2024 dependency_on_unit_never_type_fallback doesn't display during 2024 edition migration

[ehuss]

I'm not entirely certain what is wrong with this particular code, but it fails in the 2024 edition without generating any warnings.

[email protected]
https://crater-reports.s3.amazonaws.com/pr-132712/try%23da25749bf5e6ba5ed862ff361c19afff2a986b2d/reg/screenshots-0.8.10/log.txt

The error after updating to 2024 is:

error[E0277]: the trait bound `!: ReadAll` is not satisfied
   --> src/linux/wayland_screenshot.rs:70:11
    |
70  |     proxy.method_call(
    |           ^^^^^^^^^^^ the trait `ReadAll` is not implemented for `!`
    |
    = help: the following other types implement trait `ReadAll`:
              ()
              (A, B)
              (A, B, C)
              (A, B, C, D)
              (A, B, C, D, E)
              (A, B, C, D, E, F)
              (A, B, C, D, E, F, G)
              (A, B, C, D, E, F, G, H)
            and 28 others
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
    = help: did you intend to use the type `()` here instead?
note: required by a bound in `Proxy::<'a, C>::method_call`
   --> /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/dbus-0.9.7/src/blocking.rs:396:35
    |
396 |     pub fn method_call<'i, 'm, R: ReadAll, A: AppendAll, I: Into<Interface<'i>>, M: Into<Member<'m>>>(&self, i: I, m: M, args: A) -> Result<R, Error> {
    |                                   ^^^^^^^ required by this bound in `Proxy::<'a, C>::method_call`

error[E0277]: the trait bound `!: ReadAll` is not satisfied
   --> src/linux/wayland_screenshot.rs:126:11
    |
126 |     proxy.method_call(
    |           ^^^^^^^^^^^ the trait `ReadAll` is not implemented for `!`
    |
    = help: the following other types implement trait `ReadAll`:
              ()
              (A, B)
              (A, B, C)
              (A, B, C, D)
              (A, B, C, D, E)
              (A, B, C, D, E, F)
              (A, B, C, D, E, F, G)
              (A, B, C, D, E, F, G, H)
            and 28 others
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
    = help: did you intend to use the type `()` here instead?
note: required by a bound in `Proxy::<'a, C>::method_call`
   --> /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/dbus-0.9.7/src/blocking.rs:396:35
    |
396 |     pub fn method_call<'i, 'm, R: ReadAll, A: AppendAll, I: Into<Interface<'i>>, M: Into<Member<'m>>>(&self, i: I, m: M, args: A) -> Result<R, Error> {
    |                                   ^^^^^^^ required by this bound in `Proxy::<'a, C>::method_call`

The relevant types are from the dbus crate.

I would expect that if this is related to dependency_on_unit_never_type_fallback that it should generate a warning before migrating to 2024.

Meta

rustc --version --verbose:

rustc 1.84.0-nightly (143ce0920 2024-11-10)
binary: rustc
commit-hash: 143ce0920a2307b19831160a01f06f107610f1b2
commit-date: 2024-11-10
host: aarch64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.3

[compiler-errors]

the never type fallback lints are not marked as 2024 future compat, so they're not fixed in edition 2024 even though I implemented a machine applicable fix in #132383.

[compiler-errors]

I'll fix that.

[WaffleLapkin]

@ehuss the code does generate warnings:

warning: this function depends on never type fallback being `()`
  --> src/linux/wayland_capture.rs:47:1
   |
47 | / fn org_gnome_shell_screenshot(
48 | |     conn: &Connection,
49 | |     x: i32,
50 | |     y: i32,
51 | |     width: i32,
52 | |     height: i32,
53 | | ) -> XCapResult<RgbaImage> {
   | |__________________________^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
   = help: specify the types explicitly
note: in edition 2024, the requirement `!: ReadAll` will fail
  --> src/linux/wayland_capture.rs:71:11
   |
71 |     proxy.method_call(
   |           ^^^^^^^^^^^
   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
help: use `()` annotations to avoid fallback changes
   |
71 |     proxy.method_call::<_, _, (), _>(
   |                      +++++++++++++++

warning: this function depends on never type fallback being `()`
   --> src/linux/wayland_capture.rs:84:1
    |
84  | / fn org_freedesktop_portal_screenshot(
85  | |     conn: &Connection,
86  | |     x: i32,
87  | |     y: i32,
88  | |     width: i32,
89  | |     height: i32,
90  | | ) -> XCapResult<RgbaImage> {
    | |__________________________^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
    = help: specify the types explicitly
note: in edition 2024, the requirement `!: ReadAll` will fail
   --> src/linux/wayland_capture.rs:127:11
    |
127 |     proxy.method_call(
    |           ^^^^^^^^^^^
help: use `()` annotations to avoid fallback changes
    |
127 |     proxy.method_call::<_, _, (), _>(
    |                      +++++++++++++++

Not sure why they are not picked up with cargo fix --edition though 🤔 Upd: looks like @compiler-errors already figured it out.

[ehuss]

Sorry, somehow I missed that the warnings were being generated on the previous edition. I'm not sure how. I thought I ran cargo check to verify.

If @compiler-errors wants to update it to be marked as the 2024 edition, I think that should be fine. It's not absolutely necessary since our policy is that the code should be warning-free before migrating.

[compiler-errors]

🤔 yeah, but would cargo fix --edition also apply this lint unless it's attached to the edition? I guess not.

[ehuss]

Correct, it will not apply the lint (and it is suppressed so it is not displayed at all) with running cargo fix --edition.

It would probably be best to mark it with the edition so at least the warning message tells you that it changes in 2024 instead of the nonspecific "in a future release!"

[ehuss]

Oh, sorry, I just now read the conversation on #132931.

Given the comment in #132931 (review), then I would suggest we just close this.

Sorry for the confusion on my part.

[WaffleLapkin]

@ehuss I think it still makes sense to mark it with the edition -- we just need to add an option to mark it as both edition and future release (i.e. #132931 (comment)). I'll work on it tomorrow.