Summary

Closes #1121. Part of Benoît Cortier (@CBenoit)'s fuzzing umbrella #1120; this PR unblocks the downstream sub-issues #1122 (Arbitrary impls for more PDU structs), #1123 (CI feature matrix), and #1124 (advanced fuzz targets). Supersedes #1128 by rajatdahal (@razzat008) (stalled 75+ days; 2026-05-11 collaboration offer did not get a response).

Wires the arbitrary feature on ironrdp-pdu and adjusts the existing ironrdp-connector cascade so it composes correctly. This is the foundation for structure-aware fuzzing harnesses that generate valid-looking PDU inputs rather than raw bytes.

rajatdahal (@razzat008)'s exploration in #1128 was the starting point. The decisions in this PR about the ChannelName manual impl shape, the Arc<dyn LicenseCache> trade-off, the StaticChannelSet skip, and the feature-cascade wiring all build on the questions raised in that thread.

Approach

The Cargo.toml wiring uses the explicit cascade pattern:

[features]
arbitrary = ["alloc", "dep:arbitrary", "bitflags/arbitrary"]

[dependencies]
bitflags = "2.11"
arbitrary = { version = "1", features = ["derive"], optional = true }

This keeps bitflags itself free of an always-on arbitrary feature so consumers that do not opt into our arbitrary feature do not pull the arbitrary crate transitively. The cascade preserves the no_std + alloc build path the issue acceptance criterion calls for.

#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))] is applied to PDU structs and enums across the gcc, mcs, nego, capability sets, client_info, server_license, autodetect, finalization_messages, session_info, suppress_output, refresh_rectangle, server_error_info, multitransport, headers, input, basic_output, codecs/rfx, and vc/dvc/gfx modules.

ChannelName gets a hand-rolled Arbitrary impl that preserves the wire-format invariant (8-byte null-terminated, up to seven ANSI characters). LicenseExchangeSequence in ironrdp-connector gets a hand-rolled impl with NoopLicenseCache as a placeholder so the license-exchange surface stays reachable under fuzz.

Alternatives considered

• Always-on bitflags/arbitrary (as in Fix/arbitrary failing on feature flag #1128): pulls the arbitrary crate into every consumer of ironrdp-pdu unconditionally. Rejected because it defeats the optional-feature design.
• arbitrary feature on ironrdp-svc (as in Fix/arbitrary failing on feature flag #1128): rejected per the review comment that "we're probably not going to deal with 'arbitrary static virtual channels'." The trait-object types on the connector side are skipped via #[arbitrary(default)] instead.
• Populating StaticChannelSet with plausible channels under fuzz: considered but deferred. The skip path matches Fix arbitrary feature flag plumbing so it compiles #1121's compile-gate scope; channel-state fuzzing coverage is a follow-up evaluated after the initial fuzz runs surface what coverage looks like.

Trade-offs accepted

• StaticChannelSet fields on ConnectionResult and ClientConnector are empty under fuzz (skip via #[arbitrary(default)]). Connector code paths that branch on populated channel state are not exercised. Tracked for coverage iteration.
• LicenseExchangeSequence uses NoopLicenseCache under fuzz. Cache-dependent paths are not exercised. A fuzz-mode LicenseCache impl is a possible follow-up.
• Error enums carrying foreign types (ServerLicenseError, ConnectorErrorKind) are intentionally not annotated. They are not part of the wire-protocol surface fuzzing targets.

Each trade-off is recorded with an inline source comment naming the reason.

Verification

• cargo fmt --all -- --check clean
• cargo check -p ironrdp-pdu --features arbitrary succeeds
• cargo check -p ironrdp-pdu --features arbitrary,std succeeds
• cargo check -p ironrdp-pdu --no-default-features --features arbitrary,alloc succeeds
• cargo check -p ironrdp-pdu (default features) succeeds
• cargo check -p ironrdp-connector --features arbitrary succeeds
• cargo xtask check fmt clean
• cargo xtask check lints clean (pre-existing warnings unchanged)
• cargo xtask check typos clean

References

• Closes Fix arbitrary feature flag plumbing so it compiles #1121
• Supersedes Fix/arbitrary failing on feature flag #1128
• Part of the Advanced fuzzing with arbitrary #1120 fuzzing umbrella; unblocks Implement Arbitrary for more PDU structs #1122, CI: compile/test a feature matrix including arbitrary (std/no_std) #1123, Add advanced fuzz targets to verify more invariants (round-trip, framing, state-machine…) #1124