2018-Specific Changes - The Edition Guide

# 2018-Specific Changes

The following is a summary of changes that only apply to code compiled with the 2018 edition compared to the 2015 edition.

- Path changes:
  - Paths in `use` declarations work the same as other paths.
  - Paths starting with `::` must be followed with an external crate.
  - Paths in `pub(in path)` visibility modifiers must start with `crate`, `self`, or `super`.
- Anonymous trait function parameters are not allowed.
  - Trait function parameters may use any irrefutable pattern when the function has a body.
- `dyn` is a strict keyword, in 2015 it is a weak keyword.
- `async`, `await`, and `try` are reserved keywords.
- The following lints are now deny by default:
  - tyvar_behind_raw_pointer

## Cargo

- If there is a target definition in a `Cargo.toml` manifest, it no longer automatically disables automatic discovery of other targets.
- Target paths of the form `src/{target_name}.rs` are no longer inferred for targets where the `path` field is not set.
- `cargo install` for the current directory is no longer allowed, you must specify `cargo install --path .` to install the current package.