Improve error reporting for #![feature(...)] in the wrong place

[martindemello]

I spent a while wondering why I was getting this error

error[E0658]: use of unstable library feature 'array_map'
  --> src/board.rs:67:17
   |
67 |         offsets.map(|(dx, dy)| {
   |                 ^^^
   |
   = note: see issue #75243 <https://github.com/rust-lang/rust/issues/75243> for more information
   = help: add `#![feature(array_map)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.

even when I added !#[feature(array_map)] to the top of my file, and even when the same thing was working fine in main.rs. I finally found this stackoverflow post and this reddit thread that said if I were using cargo build I had to add the directive to lib.rs instead, after which everything worked.

I'm not sure where in the docs the best place for that would be (I had already read the section on attributes and crates in "rust by example" and not found the relevant hint), but it would be nice if it could be surfaced in the error message too.

[ehuss]

I think there are two issues here. One is that the error message could provide more information. It does say the crate attributes which can only appear in the root of the crate. However, it would probably be clearer if it could tell you where that is.

The other issue is that the feature attribute position is not validated (it seems to be silently ignored anywhere but the root). That should be an error (or a future-incompatible warning).

[estebank]

Split off the second part in the comment above to its own ticket.