Confusing elided-lifetimes-in-paths lint in bridge

[sbrocket]

Building the demo with -W elided-lifetimes-in-paths results in the following:

warning: hidden lifetime parameters in types are deprecated
  --> demo/src/main.rs:22:63
   |
22 |         fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
   |                                                               ^- help: indicate the anonymous lifetime: `<'_>`
   |
   = note: requested on the command line with `-W elided-lifetimes-in-paths`

warning: 1 warning emitted

However, neither cxx::UniquePtr nor BlobstoreClient have a lifetime parameter, so of course trying to add one is an error (and of course the lint still triggers too):

warning: hidden lifetime parameters in types are deprecated
  --> demo/src/main.rs:22:66
   |
22 |         fn new_blobstore_client() -> UniquePtr<BlobstoreClient<'_>>;
   |                                                                  ^^- help: indicate the anonymous lifetime: `<'_>`
   |
   = note: requested on the command line with `-W elided-lifetimes-in-paths`

error[E0106]: missing lifetime specifier
  --> demo/src/main.rs:22:64
   |
22 |         fn new_blobstore_client() -> UniquePtr<BlobstoreClient<'_>>;
   |                                                                ^^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime
   |
22 |         fn new_blobstore_client() -> UniquePtr<BlobstoreClient<'static>>;
   |                                                                ^^^^^^^

error[E0107]: this struct takes 0 lifetime arguments but 1 lifetime argument was supplied
  --> demo/src/main.rs:22:48
   |
22 |         fn new_blobstore_client() -> UniquePtr<BlobstoreClient<'_>>;
   |                                                ^^^^^^^^^^^^^^^---- help: remove these generics
   |                                                |
   |                                                expected 0 lifetime arguments
   |
note: struct defined here, with 0 lifetime parameters
  --> demo/src/main.rs:20:14
   |
20 |         type BlobstoreClient;
   |              ^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors; 1 warning emitted

Looking at the expanded output with cargo expand, I don't see any clues as to what missing anonymous lifetime the compiler might be intending to flag here. It's unclear to me if this is a lint bug, if the compiler diagnostic is just confusing due to the cxx::bridge proc macro, or something else.

This is an issue for us because our codebase uses both -W rust_2018_idioms and -D warnings by default. Worse, elided-lifetimes-in-paths can only be allowed at the crate level (rust-lang/rust#71957) so we can't just allow it for the cxx::bridge module.

[sbrocket]

Ahhhh, I see what it's trying to indicate now. It's complaining about the generated UniquePtrTarget for BlobstoreClient impl, which includes a std::fmt::Formatter type that is missing the anonymous lifetime. Ok, that should be easily fixable.

[saitoy1]

I'm running into the same issue and wondering if there is any workaround for this?

@sbrocket
How did you go about fixing it?

[dtolnay]

I am not a fan of this lint and I don't think codebases should use it, but I've published support for it in 1.0.60.

[sbrocket]

Thanks for the fix. I’m not a fan of the lint either, but it’s grouped in ` rust_2018_idioms` and we get it from there, so it fits someone’s definition of “idiomatic” 2018 edition usage.

…

On Fri, Dec 10, 2021 at 1:39 PM David Tolnay ***@***.***> wrote: I am not a fan of this lint and I don't think codebases should use it, but I've published support for it in 1.0.60. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#907 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAEIBAN6JWAP3LEL2UCW2LUQJXP7ANCNFSM5BNVQRJA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.