…r=Manishearth

Avoid or_fun_call for const_fn with no args

Based on rust-lang#5682 by @lzutao

This avoids a subset of false positives, specifically those related to `const fn`s that take no arguments.
For the rest, a much more involved fix would be needed, see rust-lang/rust-clippy#5682 (comment).

So this does *not* solve rust-lang#5658

changelog: Avoid triggering [`or_fun_call`] with `const fn`s that take no arguments.

Fixes rust-lang#5886

or_fn_call: ignore nullary associated const fns

The fix in rust-lang#5889 was missing associated functions.

changelog: Ignore also `const fn` methods in [`or_fun_call`]

Fixes rust-lang#5693

…thiaskrgr

Revert: or_fun_call should lint calls to `const fn`s with no args

The changes in rust-lang#5889 and rust-lang#5984 were done under the incorrect assumption that a `const fn` with no args was guaranteed to be evaluated at compile time.  A `const fn` is only guaranteed to be evaluated at compile time if it's inside a const context (the initializer of a `const` or a `static`).

See this [zulip conversation](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Common.20misconception.3A.20.60const.20fn.60.20and.20its.20effect.20on.20codegen/near/208059113) for more details on this common misconception.

Given that none of the linted methods by `or_fun_call` can be called in const contexts, the lint should make no exceptions.

changelog: [`or_fun_call`] lints again calls to `const fn` with no args