Adding doc(hidden) non-defaulted item is permissible if other doc(hidden) non-defaulted items exist

[jswrenn]

Which lint or lints are the issue

trait_associated_type_added

Known issues that might be causing this

• Is the flagged item defined in another crate, or a re-export of such an item? (#638)

Steps to reproduce the bug with the above code

We are experiencing this issue with the KnownLayout trait in zerocopy, specifically from this commit: google/zerocopy@b573d83

In isolation, this bug may be reproduced by defining a trait with doc(hidden) non-defaulted items:

trait KnownLayout {
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;
}

Then, add an associated type:

  trait KnownLayout {
+     type MaybeUninit;
  
      #[doc(hidden)]
      fn only_derive_is_allowed_to_implement_this_trait()
      where
          Self: Sized;
  }

Actual Behaviour

cargo-semver-checks registers this delta as a breaking change.

Expected Behaviour

Because MyTrait already contains doc(hidden) non-defaulted items, its consumers have opted out of SemVer stability. In practice, this situation arises when the trait is documented as being implementable by a particular proc macro (e.g., zerocopy's KnownLayout, so no breakage occurs for any users who are respecting this contract.

Verbose Lint Output

$ cargo semver-checks check-release --package zerocopy --only-explicit-features --features __internal_use_only_features_that_work_on_stable --verbose
     Parsing zerocopy v0.8.10 (current)
             Features: __internal_use_only_features_that_work_on_stable
    Updating crates.io index
     Locking 6 packages to latest compatible versions
 Documenting zerocopy v0.8.10 (/home/ubuntu/projects/zerocopy)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.92s
      Parsed [   1.189s] (current)
     Parsing zerocopy v0.8.10 (baseline)
             Features: __internal_use_only_features_that_work_on_stable
   Compiling proc-macro2 v1.0.89
   Compiling unicode-ident v1.0.14
   Compiling zerocopy v0.8.10
   Compiling quote v1.0.37
   Compiling syn v2.0.87
   Compiling zerocopy-derive v0.8.10
 Documenting zerocopy v0.8.10
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.00s
      Parsed [   4.324s] (baseline)
    Checking zerocopy v0.8.10 -> v0.8.10 (no change)
    Starting 94 checks, 0 unnecessary on 72 threads
        PASS [   0.018s]       major        auto_trait_impl_removed
        PASS [   0.002s]       major        constructible_struct_adds_field
        PASS [   0.006s]       major        constructible_struct_adds_private_field
        PASS [   0.003s]       major        constructible_struct_changed_type
        PASS [   0.011s]       major        declarative_macro_missing
        PASS [   0.026s]       major        derive_trait_impl_removed
        PASS [   0.002s]       major        enum_marked_non_exhaustive
        PASS [   0.003s]       major        enum_missing
        PASS [   0.002s]       minor        enum_must_use_added
        PASS [   0.004s]       major        enum_no_repr_variant_discriminant_changed
        PASS [   0.002s]       major        enum_now_doc_hidden
        PASS [   0.005s]       major        enum_repr_int_changed
        PASS [   0.003s]       major        enum_repr_int_removed
        PASS [   0.003s]       major        enum_repr_transparent_removed
        PASS [   0.003s]       major        enum_struct_variant_field_added
        PASS [   0.003s]       major        enum_struct_variant_field_missing
        PASS [   0.003s]       major        enum_struct_variant_field_now_doc_hidden
        PASS [   0.003s]       major        enum_tuple_variant_changed_kind
        PASS [   0.004s]       major        enum_tuple_variant_field_added
        PASS [   0.003s]       major        enum_tuple_variant_field_missing
        PASS [   0.003s]       major        enum_tuple_variant_field_now_doc_hidden
        PASS [   0.003s]       major        enum_unit_variant_changed_kind
        PASS [   0.004s]       major        enum_variant_added
        PASS [   0.003s]       major        enum_variant_marked_non_exhaustive
        PASS [   0.003s]       major        enum_variant_missing
        PASS [   0.003s]       major        exported_function_changed_abi
        PASS [   0.004s]       major        function_abi_no_longer_unwind
        PASS [   0.003s]       major        function_changed_abi
        PASS [   0.002s]       major        function_const_removed
        PASS [   0.003s]       major        function_export_name_changed
        PASS [   0.003s]       major        function_missing
        PASS [   0.004s]       minor        function_must_use_added
        PASS [   0.003s]       major        function_now_doc_hidden
        PASS [   0.003s]       major        function_parameter_count_changed
        PASS [   0.003s]       major        function_unsafe_added
        PASS [   0.005s]       major        inherent_associated_const_now_doc_hidden
        PASS [   0.006s]       major        inherent_associated_pub_const_missing
        PASS [   0.015s]       major        inherent_method_const_removed
        PASS [   0.010s]       major        inherent_method_missing
        PASS [   0.011s]       minor        inherent_method_must_use_added
        PASS [   0.010s]       major        inherent_method_now_doc_hidden
        PASS [   0.017s]       major        inherent_method_unsafe_added
        PASS [   0.019s]       major        method_parameter_count_changed
        PASS [   0.002s]       major        module_missing
        PASS [   0.004s]       major        non_exhaustive_struct_changed_type
        PASS [   0.002s]       major        pub_module_level_const_missing
        PASS [   0.003s]       major        pub_module_level_const_now_doc_hidden
        PASS [   0.002s]       major        pub_static_missing
        PASS [   0.002s]       major        pub_static_mut_now_immutable
        PASS [   0.002s]       major        pub_static_now_doc_hidden
        PASS [   0.004s]       major        repr_c_removed
        PASS [   0.004s]       major        repr_packed_added
        PASS [   0.005s]       major        repr_packed_removed
        PASS [   0.008s]       major        sized_impl_removed
        PASS [   0.004s]       major        struct_marked_non_exhaustive
        PASS [   0.003s]       major        struct_missing
        PASS [   0.004s]       minor        struct_must_use_added
        PASS [   0.004s]       major        struct_now_doc_hidden
        PASS [   0.003s]       major        struct_pub_field_missing
        PASS [   0.003s]       major        struct_pub_field_now_doc_hidden
        PASS [   0.004s]       major        struct_repr_transparent_removed
        PASS [   0.004s]       major        struct_with_no_pub_fields_changed_type
        PASS [   0.004s]       major        struct_with_pub_fields_changed_type
        PASS [   0.004s]       major        trait_added_supertrait
        PASS [   0.004s]       major        trait_associated_const_added
        PASS [   0.002s]       major        trait_associated_const_default_removed
        PASS [   0.003s]       major        trait_associated_const_now_doc_hidden
        FAIL [   0.003s]       major        trait_associated_type_added
        PASS [   0.002s]       major        trait_associated_type_default_removed
        PASS [   0.003s]       major        trait_associated_type_now_doc_hidden
        PASS [   0.003s]       major        trait_method_added
        PASS [   0.005s]       major        trait_method_default_impl_removed
        PASS [   0.002s]       major        trait_method_missing
        PASS [   0.003s]       major        trait_method_now_doc_hidden
        PASS [   0.002s]       major        trait_method_unsafe_added
        PASS [   0.003s]       major        trait_method_unsafe_removed
        PASS [   0.003s]       major        trait_missing
        PASS [   0.003s]       minor        trait_must_use_added
        PASS [   0.003s]       major        trait_newly_sealed
        PASS [   0.002s]       major        trait_no_longer_object_safe
        PASS [   0.003s]       major        trait_now_doc_hidden
        PASS [   0.003s]       major        trait_removed_associated_constant
        PASS [   0.002s]       major        trait_removed_associated_type
        PASS [   0.004s]       major        trait_removed_supertrait
        PASS [   0.003s]       major        trait_unsafe_added
        PASS [   0.002s]       major        trait_unsafe_removed
        PASS [   0.001s]       major        tuple_struct_to_plain_struct
        PASS [   0.002s]       minor        type_marked_deprecated
        PASS [   0.002s]       major        union_field_missing
        PASS [   0.002s]       major        union_missing
        PASS [   0.004s]       minor        union_must_use_added
        PASS [   0.002s]       major        union_now_doc_hidden
        PASS [   0.003s]       major        union_pub_field_now_doc_hidden
        PASS [   0.003s]       major        unit_struct_changed_kind
     Checked [   0.032s] 94 checks: 93 pass, 1 fail, 0 warn, 0 skip

--- failure trait_associated_type_added: non-sealed public trait added associated type without default value ---

Description:
A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/trait_associated_type_added.ron

Failed in:
  trait associated type zerocopy::KnownLayout::MaybeUninit in file /home/ubuntu/projects/zerocopy/src/lib.rs:742

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [   5.608s] zerocopy

Generated System Information

System information:

Software version

cargo-semver-checks 0.36.0

Operating system

Linux 5.15.0-1064-aws

Command-line

/home/ubuntu/.cargo/bin/cargo-semver-checks semver-checks --bugreport 

cargo version

> cargo -V 
cargo 1.83.0-nightly (80d82ca22 2024-09-27)

Compile time information

• Profile: release
• Target triple: x86_64-unknown-linux-gnu
• Family: unix
• OS: linux
• Architecture: x86_64
• Pointer width: 64
• Endian: little
• CPU features: fxsr,sse,sse2
• Host: x86_64-unknown-linux-gnu

Cargo build configuration:

[build]
rustflags = ["--cfg=zerocopy_unstable"]
rustdocflags = ["--cfg=zerocopy_unstable"]

[doc]
browser = ["/home/ubuntu/.vscode-server/cli/servers/Stable-4849ca9bdf9666755eb463db297b69e5385090e3/server/bin/helpers/browser.sh"]

Build Configuration

No response

Additional Context

google/zerocopy#2055 (comment)

[obi1kenobi]

Thanks! This is an instance of a broader issue around sealed traits and SemVer. It's definitely fixable, though will require some (relatively intricate) new analysis.

I made #999 for the broader problem.