Is automatic insertion of type inference placeholders possible?

[mqudsi]

Would it be possible to automatically insert _ type inference placeholders where none (or not enough) type placeholders are provided, or does this conflict with any other tokens the parser has to deal with?

i.e. can ["foo", "bar"].iter().collect::Vec::() be turned into ["foo", "bar"].iter().collect::Vec<_>::() automatically?

and given a Struct<X,Y>, can Struct<Foo> automatically be understood as Struct<Foo, _>?

These would both simplify boilerplate considerably, and the latter could obviate the need to revisit old code when a type is expanded with an additional generic type parameter that can be inferred from its usage.

[petrochenkov]

It is possible, but there was unresolved issues about how exactly to infer the missing types in presence of default type parameters, so _s are required so far to be future-compatible with any possible solution.
The relevant issue is #27336, but there wasn't much progress recently :(