[Evgenii (Vaiz)]

🤖 Clawpilot here! Posted automatically by Clawpilot (an AI agent), not by a human. Please verify before acting.

What this changes

#[event(...)] now rejects a field that holds a mutable reference. The check runs while the macro parses fields, before it generates code. The error points at the field type. Fixes AB#7757620.

What you get

A classified field was rejected before, but the error named generated code:

#[event("req")]
#[info]
struct Req<'a> {
    #[data_class(DC)]
    v: &'a mut u64,
}

Before — rustc reports E0507 against the attribute:

error[E0507]: cannot move out of value, as `self` is a captured variable in an `Fn` closure
  --> src/lib.rs:1:1
   |
1  | #[event("req")]
   | ^^^^^^^^^^^^^^^ move occurs because value has type `&mut u64`,
   |                 which does not implement the `Copy` trait
   = note: this error originates in the attribute macro `event`

After — the error names the field and the rule:

error: an event field cannot hold a mutable reference; event fields are read
       through `&self` when the event is visited, so they support only shared
       references. Use `&T` instead of `&mut T`
 --> src/lib.rs:5:8
  |
5 |     v: &'a mut u64,
  |        ^^^^^^^^^^^

An #[unredacted] field behaves differently. It compiled before this change:

#[event("req")]
#[info]
struct Req<'a> {
    #[unredacted]
    v: &'a mut u64,   // compiled before; rejected now
}

The generated body read it through self.v.clone(), which auto-dereferences to u64. The exclusive borrow bought nothing, because visit_fields takes &self and never writes. This case is now rejected as well.

Fix either case by removing mut: v: &'a u64.

Details

• is_mutable_reference_type inspects syn::TypeReference::mutability. It recurses like strip_reference, so &&mut T is caught.
• reject_mutable_reference runs in parse_field_def. It also reads the inner type of an Option<T> field, and names the optional form in the message.

Effects

• #[unredacted] v: &'a mut u64 compiled before this change and does not compile after it. An owner of such a field must write &'a u64.
• A classified or default-redaction &mut T field failed before with E0507 spanned on #[event(...)]. It now fails with a message spanned on the field type.
• Option<&mut T> is rejected, with wording that names the optional form.
• Shared references are unchanged. &'a T, &'a &'a T and Option<&'a T> expand exactly as before, which a snapshot pins.
• #[derive(Enrichment)] is deliberately unchanged. into_entries(self) takes self by value, so a &mut T enrichment field still compiles. Only #[event(...)] gained the check.
• No new dependency and no Cargo.lock change.
• The observed public API is unchanged. Both new functions are internal to observed_macros_impl.

Validation

Manual before-and-after repro, not committed: three event structs were compiled against the macro with the check disabled, then enabled. This confirmed that the #[unredacted] case compiled before the fix, that the classified case produced E0507 against the attribute, and that a shared-reference classified field still compiles after the fix.

[Copilot]

🟢 Approval recommended

The change is narrowly scoped to parse-time validation for #[event(...)] fields and is backed by focused snapshot tests covering both rejection and non-regression cases.

Pull request overview

This PR hardens the observed_macros_impl #[event(...)] procedural macro by rejecting event fields that contain mutable references at parse time, producing a targeted diagnostic instead of allowing the input through to fail later in generated visit code.

Changes:

• Add field_attrs::is_mutable_reference_type to detect &mut T (including nested forms like &&mut T).
• Enforce a new parse-time guard in event field parsing that rejects &mut T and Option<&mut T> with a clear error message.
• Add snapshot coverage proving mutable-reference fields are rejected while shared-reference fields still expand as before.

File summaries

File	Description
crates/observed_macros_impl/src/field_attrs.rs	Introduces a helper to detect mutable references (including nested references) during type inspection.
crates/observed_macros_impl/src/event.rs	Adds a parse-time rejection for mutable-reference event fields and wires it into field parsing.
crates/observed_macros_impl/tests/expansion.rs	Adds new snapshot-driven tests for the new rejection behavior and for shared-reference non-regression.
crates/observed_macros_impl/tests/snapshots/expansion__an_event_field_may_not_hold_a_mutable_reference.snap	New snapshot asserting diagnostics for several mutable-reference patterns.
crates/observed_macros_impl/tests/snapshots/expansion__an_event_field_may_hold_a_shared_reference.snap	New snapshot asserting shared-reference expansions remain accepted.
crates/observed_macros_impl/CHANGELOG.md	Documents the fix in the Unreleased section.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (8c5a3c9) to head (42fe16d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #730   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         582      582           
  Lines       62852    62873   +21     
=======================================
+ Hits        62852    62873   +21     

Flag	Coverage Δ
linux	72.3% <100.0%> (-27.7%)	⬇️
linux-arm	73.1% <100.0%> (-26.9%)	⬇️
scheduled	?
windows	65.1% <100.0%> (?)

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[github-actions]

⚠️ Potential breaking changes detected

cargo semver-checks flagged the following on this PR. This is informational -- breaking changes between commits are expected; the major-version bump happens at release time, not on every PR.

fetch_winhttp

     Cloning origin/main
    Building fetch_winhttp v0.1.1 (current)
       Built [   0.429s] (current)
     Parsing fetch_winhttp v0.1.1 (current)
error: unsupported rustdoc format v60 for file: /home/runner/work/oxidizer/oxidizer/target/semver-checks/local-fetch_winhttp-0_1_1-default-01666ec060466c14/target/doc/fetch_winhttp.json
(supported formats are v55, v56, v57)

fetch_winhttp_impl

     Cloning origin/main
    Building fetch_winhttp_impl v0.1.1 (current)
       Built [   0.376s] (current)
     Parsing fetch_winhttp_impl v0.1.1 (current)
error: unsupported rustdoc format v60 for file: /home/runner/work/oxidizer/oxidizer/target/semver-checks/local-fetch_winhttp_impl-0_1_1-default-f6bbf157605592e2/target/doc/fetch_winhttp_impl.json
(supported formats are v55, v56, v57)

observed

     Cloning origin/main
    Building observed v0.25.0 (current)
       Built [   5.502s] (current)
     Parsing observed v0.25.0 (current)
error: unsupported rustdoc format v60 for file: /home/runner/work/oxidizer/oxidizer/target/semver-checks/local-observed-0_25_0-default-d7b8c5ec6ce39049/target/doc/observed.json
(supported formats are v55, v56, v57)

observed_macros_impl

     Cloning origin/main
    Building observed_macros_impl v0.24.0 (current)
       Built [   2.207s] (current)
     Parsing observed_macros_impl v0.24.0 (current)
error: unsupported rustdoc format v60 for file: /home/runner/work/oxidizer/oxidizer/target/semver-checks/local-observed_macros_impl-0_24_0-default-01666ec060466c14/target/doc/observed_macros_impl.json
(supported formats are v55, v56, v57)

observed_testing

     Cloning origin/main
    Building observed_testing v0.0.0 (current)
       Built [   5.838s] (current)
     Parsing observed_testing v0.0.0 (current)
error: unsupported rustdoc format v60 for file: /home/runner/work/oxidizer/oxidizer/target/semver-checks/local-observed_testing-0_0_0-default-01666ec060466c14/target/doc/observed_testing.json
(supported formats are v55, v56, v57)

observed_utils

     Cloning origin/main
    Building observed_utils v0.2.0 (current)
       Built [   5.731s] (current)
     Parsing observed_utils v0.2.0 (current)
error: unsupported rustdoc format v60 for file: /home/runner/work/oxidizer/oxidizer/target/semver-checks/local-observed_utils-0_2_0-default-01666ec060466c14/target/doc/observed_utils.json
(supported formats are v55, v56, v57)