Breaking change from 2.1.3 to 2.2.1 due to new RuleType bound on Error types

[dennis-hamester]

Describe the bug
In commit 5b807be, Error<R> was changed to Error<R: RuleType>, which is a breaking change. As far as I can see, this also extends to ErrorVariant.

To Reproduce
Compiles with Pest 2.1.3, but not with 2.2.1:

fn inspect_pest_error<R>(err: &pest::error::Error<R>) {
}

To make it compile with 2.2.1, a bound on R must be added:

fn inspect_pest_error<R: pest::RuleType>(err: &pest::error::Error<R>) {
}

Additional context
For me personally, this is not a huge deal. My code didn't require any bigger changes to make it compile again. But it can certainly be surprising.

[tomtau]

@dennis-hamester thanks for the report.

@pest-parser/maintainers @pest-parser/triage any thoughts on how to resolve it? Revert that commit and release a new version? Or keep it as it is and be more careful next time (E.g. with https://crates.io/crates/cargo-semver-checks)? Or release it as v3.0.0 (despite it not being the "mythical pest3" with the new grammar etc.)?

[nathanielknight]

I think leaving it as is and adding a check is probably reasonable, perhaps with a note on the release?

[glyn]

I could go either way. I agree it's not a huge deal. Maybe a comment on the offending type would help?

Note that, ultimately, we'll probably need to go to v3.0.0 before "mythical pest3", so the sooner we get that over and done with, the better?

[CAD97]

This is simple enough to fix, though. Just remove the trait bound from the type. Not bounding on the struct is best practice.

[CAD97]

5b807be also removes the fmt::Display impl for cfg(not(feature = "std")), so I think #694 is warranted to keep the best-effort no-std support.

[tomtau]

The patch looks all right, so I guess we'll yank 2.2.1 and release 2.2.2 after merging that?

[CAD97]

👍 sounds accurate to me

[tomtau]

Actually, the release may be 2.3.0 due to new features since 2.2.x. anyway, probably good to run cargo semver-checks check-release against 2.1.3 before going with it

[tomtau]

FYI I haven't checked all crates, but pest on the current branch seems fine:

$ cargo semver-checks check-release --current current-all-features.json --baseline 2.1.3-all-features.json 
    Starting 18 checks, version 2.1.3 -> 2.2.1 (minor change)
        PASS [   0.029s]       major        auto_trait_impl_removed
        PASS [   0.018s]       major        derive_trait_impl_removed
        PASS [   0.002s]       major        enum_missing
        PASS [   0.000s]       major        enum_repr_c_removed
        PASS [   0.000s]       major        enum_repr_int_changed
        PASS [   0.000s]       major        enum_repr_int_removed
        PASS [   0.005s]       major        enum_variant_added
        PASS [   0.005s]       major        enum_variant_missing
        PASS [   0.001s]       major        function_missing
        PASS [   0.019s]       major        inherent_method_missing
        PASS [   0.005s]       major        sized_impl_removed
        PASS [   0.000s]       major        struct_marked_non_exhaustive
        PASS [   0.002s]       major        struct_missing
        PASS [   0.001s]       major        struct_pub_field_missing
        PASS [   0.000s]       major        struct_repr_c_removed
        PASS [   0.000s]       major        struct_repr_transparent_removed
        PASS [   0.000s]       major        unit_struct_changed_kind
        PASS [   0.005s]       major        variant_marked_non_exhaustive
     Summary [   0.092s] 18 checks run: 18 passed, 0 skipped
$ cargo semver-checks check-release --current current-default-features.json --baseline 2.1.3-default-features.json 
    Starting 18 checks, version 2.1.3 -> 2.2.1 (minor change)
        PASS [   0.023s]       major        auto_trait_impl_removed
        PASS [   0.016s]       major        derive_trait_impl_removed
        PASS [   0.002s]       major        enum_missing
        PASS [   0.000s]       major        enum_repr_c_removed
        PASS [   0.001s]       major        enum_repr_int_changed
        PASS [   0.000s]       major        enum_repr_int_removed
        PASS [   0.004s]       major        enum_variant_added
        PASS [   0.004s]       major        enum_variant_missing
        PASS [   0.001s]       major        function_missing
        PASS [   0.015s]       major        inherent_method_missing
        PASS [   0.005s]       major        sized_impl_removed
        PASS [   0.000s]       major        struct_marked_non_exhaustive
        PASS [   0.002s]       major        struct_missing
        PASS [   0.001s]       major        struct_pub_field_missing
        PASS [   0.000s]       major        struct_repr_c_removed
        PASS [   0.000s]       major        struct_repr_transparent_removed
        PASS [   0.000s]       major        unit_struct_changed_kind
        PASS [   0.004s]       major        variant_marked_non_exhaustive
     Summary [   0.078s] 18 checks run: 18 passed, 0 skipped