Split elided_lifetime_in_paths into finer-grained lints

[shepmaster]

Description

Converts the existing elided_lifetime_in_paths lint into three smaller pieces:

• hidden_lifetimes_in_input_paths — fires for fn(ContainsLifetime) -> ...
• hidden_lifetimes_in_output_paths — fires for fn(...) -> ContainsLifetime
• hidden_lifetimes_in_type_paths — fires for many other usages of ContainsLifetime, such as in a static or in a turbofish

A new group hidden_lifetimes_in_paths is created with the three smaller lints and places that use the old elided_lifetime_in_paths name are updated to match.

#![allow(dead_code)]
#![warn(hidden_lifetimes_in_paths)]

struct ContainsLifetime<'a>(&'a u8);

fn inputs_and_outputs(v: ContainsLifetime) -> ContainsLifetime { v }
static CL: ContainsLifetime = ContainsLifetime(&42);

fn main() {}

warning: paths containing hidden lifetime parameters are deprecated
 --> a.rs:6:26
  |
6 | fn inputs_and_outputs(v: ContainsLifetime) -> ContainsLifetime { v }
  |                          ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(hidden_lifetimes_in_input_paths)]` implied by `#[warn(hidden_lifetimes_in_paths)]`
help: indicate the anonymous lifetime
  |
6 | fn inputs_and_outputs(v: ContainsLifetime<'_>) -> ContainsLifetime { v }
  |                                          ++++

warning: paths containing hidden lifetime parameters are deprecated
 --> a.rs:6:47
  |
6 | fn inputs_and_outputs(v: ContainsLifetime) -> ContainsLifetime { v }
  |                                               ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(hidden_lifetimes_in_output_paths)]` implied by `#[warn(hidden_lifetimes_in_paths)]`
help: indicate the anonymous lifetime
  |
6 | fn inputs_and_outputs(v: ContainsLifetime) -> ContainsLifetime<'_> { v }
  |                                                               ++++

warning: paths containing hidden lifetime parameters are deprecated
 --> a.rs:7:12
  |
7 | static CL: ContainsLifetime = ContainsLifetime(&42);
  |            ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(hidden_lifetimes_in_type_paths)]` implied by `#[warn(hidden_lifetimes_in_paths)]`
help: indicate the anonymous lifetime
  |
7 | static CL: ContainsLifetime<'_> = ContainsLifetime(&42);
  |                            ++++

Background

In general, we want to discourage function signatures like fn (&T) -> ContainsLifetime, fn (ContainsLifetime) -> &T , and fn (ContainsLifetime) -> ContainsLifetime as it is not obvious that a lifetime flows through the function and back out as there is no visual indication that ContainsLifetime has a lifetime generic (such types are not usually literally called "contains lifetime" 😃).

In #120808 (comment) (and multiple followup comments, e.g. #120808 (comment)), the lang team decided on a plan where we introduce a new warn-by-default lint mismatched_lifetime_syntaxes (superseding elided_named_lifetimes) which helps insure consistency between input and output lifetime syntaxes and then split elided_lifetime_in_paths into three parts. The combination of these lints should be enough to accomplish the original goal.

History

Note that this PR has substantially changed from its original version. Check the (copious) comments below as well as the edit history of this message.