Upgrade `#![feature(…)]` in non-root modules from lint warning `unused_attributes` to a hard error (requires crater run)

[nelhage]

Putting #![feature(…)] in a mod is silently ignored.

I tried this code (minimized version):

https://github.com/nelhage/feature-in-mod

I'm writing some toy Rust code that needs to perform pattern-matching over ASTs (that use Box to reference children), and wanted to use box patterns. On advice of https://doc.rust-lang.org/1.14.0/book/box-syntax-and-patterns.html I added #![feature(box_patterns)] to the top of my .rs file (which is a mod inside the larger executable).

I expected to see this happen: Either:

• The feature() directive would work and I would get access to the feature
• The feature() directive would yield an error

Instead, this happened:
The feature() directive had no observable effect at all; cargo build output did not change after adding the line, either to show a new error or to suppress the "box pattern syntax is experimental" error.

Meta

rustc --version --verbose:

rustc 1.25.0 (84203cac6 2018-03-25)
binary: rustc
commit-hash: 84203cac67e65ca8640b8392348411098c856985
commit-date: 2018-03-25
host: x86_64-unknown-linux-gnu
release: 1.25.0
LLVM version: 6.0

[ExpHP]

I see the following:

   Compiling feature v0.1.0 (file:///home/lampam/cpp/throwaway/feature)
warning: unused attribute
 --> src/m.rs:1:1
  |
1 | #![feature(box_patterns)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_attributes)] on by default

warning: crate-level attribute should be in the root module
 --> src/m.rs:1:1
  |
1 | #![feature(box_patterns)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^

    Finished dev [unoptimized + debuginfo] target(s) in 0.28 secs

[ExpHP]

ahhhhh, I see. When I try your example, I get:

   Compiling feature-in-mod v0.1.0 (file:///home/lampam/cpp/throwaway/feature/feature-in-mod)
error[E0658]: box pattern syntax is experimental (see issue #29641)
 --> src/transform.rs:7:18
  |
7 |         AST::Add(box AST::Add(ll, lr), r) => AST::Add(ll, Box::new(AST::Add(lr, r))),
  |                  ^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(box_patterns)] to the crate attributes to enable

error: aborting due to previous error

So the existing warnings rather fail to be helpful, since failing to put a feature flag in the right place tends to cause errors (and those take precedence).

[abonander]

The existing warning is just the generic unused_attributes lint. It should be a hard error for #![feature(..)] to appear anywhere else than the crate root but this would be a breaking change now.

[alex]

Does a breaking change mean (a) fixing this is impossible, (b) it'd require a release where it's a warning first (it's already a warning though!), (c) it could potentially be done if a crater run confirmed nothing would be broken by it?

[Mark-Simulacrum]

I would argue that we can probably make this change with a crater run. I doubt it would affect anyone who is using stable today; it seems unlikely to have unused feature annotations in submodules that have no effect and be on stable.

[abonander]

If the crate is compiling already then the #![feature] attribute will be warned about by the unused_attributes lint so it's unlikely that this change would break any compiling crate unless the author is ignoring warnings as well.