Show actual type parameters of generic functions and methods

[Veetaha]

The feature is to put actual type parameters (i.e. resolved, inferred ones) in onHover function signature hint. This lets you visually debug the type inference and find out what the formal parameters ended up to be for the specific function call expression.

I really miss the feature of inspecting the resolved types of a generic function from TypeScript where it looks the following way:

I'd like to bother implementing this if you don't mind.
@flodiebold @matklad I'd be grateful if you give any pointers to the code!

[flodiebold]

For simple function calls, you can already get these from the inference result; the callee itself will have a type like Ty::Apply(ApplicationTy { ctor: TypeCtor::FnDef(.. the fn id..), parameters: .. the type parameters .. }). You can see this in the tests as well:
https://github.com/rust-analyzer/rust-analyzer/blob/bf0c3ec67e0f2bc232924923ef3bd55aa3bd0cbd/crates/ra_hir_ty/src/tests/simple.rs#L1553

For method calls, we don't record the parameters yet. Here's the point where we've got the method type, so we should record it in the InferenceResult. The parameters will often contain variables, which will need to be resolved after inference (before you do this, you'll probably see lots of _ in the substs). This happens here for other types recorded during inference.

As for actually printing the function definition with parameters, there's two approaches: Trying to print the whole definition from HIR, or adapting the current way and trying to substitute type parameters in it syntactically. Both have their complications. Printing full definitions from HIR will require a bunch of code we don't have yet (basically we only have printing of types). For substituting parameters syntactically, we have some code, but it's not general enough yet.

[Veetaha]

Thanks for the pointers! I'll look into this, but it may take some time!