Document "Limitations" and "Workarounds".

[taiki-e]

pin-project-lite has some limitations and workarounds that are only described in the issue or code. It would be nice if we could provide them as part of the documentation.

Issues:

• no support for multiple trait bounds Support multiple trait bounds on generics and where clause #2 (workaround: Support multiple trait bounds on generics and where clause #2 (comment))
• no interoperability with other field attributes Cannot interoperate with other field attributes #3 (workaround: Cannot interoperate with other field attributes #3 (comment))
• Cannot interoperate with other field attributes #3 (comment)
• missing_debug_implementations does not lint on pin projected type #52

Code comments:

pin-project-lite/src/lib.rs

Lines 401 to 408 in 416be96

	// limitations:
	// * no support for tuple structs and tuple variant (wontfix).
	// * no support for multiple trait/lifetime bounds.
	// * no support for `Self` in where clauses. (wontfix)
	// * no support for overlapping lifetime names. (wontfix)
	// * no interoperability with other field attributes.
	// * no useful error messages. (wontfix)
	// etc...

pin-project-lite/tests/test.rs

Lines 577 to 584 in 6f7a40d

	// pin_project! {
	// pub struct S4<
	// 'a,
	// 'b: 'a, // <-----
	// > {
	// f: &'a &'b (),
	// }
	// }