ridl_diff/

classify.rs

//! The breaking/compatible classifier.
//!
//! [`classify`] turns one structural [`Change`] from the walk into a directional
//! [`Verdict`]. Direction is judged from the consumer's side (ADR-0008 decision
//! 14): a change is breaking when it shifts or reuses a wire identity or narrows
//! a consumer-visible guarantee, and compatible when it only relaxes or appends.
//!
//! The classifier reads the resolved IR of both sides, never source, so every
//! bound it compares is the bound a backend sees. That is what makes the
//! `[defaults].timing` rule need no special case: the manifest default is
//! already resolved into every untimed interaction (ADR-0008 decision 12), so
//! editing it arrives here as an ordinary [`Category::TimingChanged`] on each
//! defaulted interaction and classifies by the bound rules below (ridl §9.1).
//!
//! Two properties are deliberate:
//!
//! - **No implication proving.** Contract clauses are carried as canonical
//!   source text (ADR-0008 decision 14), so the classifier compares text. Any
//!   `require` text change is breaking, and any `ensure` text change is
//!   breaking; it never tries to show that one clause implies another.
//! - **Unlisted is breaking.** A shape the table does not name is classified
//!   breaking. A false "breaking" costs a maintainer one review; a false
//!   "compatible" ships a wire break.

use ridl_ir::v2;

use crate::{Category, Change, Verdict, frozen};

#[cfg(test)]
mod classify_tests;

/// Classifies one change against the two snapshots it was drawn from.
///
/// `old` and `new` are the matched packages the change belongs to; the change's
/// path is resolved against them to recover the typed IR the direction is read
/// from. For a package present on only one side the caller passes that package
/// as both arguments — those changes classify on their category alone.
///
/// A reference to a declaration of another package is not resolved here, and a
/// type the diff cannot resolve is reported as breaking: an appended struct
/// field typed from another package classifies breaking.
/// [`diff_sets_in`](crate::diff_sets_in) resolves such a reference against every
/// package of the new snapshot and the context packages, such as `ridl.std`.
pub fn classify(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    classify_in(change, old, new, &[])
}

/// [`classify`], with `scope` — every package of the new snapshot, and the
/// context packages such as `ridl.std` — to resolve a reference to another
/// package's declaration. Only the appended struct field reads it: whether its
/// type is legal at 0 is decided by the declaration the new side names
/// (driftsys/ridl#598).
// A new variant must be given a real arm here, not swept into a
// catch-all: rustc forces *an* arm, and the arm its `help:` text
// proposes is `_ =>`, which classifies the new variant silently. The
// two lints below reject a wildcard over `Category` — the first when
// it covers several variants, the second when it covers exactly one,
// which is the case one added variant creates.
#[deny(
    clippy::wildcard_enum_match_arm,
    clippy::match_wildcard_for_single_variants
)]
pub(crate) fn classify_in(
    change: &Change,
    old: &v2::Package,
    new: &v2::Package,
    scope: &[&v2::Package],
) -> Verdict {
    match change.category {
        // Shifts or reuses a wire identity, or replaces a wire-carrying type.
        // Every one of these is breaking in either direction.
        Category::InteractionInserted
        | Category::InteractionReordered
        | Category::MemberReordered
        | Category::InteractionRemoved
        | Category::ReservedNameRedeclared
        | Category::KindChanged
        | Category::PayloadChanged
        | Category::ReturnChanged
        | Category::ParamsChanged
        | Category::WidthChanged
        | Category::ServiceChanged
        | Category::DeclRemoved => Verdict::Breaking,

        // An init is part of the published contract (ridl §9.1): consumers read
        // the pre-publish value. Neither reference states a compatible
        // direction, so the change is breaking.
        Category::InitChanged => Verdict::Breaking,

        // Doc comments, labels, and deprecation notes reach no consumer's build.
        // Visibility is not among them — it has its own category below.
        //
        // A tombstone in the retired interaction's own slot is the sanctioned
        // retirement (ridl §11); the walk only emits `InteractionRetired` when
        // the slot is preserved.
        Category::DocOnly | Category::InteractionRetired => Verdict::Compatible,

        // An interface's identity is its `interfaces.lock` number (lock
        // design §7). A rename that keeps the number moves nothing on the
        // wire — the routing key is the number — and is visible in source,
        // which the text report's heading says of it. A number the new side's
        // lock retires is the sanctioned removal: the entry keeps the number
        // forever, so it is never allocated again.
        Category::InterfaceRenamed | Category::InterfaceRetired => Verdict::Compatible,

        // `ridl lock` freezing a provisional number in place keeps the
        // routing key, so nothing moves on the wire; the catalog hash, which
        // covers the flag, changes, which the text report's heading says of
        // it. A number that differs between the two sides is a new routing
        // key for the same interface.
        Category::InterfaceFrozen => Verdict::Compatible,
        Category::InterfaceNumberChanged => Verdict::Breaking,

        // An enum value or enum-set bit takes its identity from its explicit
        // number (typl §8, §9), so a body that differs only in order moves
        // nothing on the wire. The catalog hash covers that order when an
        // interface reaches the enum, which the text report's heading says of
        // it.
        Category::EnumReordered => Verdict::Compatible,

        // A service's list is a set of interface references (ADR-0015
        // decision 19 as amended on 2026-09-15). The routing key does not
        // contain the service, so an interface joining or leaving the set
        // moves no wire identity: both directions are compatible. A removal
        // is still visible in source — the `service.member` addresses of that
        // interface stop resolving under the service — which is what the text
        // report's heading says of it.
        Category::ServiceInterfaceAdded | Category::ServiceInterfaceRemoved => Verdict::Compatible,

        Category::VisibilityChanged => visibility(change, old, new),
        Category::InteractionAppended => appended(change, old, new),
        Category::DeclAdded => added(change, old, new, scope),
        Category::ConstraintChanged => constraint(change, old, new),
        Category::TimingChanged => timing(change, old, new),
        Category::RpcBoundChanged => rpc_bound(change, old, new),
        Category::ContractChanged => contract(change, old, new),
    }
}

// ==========================================================================
// Visibility.
// ==========================================================================

/// A change to the visibility a declaration is published at.
///
/// Visibility rides the surface grammar next to doc comments, but it is not
/// metadata to a consumer. `internal` maps to the target's package-private
/// mechanism — Rust `pub(crate)`, a non-exported TypeScript member (ADR-0002
/// §8) — so narrowing `public` to `internal` deletes the declaration from every
/// out-of-package consumer's build. That is the plainest form of "narrows a
/// consumer-visible guarantee" in ADR-0008 decision 14, even though the byte
/// layout on the wire never moves.
///
/// Widening `internal` to `public` only offers more, so it is compatible. Any
/// direction involving an unset visibility is breaking, following the module's
/// unlisted-is-breaking rule.
fn visibility(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    let (Some(old_visibility), Some(new_visibility)) = (
        find_visibility(old, &change.path, |name| find_old_shape(old, new, name)),
        find_visibility(new, &change.path, |name| find_shape(new, name)),
    ) else {
        return Verdict::Breaking;
    };

    match (
        v2::Visibility::try_from(old_visibility),
        v2::Visibility::try_from(new_visibility),
    ) {
        (Ok(v2::Visibility::Internal), Ok(v2::Visibility::Public)) => Verdict::Compatible,
        _ => Verdict::Breaking,
    }
}

/// The visibility published at a change's path: an interaction inside an
/// interface or service shape, or a package-level declaration, interface, or
/// service. `shape_of` resolves the container name the path carries to this
/// side's shape — by name on the new side, and on the old side the way the
/// walk matched it ([`find_old_shape`]), since a renamed interface's path
/// carries only its new name.
fn find_visibility<'a>(
    package: &'a v2::Package,
    path: &str,
    shape_of: impl Fn(&str) -> Option<v2::InterfaceShape<'a>>,
) -> Option<i32> {
    let mut segments = path.split('/').skip(1);
    let name = segments.next()?;

    if let Some(member) = segments.next() {
        return shape_of(name)?
            .interface
            .interactions
            .iter()
            .find(|decl| decl.name == member)
            .map(|decl| decl.visibility);
    }
    if let Some(decl) = find_decl(package, name) {
        return Some(decl.visibility);
    }
    // `Package::shapes` answers for a named interface and for a service with an
    // inline shape, and `InterfaceShape::visibility` reads the authoritative
    // field in each case — the owning service's for an inline shape, whose own
    // `Interface.visibility` is `VISIBILITY_UNSPECIFIED` by construction.
    if let Some(shape) = shape_of(name) {
        return Some(shape.visibility());
    }
    // A service naming an interface after `:` carries no shape of its own, so
    // it is not in `shapes()`; its visibility is still the service's.
    package
        .services
        .iter()
        .find(|service| service.name == name)
        .map(|service| service.visibility)
}

// ==========================================================================
// Interaction append — the identity-reuse guard.
// ==========================================================================

/// An interaction (or a freshly minted tombstone) added after every slot that
/// existed before. Appending is compatible, but only when the slot it takes was
/// never occupied: an ordinal freed by an untombstoned removal and handed to a
/// new name **reuses a wire identity**, which ADR-0008 decision 14 lists first
/// among breaking changes.
///
/// The walk labels that case `InteractionAppended` because the new name does sit
/// after every surviving slot; the reuse is only visible by looking back at what
/// the old snapshot held at that ordinal, which is why the check lives here and
/// not in the walk.
fn appended(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    let Some((container, member)) = member_path(change) else {
        return Verdict::Breaking;
    };
    let (Some(old_iface), Some(new_iface)) = (
        find_old_interface(old, new, container),
        find_interface(new, container),
    ) else {
        return Verdict::Breaking;
    };
    let Some(ordinal) = slot_ordinal(new_iface, member) else {
        return Verdict::Breaking;
    };

    for (name, old_ordinal) in slots(old_iface) {
        if old_ordinal == ordinal && name != member {
            return Verdict::Breaking;
        }
    }
    Verdict::Compatible
}

// ==========================================================================
// Additions — package level, and the append-only composite bodies.
// ==========================================================================

/// A declaration present only in the new snapshot.
///
/// A package-level addition — a new decl, interface, or service — is compatible:
/// nothing that existed moved. A composite member addition is compatible only
/// when it is a genuine append: typl §7.4 makes struct fields and union arms one
/// append-only rule ("new fields are added at the end of the struct or union"),
/// and an enum value appends by taking a number above every live and every
/// retired one.
///
/// An appended struct field must also be readable from an old payload, which
/// does not carry it ([`absence_reads_as_legal`], driftsys/ridl#598).
fn added(change: &Change, old: &v2::Package, new: &v2::Package, scope: &[&v2::Package]) -> Verdict {
    let Some((container, member)) = member_path(change) else {
        // One or two segments: a whole package, or a package-level decl,
        // interface, or service.
        return Verdict::Compatible;
    };
    let (Some(old_decl), Some(new_decl)) = (find_decl(old, container), find_decl(new, container))
    else {
        return Verdict::Breaking;
    };

    // The append test is a property of the whole body, and reading the body is
    // what catches a *surviving* member whose slot moved — which the walk does
    // not report alongside an addition. The member name is read only for a
    // struct field, whose type decides whether an old payload stays readable.
    use v2::decl::Kind;
    let appended = match (&old_decl.kind, &new_decl.kind) {
        (Some(Kind::StructDef(old_def)), Some(Kind::StructDef(new_def))) => {
            appended_slot(
                &struct_slots(old_def),
                &struct_reserved(old_def),
                &struct_slots(new_def),
            ) && absence_reads_as_legal(new_def, member, new, scope)
        }
        (Some(Kind::UnionDef(old_def)), Some(Kind::UnionDef(new_def))) => {
            // A result union's arms are its transport identity (ADR-0008
            // decision 4): any arm change flips it.
            !old_def.is_result
                && !new_def.is_result
                && appended_slot(
                    &union_slots(old_def),
                    &union_reserved(old_def),
                    &union_slots(new_def),
                )
        }
        (Some(Kind::EnumDef(old_def)), Some(Kind::EnumDef(new_def))) => appended_slot(
            &value_slots(&old_def.values),
            &reserved_values(&old_def.reserved),
            &value_slots(&new_def.values),
        ),
        (Some(Kind::EnumSetDef(old_def)), Some(Kind::EnumSetDef(new_def))) => appended_slot(
            &value_slots(&old_def.bits),
            &[],
            &value_slots(&new_def.bits),
        ),
        // A shape the table does not name classifies breaking.
        _ => false,
    };

    if appended {
        Verdict::Compatible
    } else {
        Verdict::Breaking
    }
}

/// Whether a reader built against `new_set` refuses a payload in which the
/// field `member` of struct `container`, in package `package`, is absent —
/// the rule that makes an appended field breaking for its type
/// (driftsys/ridl#598). `context` holds packages that references resolve
/// against without being compared, such as `ridl.std`. `false` when
/// `container` is not a struct of that package. The `ridl check` desk check
/// reads it to say so in its RIDL-407 message for an append that also sits
/// beside a moved sibling.
pub fn absence_refused(
    new_set: &[v2::Package],
    context: &[v2::Package],
    package: &str,
    container: &str,
    member: &str,
) -> bool {
    let scope: Vec<&v2::Package> = new_set.iter().chain(context).collect();
    let Some(home) = new_set.iter().find(|candidate| candidate.name == package) else {
        return false;
    };
    match find_decl(home, container).and_then(|decl| decl.kind.as_ref()) {
        Some(v2::decl::Kind::StructDef(def)) => !absence_reads_as_legal(def, member, home, &scope),
        _ => false,
    }
}

/// Whether a reader built against the new struct body accepts a payload the old
/// body wrote, in which the appended field `member` is absent
/// (driftsys/ridl#598).
///
/// An optional field reads as absent. A non-optional field reads as the
/// FlatBuffers default, 0, when its type is a scalar, an enum or an enum set and
/// 0 is a legal value of that type. That is the condition under which the
/// FlatBuffers codec of `ridl-backend-rust` reads the absent field instead of
/// refusing it as a missing required field (driftsys/ridl#472), and the two
/// share its definition, [`ridl_ir::zero`]. Every other absent non-optional
/// field is refused: a string, bytes, struct, union, tuple, array or map field
/// is an offset with no default, and a scalar or enum whose type excludes 0 has
/// no legal value to read.
///
/// A type the classifier cannot resolve reads as refused, following the module's
/// unlisted-is-breaking rule: a type the diff cannot resolve is reported as
/// breaking. That includes a reference to another package when `scope` does
/// not hold it. No snapshot carries `ridl.std`, so it resolves only when the
/// caller passes it as context, as the `ridl` CLI does.
fn absence_reads_as_legal(
    def: &v2::StructDef,
    member: &str,
    home: &v2::Package,
    scope: &[&v2::Package],
) -> bool {
    let Some(r#type) = def.members.iter().find_map(|slot| match &slot.member {
        Some(v2::struct_member::Member::Field(field)) if field.name == member => {
            field.r#type.as_ref()
        }
        _ => None,
    }) else {
        return false;
    };
    if r#type.optional {
        return true;
    }
    use v2::field_type::Kind;
    match &r#type.kind {
        Some(Kind::Primitive(primitive)) => matches!(
            v2::PrimitiveType::try_from(*primitive),
            Ok(v2::PrimitiveType::Boolean | v2::PrimitiveType::Integer | v2::PrimitiveType::Float)
        ),
        Some(Kind::InlineScalar(def)) => scalar_holds_zero(def),
        Some(Kind::Named(reference)) => {
            use v2::decl::Kind as Decl;
            match resolve(home, scope, reference).and_then(|decl| decl.kind.as_ref()) {
                Some(Decl::TypeDef(def)) => scalar_holds_zero(def),
                Some(Decl::EnumDef(def)) => ridl_ir::zero::enum_zero_member(&def.values).is_some(),
                // An enum set at 0 is the empty set.
                Some(Decl::EnumSetDef(_)) => true,
                _ => false,
            }
        }
        _ => false,
    }
}

/// Whether a named or inline scalar is legal at 0: a boolean, integer, float or
/// unit backing whose constraint holds 0. A string or bytes backing has no
/// default at all.
fn scalar_holds_zero(def: &v2::TypeDef) -> bool {
    let numeric = match def
        .backing
        .as_ref()
        .and_then(|backing| backing.kind.as_ref())
    {
        Some(v2::backing::Kind::Unit(_)) => true,
        Some(v2::backing::Kind::Primitive(primitive)) => matches!(
            v2::PrimitiveType::try_from(*primitive),
            Ok(v2::PrimitiveType::Boolean | v2::PrimitiveType::Integer | v2::PrimitiveType::Float)
        ),
        None => false,
    };
    numeric
        && def.constraint.as_ref().is_none_or(|constraint| {
            ridl_ir::zero::range_holds_zero(
                constraint.min.as_deref(),
                constraint.max.as_deref(),
                constraint.step.as_deref(),
            )
        })
}

/// The declaration a type reference names on the new side, in the IR's
/// canonical form: a bare `Name` in `home`, and `pkg.Name` in the package of
/// that name — `home` itself or one of `scope`.
fn resolve<'a>(
    home: &'a v2::Package,
    scope: &[&'a v2::Package],
    reference: &str,
) -> Option<&'a v2::Decl> {
    match reference.rsplit_once('.') {
        Some((package, name)) => std::iter::once(home)
            .chain(scope.iter().copied())
            .find(|candidate| candidate.name == package)
            .and_then(|candidate| find_decl(candidate, name)),
        None => find_decl(home, reference),
    }
}

/// Whether the new body is the old body with every pre-existing slot untouched
/// and every addition sitting above the highest slot ever used — live or
/// retired.
///
/// Both halves matter. A member whose slot number moved has had its wire
/// identity shifted even though its name survived, and the walk's composite
/// comparison does not report that alongside an addition. A member taking a
/// number at or below the old high-water mark is an insertion, or a reuse of a
/// retired number, which typl §7.4 forbids so a wire value never carries a new
/// meaning.
fn appended_slot(old: &[(String, i64)], old_retired: &[i64], new: &[(String, i64)]) -> bool {
    for (name, old_slot) in old {
        match new.iter().find(|(new_name, _)| new_name == name) {
            Some((_, new_slot)) if new_slot == old_slot => {}
            // A surviving member moved, or vanished alongside the addition.
            _ => return false,
        }
    }

    let high_water = old
        .iter()
        .map(|(_, slot)| *slot)
        .chain(old_retired.iter().copied())
        .max();
    let old_names: Vec<&String> = old.iter().map(|(name, _)| name).collect();
    for (name, slot) in new {
        if old_names.contains(&name) {
            continue;
        }
        match high_water {
            Some(mark) if *slot <= mark => return false,
            _ => {}
        }
    }
    true
}

// ==========================================================================
// Constraints — narrowed versus widened.
// ==========================================================================

/// A scalar constraint change on a named type. Widening keeps every value the
/// old contract admitted legal, so it is compatible; narrowing rejects values a
/// consumer may already be sending.
///
/// A composite body changed in place reaches here with no member path and no
/// rendered values — the walk does not say which member changed inside it — and
/// classifies breaking.
fn constraint(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    let mut segments = change.path.split('/').skip(1);
    let (Some(name), None) = (segments.next(), segments.next()) else {
        return Verdict::Breaking;
    };
    let (Some(old_decl), Some(new_decl)) = (find_decl(old, name), find_decl(new, name)) else {
        return Verdict::Breaking;
    };

    use v2::decl::Kind;
    let (Some(Kind::TypeDef(old_def)), Some(Kind::TypeDef(new_def))) =
        (&old_decl.kind, &new_decl.kind)
    else {
        return Verdict::Breaking;
    };

    if narrows(old_def.constraint.as_ref(), new_def.constraint.as_ref()) {
        Verdict::Breaking
    } else {
        Verdict::Compatible
    }
}

/// Whether the constraint moved in the narrowing direction on any facet.
///
/// Each facet is judged independently and any narrowing decides the whole
/// change, so a mixed edit — a lowered `min` with a lowered `max` — is breaking
/// on the half that narrows.
fn narrows(old: Option<&v2::Constraint>, new: Option<&v2::Constraint>) -> bool {
    let (Some(old), Some(new)) = (old, new) else {
        // A constraint appearing where there was none bounds a previously
        // unbounded value; dropping one entirely only widens.
        return old.is_none() && new.is_some();
    };

    // A raised floor or a lowered ceiling rejects values that were legal.
    if raised(old.min.as_deref(), new.min.as_deref())
        || lowered(old.max.as_deref(), new.max.as_deref())
    {
        return true;
    }
    // Length bounds are the same rule over character and byte counts.
    if raised_u64(old.len_min, new.len_min) || lowered_u64(old.len_max, new.len_max) {
        return true;
    }
    // A step quantizes: added or changed at all it can exclude values that were
    // legal, and the classifier does not prove divisibility. Removed, it widens.
    if new.step.is_some() && old.step != new.step {
        return true;
    }
    // A pattern added or rewritten can reject strings that matched before.
    // Removed, it widens.
    if (new.pattern.is_some() && old.pattern != new.pattern)
        || (new.pattern_const.is_some() && old.pattern_const != new.pattern_const)
    {
        return true;
    }
    false
}

/// Whether a lower bound was added or moved up.
fn raised(old: Option<&str>, new: Option<&str>) -> bool {
    match (old, new) {
        (None, Some(_)) => true,
        (Some(old), Some(new)) => cmp_decimal(new, old).is_none_or(std::cmp::Ordering::is_gt),
        _ => false,
    }
}

/// Whether an upper bound was added or moved down.
fn lowered(old: Option<&str>, new: Option<&str>) -> bool {
    match (old, new) {
        (None, Some(_)) => true,
        (Some(old), Some(new)) => cmp_decimal(new, old).is_none_or(std::cmp::Ordering::is_lt),
        _ => false,
    }
}

fn raised_u64(old: Option<u64>, new: Option<u64>) -> bool {
    match (old, new) {
        (None, Some(_)) => true,
        (Some(old), Some(new)) => new > old,
        _ => false,
    }
}

fn lowered_u64(old: Option<u64>, new: Option<u64>) -> bool {
    match (old, new) {
        (None, Some(_)) => true,
        (Some(old), Some(new)) => new < old,
        _ => false,
    }
}

/// Orders two canonical decimal strings exactly, without going through a float
/// (the IR carries exact decimals for precisely this reason, ADR-0007 decision
/// 9). Returns `None` when either side is not a plain decimal, which callers
/// read as "cannot prove this relaxes".
///
/// The `None` case is not dead. `ridlc` only ever writes canonical decimals, but
/// [`load_ir_json`](crate::load_ir_json) deserializes a snapshot off disk and a
/// bound is a plain string there, so a hand-edited or foreign `.ir.json` can
/// carry an exponent form or any other spelling this function does not read.
fn cmp_decimal(left: &str, right: &str) -> Option<std::cmp::Ordering> {
    use std::cmp::Ordering;

    let (left_negative, left_digits) = split_sign(left)?;
    let (right_negative, right_digits) = split_sign(right)?;
    if left_negative != right_negative {
        // Zero is signless in canonical form, so a sign disagreement is a real
        // ordering: the negative side is the smaller one.
        return Some(if left_negative {
            Ordering::Less
        } else {
            Ordering::Greater
        });
    }

    let magnitude = cmp_magnitude(&left_digits, &right_digits)?;
    Some(if left_negative {
        magnitude.reverse()
    } else {
        magnitude
    })
}

/// Splits an optional leading sign from a decimal, rejecting anything that is
/// not sign-digits-optional-fraction.
fn split_sign(text: &str) -> Option<(bool, String)> {
    let (negative, rest) = match text.strip_prefix('-') {
        Some(rest) => (true, rest),
        None => (false, text.strip_prefix('+').unwrap_or(text)),
    };
    if rest.is_empty() || !rest.chars().all(|c| c.is_ascii_digit() || c == '.') {
        return None;
    }
    if rest.matches('.').count() > 1 {
        return None;
    }
    Some((negative, rest.to_string()))
}

/// Orders two unsigned decimal magnitudes by integer part then fraction, so
/// `9` < `10` and `1.5` < `1.50001`.
fn cmp_magnitude(left: &str, right: &str) -> Option<std::cmp::Ordering> {
    use std::cmp::Ordering;

    let (left_int, left_frac) = left.split_once('.').unwrap_or((left, ""));
    let (right_int, right_frac) = right.split_once('.').unwrap_or((right, ""));

    let left_int = left_int.trim_start_matches('0');
    let right_int = right_int.trim_start_matches('0');
    let by_length = left_int.len().cmp(&right_int.len());
    if by_length != Ordering::Equal {
        return Some(by_length);
    }
    let by_int = left_int.cmp(right_int);
    if by_int != Ordering::Equal {
        return Some(by_int);
    }

    // Compare fractions digit by digit, padding the shorter with zeros.
    let width = left_frac.len().max(right_frac.len());
    let pad = |frac: &str| format!("{frac:0<width$}");
    Some(pad(left_frac).cmp(&pad(right_frac)))
}

// ==========================================================================
// Timing.
// ==========================================================================

/// A resolved timing change on a signal or event (ADR-0008 decision 12).
///
/// `min` is the rate floor and `max` the staleness bound, so the consumer-facing
/// guarantee strengthens when `min` rises or `max` falls and weakens when either
/// moves the other way. A bound added or removed, or the strict-periodic/range
/// mode flipped, changes what a consumer may assume at all — rmdl clocks key on
/// strict — and is breaking in both directions.
fn timing(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    let Some((container, member)) = member_path(change) else {
        return Verdict::Breaking;
    };
    let (Some(old_decl), Some(new_decl)) = (
        find_old_interaction(old, new, container, member),
        find_interaction(new, container, member),
    ) else {
        return Verdict::Breaking;
    };
    let (Some(old_timing), Some(new_timing)) =
        (interaction_timing(old_decl), interaction_timing(new_decl))
    else {
        // An interaction kind that carries no timing at all. The walk cannot
        // produce this: it emits `TimingChanged` only inside its signal and
        // event arms, so both sides are already a timed kind by the time a
        // change reaches here. It is still reachable, because [`classify`] is
        // public and takes any hand-built `Change` — so the arm stays, follows
        // the module's unlisted-is-breaking rule, and carries a test of its own.
        // It is deliberately not a `debug_assert!`: a caller passing a category
        // the walk would not have emitted is asking a question, not committing
        // a bug, and aborting a debug build over it would be wrong.
        return Verdict::Breaking;
    };

    let (Some(old_timing), Some(new_timing)) = (old_timing, new_timing) else {
        // Timing appearing where there was none, or dropped, is a bound added
        // or removed.
        return Verdict::Breaking;
    };

    if old_timing.mode != new_timing.mode {
        return Verdict::Breaking;
    }
    // A floor lowered, a ceiling raised, or either bound added or removed.
    if lowered(old_timing.min_us.as_deref(), new_timing.min_us.as_deref())
        || dropped(old_timing.min_us.as_deref(), new_timing.min_us.as_deref())
        || raised(old_timing.max_us.as_deref(), new_timing.max_us.as_deref())
        || dropped(old_timing.max_us.as_deref(), new_timing.max_us.as_deref())
    {
        return Verdict::Breaking;
    }
    // What remains is a floor raised, a ceiling lowered, or only
    // `default_applied` flipped over identical bounds — a default made explicit.
    Verdict::Compatible
}

/// A declared RPC-bound change on a command or query (ADR-0015 decision 8).
///
/// The two bounds keep their generic §9 meaning — `min` the rate floor, `max`
/// the staleness bound — but on an RPC `min` is the **call throttle** and
/// constrains the consumer, so its direction inverts against [`timing`]'s
/// convention: raising it withdraws a call rate the caller was entitled to
/// use (breaking), and lowering it leaves the caller less constrained
/// (compatible). `max` is the response bound, a provider promise, and keeps
/// the provider-side rule: raised is a weaker promise (breaking), lowered a
/// stronger one (compatible). A bound added or removed — the whole annotation
/// included — is breaking in both directions, exactly as in [`timing`]:
/// callers size timeouts against a declared bound, so its appearance and its
/// disappearance both change what a consumer may assume at all.
///
/// This is a distinct category rather than a kind-aware branch inside
/// [`timing`] so that a missed branch fails closed (ADR-0012 decision 9): the
/// deny lints above turn a missing arm into a compile error rather than a
/// silently inherited signal rule that calls a raised RPC `min` compatible.
fn rpc_bound(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    let Some((container, member)) = member_path(change) else {
        return Verdict::Breaking;
    };
    let (Some(old_decl), Some(new_decl)) = (
        find_old_interaction(old, new, container, member),
        find_interaction(new, container, member),
    ) else {
        return Verdict::Breaking;
    };
    let (Some(old_timing), Some(new_timing)) = (rpc_timing(old_decl), rpc_timing(new_decl)) else {
        // An interaction kind that is not an RPC. The walk cannot produce
        // this — it emits `RpcBoundChanged` only inside its command and query
        // arms — but [`classify`] is public and takes any hand-built
        // [`Change`], so the arm stays and follows the module's
        // unlisted-is-breaking rule (see [`timing`] for why it is not a
        // `debug_assert!`).
        return Verdict::Breaking;
    };

    let (Some(old_timing), Some(new_timing)) = (old_timing, new_timing) else {
        // The annotation appearing where there was none, or dropped, is a
        // bound added or removed — breaking in both directions.
        return Verdict::Breaking;
    };

    // The mode is always `Range` on an RPC (ADR-0015 decision 7); anything
    // else in a snapshot is erroneous IR, and a flip is breaking regardless.
    if old_timing.mode != new_timing.mode {
        return Verdict::Breaking;
    }
    // A throttle raised, a response bound raised, or either bound added or
    // removed. `raised` covers the added case (`None` → `Some`), `dropped` the
    // removed one. This is [`timing`]'s predicate with the `min` direction
    // inverted — `raised` where the signal rule reads `lowered`.
    if raised(old_timing.min_us.as_deref(), new_timing.min_us.as_deref())
        || dropped(old_timing.min_us.as_deref(), new_timing.min_us.as_deref())
        || raised(old_timing.max_us.as_deref(), new_timing.max_us.as_deref())
        || dropped(old_timing.max_us.as_deref(), new_timing.max_us.as_deref())
    {
        return Verdict::Breaking;
    }
    // What remains is a throttle lowered, a response bound lowered, or only
    // `default_applied` flipped over identical bounds — a default made
    // explicit (ADR-0015 decision 7), the rule [`timing`] gives a signal. The
    // caller is less constrained, or the provider promises more, or neither
    // changed.
    Verdict::Compatible
}

/// Whether a bound present before is absent now.
fn dropped(old: Option<&str>, new: Option<&str>) -> bool {
    old.is_some() && new.is_none()
}

/// The resolved timing of a signal or event; `None` for interaction kinds that
/// carry none, and `Some(None)` for a timed kind with the field unset.
fn interaction_timing(decl: &v2::Decl) -> Option<Option<&v2::Timing>> {
    use v2::decl::Kind;
    match &decl.kind {
        Some(Kind::SignalDef(def)) => Some(def.timing.as_ref()),
        Some(Kind::EventDef(def)) => Some(def.timing.as_ref()),
        _ => None,
    }
}

/// The declared timing of a command or query; `None` for other kinds, and
/// `Some(None)` for an RPC with no bound at all. An RPC bound may come from a
/// default (ADR-0015 decisions 4 and 7, as amended).
fn rpc_timing(decl: &v2::Decl) -> Option<Option<&v2::Timing>> {
    use v2::decl::Kind;
    match &decl.kind {
        Some(Kind::CommandDef(def)) => Some(def.timing.as_ref()),
        Some(Kind::QueryDef(def)) => Some(def.timing.as_ref()),
        _ => None,
    }
}

// ==========================================================================
// Contracts.
// ==========================================================================

/// A `require`/`ensure` clause-set change on a command or query.
///
/// A `require` is a precondition the caller must meet: adding one, or rewriting
/// one, rejects callers that were legal. An `ensure` is a postcondition the
/// caller may rely on: removing one, or rewriting one, withdraws a guarantee.
/// Clause text is compared verbatim — the classifier never tries to prove that
/// one clause implies another (ADR-0008 decision 14), so a rewrite reads as an
/// addition on the `require` side and as a removal on the `ensure` side, and
/// both are breaking.
fn contract(change: &Change, old: &v2::Package, new: &v2::Package) -> Verdict {
    let Some((container, member)) = member_path(change) else {
        return Verdict::Breaking;
    };
    let (Some(old_decl), Some(new_decl)) = (
        find_old_interaction(old, new, container, member),
        find_interaction(new, container, member),
    ) else {
        return Verdict::Breaking;
    };
    let (Some(old_clauses), Some(new_clauses)) = (contracts(old_decl), contracts(new_decl)) else {
        return Verdict::Breaking;
    };

    let kind = |want: v2::ContractKind| {
        move |clause: &&v2::Contract| v2::ContractKind::try_from(clause.kind) == Ok(want)
    };
    let sources = |clauses: &[v2::Contract], want: v2::ContractKind| -> Vec<String> {
        let mut out: Vec<String> = clauses
            .iter()
            .filter(kind(want))
            .map(|clause| clause.source.clone())
            .collect();
        out.sort();
        out
    };

    let old_require = sources(old_clauses, v2::ContractKind::Require);
    let new_require = sources(new_clauses, v2::ContractKind::Require);
    let old_ensure = sources(old_clauses, v2::ContractKind::Ensure);
    let new_ensure = sources(new_clauses, v2::ContractKind::Ensure);

    if !covers(&old_require, &new_require) || !covers(&new_ensure, &old_ensure) {
        return Verdict::Breaking;
    }
    Verdict::Compatible
}

/// Whether every clause in `subset` appears in `superset`, counting duplicates.
fn covers(superset: &[String], subset: &[String]) -> bool {
    let mut remaining: Vec<&String> = superset.iter().collect();
    for clause in subset {
        match remaining.iter().position(|held| *held == clause) {
            Some(index) => {
                remaining.swap_remove(index);
            }
            None => return false,
        }
    }
    true
}

/// The contract clauses of a command or query; `None` for kinds that carry none.
fn contracts(decl: &v2::Decl) -> Option<&[v2::Contract]> {
    use v2::decl::Kind;
    match &decl.kind {
        Some(Kind::CommandDef(def)) => Some(&def.contracts),
        Some(Kind::QueryDef(def)) => Some(&def.contracts),
        _ => None,
    }
}

// ==========================================================================
// Path resolution and IR lookups.
// ==========================================================================

/// Splits a `package/container/member` path into its container and member.
/// Returns `None` for the shorter package-level paths.
fn member_path(change: &Change) -> Option<(&str, &str)> {
    let mut segments = change.path.split('/').skip(1);
    let container = segments.next()?;
    let member = segments.next()?;
    if segments.next().is_some() {
        return None;
    }
    Some((container, member))
}

fn find_decl<'a>(package: &'a v2::Package, name: &str) -> Option<&'a v2::Decl> {
    package.decls.iter().find(|decl| decl.name == name)
}

/// The shape a name refers to — a top-level interface, or the inline shape of
/// a service, which the walk descends into under the service's own dotted
/// name. `Package::shapes` keys both on exactly that identity, so one lookup
/// covers them.
fn find_shape<'a>(package: &'a v2::Package, name: &str) -> Option<v2::InterfaceShape<'a>> {
    package.shapes().find(|shape| shape.name == name)
}

/// The old side of the shape a diff path names, found the way the walk
/// matched it (lock design §7; plan decision PD-14): by the frozen
/// `interfaces.lock` number of the new side's shape when it has one, and by
/// name otherwise — the new side has no identity, or does not hold the name
/// at all. A renamed interface's member changes therefore classify as any
/// other interface's, although their paths carry only the new name.
fn find_old_shape<'a>(
    old: &'a v2::Package,
    new: &v2::Package,
    name: &str,
) -> Option<v2::InterfaceShape<'a>> {
    let by_number = find_shape(new, name)
        .filter(|shape| frozen(shape.interface))
        .and_then(|shape| {
            old.shapes().find(|candidate| {
                frozen(candidate.interface) && candidate.interface.number == shape.interface.number
            })
        });
    by_number.or_else(|| find_shape(old, name))
}

fn find_interface<'a>(package: &'a v2::Package, name: &str) -> Option<&'a v2::Interface> {
    find_shape(package, name).map(|shape| shape.interface)
}

fn find_old_interface<'a>(
    old: &'a v2::Package,
    new: &v2::Package,
    name: &str,
) -> Option<&'a v2::Interface> {
    find_old_shape(old, new, name).map(|shape| shape.interface)
}

fn find_interaction<'a>(
    package: &'a v2::Package,
    container: &str,
    member: &str,
) -> Option<&'a v2::Decl> {
    find_interface(package, container)?
        .interactions
        .iter()
        .find(|decl| decl.name == member)
}

/// The old side's interaction under a `<package>/<container>/<member>` path,
/// with the container resolved by [`find_old_shape`].
fn find_old_interaction<'a>(
    old: &'a v2::Package,
    new: &v2::Package,
    container: &str,
    member: &str,
) -> Option<&'a v2::Decl> {
    find_old_interface(old, new, container)?
        .interactions
        .iter()
        .find(|decl| decl.name == member)
}

/// Every slot of an interface body as (name, ordinal), tombstones included — a
/// tombstone occupies its ordinal exactly so the slot is never reused
/// (ridl §11).
///
/// A tombstone whose name is unset still holds its slot. The interface-body
/// `reserved` form accepts a bare ordinal or a string literal as well as a
/// name, and both lower to `Reserved { name: None }`; dropping those from the
/// slot list would leave their ordinals looking free, and a new interaction
/// taking one would classify as a clean append. The empty name never equals a
/// real interaction name, so the slot is held against every reuse without
/// matching anything.
fn slots(interface: &v2::Interface) -> Vec<(&str, u32)> {
    interface
        .interactions
        .iter()
        .filter_map(|decl| match &decl.kind {
            Some(v2::decl::Kind::ReservedSlot(reserved)) => {
                Some((reserved.name.as_deref().unwrap_or(""), decl.ordinal))
            }
            Some(_) => Some((decl.name.as_str(), decl.ordinal)),
            None => None,
        })
        .collect()
}

fn slot_ordinal(interface: &v2::Interface, member: &str) -> Option<u32> {
    slots(interface)
        .into_iter()
        .find(|(name, _)| *name == member)
        .map(|(_, ordinal)| ordinal)
}

/// Every live struct field with its ordinal — the 1-based place in the body,
/// counting tombstones, that typl §7.4 makes the wire identity. Shared with the
/// walk, which reports a reorder by these ordinals.
pub(crate) fn struct_slots(def: &v2::StructDef) -> Vec<(String, i64)> {
    def.members
        .iter()
        .filter_map(|member| match &member.member {
            Some(v2::struct_member::Member::Field(field)) => {
                Some((field.name.clone(), i64::from(field.ordinal)))
            }
            _ => None,
        })
        .collect()
}

fn struct_reserved(def: &v2::StructDef) -> Vec<i64> {
    def.members
        .iter()
        .filter_map(|member| match &member.member {
            Some(v2::struct_member::Member::Reserved(reserved)) => {
                Some(i64::from(reserved.ordinal))
            }
            _ => None,
        })
        .collect()
}

/// Every union arm with its ordinal, on the same rule as [`struct_slots`].
pub(crate) fn union_slots(def: &v2::UnionDef) -> Vec<(String, i64)> {
    def.arms
        .iter()
        .map(|arm| (arm.name.clone(), i64::from(arm.ordinal)))
        .collect()
}

fn union_reserved(def: &v2::UnionDef) -> Vec<i64> {
    def.reserved
        .iter()
        .map(|reserved| i64::from(reserved.ordinal))
        .collect()
}

/// Enum values and enum-set bits key on their integer value, not a declaration
/// ordinal: the number is the wire identity (typl §7.4, §8).
pub(crate) fn value_slots(values: &[v2::EnumValue]) -> Vec<(String, i64)> {
    values
        .iter()
        .map(|value| (value.name.clone(), value.value))
        .collect()
}

fn reserved_values(reserved: &[v2::Reserved]) -> Vec<i64> {
    reserved.iter().filter_map(|entry| entry.value).collect()
}

// ==========================================================================
// `--explain` — the rule row for one category.
// ==========================================================================

/// Parses the snake_case word a report prints back into its category, so
/// `ridl diff --explain <category>` takes exactly what the report shows.
pub fn category_from_word(word: &str) -> Option<Category> {
    crate::CATEGORIES
        .into_iter()
        .find(|category| crate::category_word(*category) == word)
}

/// The rule row for a category: the classification table of ADR-0008 decision
/// 14 as text. This is the CI-facing documentation of record until the E4 error
/// index publishes it.
// A new variant must be given a real arm here, not swept into a
// catch-all: rustc forces *an* arm, and the arm its `help:` text
// proposes is `_ =>`, which classifies the new variant silently. The
// two lints below reject a wildcard over `Category` — the first when
// it covers several variants, the second when it covers exactly one,
// which is the case one added variant creates.
#[deny(
    clippy::wildcard_enum_match_arm,
    clippy::match_wildcard_for_single_variants
)]
pub fn explain(category: Category) -> &'static str {
    match category {
        Category::DeclAdded => concat!(
            "A declaration, interface, or service present only in the new snapshot.\n",
            "  compatible  a new package-level decl, interface, or service; an enum\n",
            "              value appended above every live and retired number; a\n",
            "              union arm appended at the end of the body (typl 7.4,\n",
            "              append-only); a struct field appended at the end of the body\n",
            "              when it is optional, or when it is a scalar, enum, or enum set\n",
            "              whose type allows the value 0\n",
            "  breaking    a member inserted below the highest slot ever used, a member\n",
            "              taking a retired number, any addition that moves a surviving\n",
            "              member's slot, or any arm of a result union (ADR-0008 d4); a\n",
            "              non-optional struct field appended whose type does not allow\n",
            "              0 — a string, bytes, struct, union, tuple, array, or map, a\n",
            "              scalar whose range or step excludes 0, an enum with no zero\n",
            "              member, or a type the diff cannot resolve (ridl diff\n",
            "              resolves ridl.std types). A reader of the new version\n",
            "              refuses every payload of the old one, which does not carry\n",
            "              the field (typl 7.4). Declare the new field optional instead\n",
            "              (`field : T?`): an absent optional field reads as absent\n",
            "  note        an interface is matched by its interfaces.lock number (lock\n",
            "              design 7): a frozen number the old side never held is a new\n",
            "              interface, and so is every provisional one — a declaration\n",
            "              with no lock entry carries no identity, and a rename keeps its\n",
            "              number, so it is never a renamed interface (interface_renamed)"
        ),
        Category::DeclRemoved => concat!(
            "A declaration, interface, or service present only in the old snapshot.\n",
            "  breaking    a removed service, interface, decl, enum value, struct\n",
            "              field, or union arm withdraws something a consumer compiled\n",
            "              against\n",
            "  note        an interface is matched by its interfaces.lock number (lock\n",
            "              design 7), so an interface here is one whose number is gone\n",
            "              from the new side and not retired there — a lock line deleted\n",
            "              by hand, or a number changed by hand, which is this row plus\n",
            "              decl_added. A number the new side's lock retires is\n",
            "              interface_retired instead, and `ridl baseline` refuses to\n",
            "              publish the removal of a number it does not find retired\n",
            "              (RIDL-412)\n",
            "  caveat      a composite member retired the sanctioned way — replaced by\n",
            "              a `reserved` tombstone in its own slot (typl 7.4) — is also\n",
            "              reported breaking today. The body comparison is keyed on\n",
            "              member names and does not read the `reserved` list, so it\n",
            "              cannot yet tell that retirement from a bare deletion. This\n",
            "              errs on the safe side; carried as debt, see the note on\n",
            "              `diff_composite`. The interaction-level tombstone IS\n",
            "              recognised — see interaction_retired"
        ),
        Category::InterfaceRenamed => concat!(
            "An interface whose interfaces.lock number is the same on both sides and\n",
            "whose name changed.\n",
            "  compatible  always — the number is the interface's identity and its\n",
            "              routing key (lock design 7, rsdl decision D-7), so nothing\n",
            "              moves on the wire. Visible in source: a rename changes the\n",
            "              generated identity-table names in both wire backends, which\n",
            "              is why the text report lists it under the heading\n",
            "              \"compatible on the wire, visible in source\". The path\n",
            "              carries the new name and the detail old -> new; changes\n",
            "              inside the interface are reported under the new name and\n",
            "              classified as any other interface's\n",
            "  note        a provisional number is no identity: a declaration with no\n",
            "              lock entry is never matched, so a rename the lock does not\n",
            "              record is decl_removed plus decl_added, breaking, until\n",
            "              `ridl lock <pkg> --rename Old=New` records it"
        ),
        Category::InterfaceRetired => concat!(
            "An interface whose number the old snapshot held is gone from the new\n",
            "snapshot, whose interfaces.lock retires it.\n",
            "  compatible  always — the sanctioned removal of an interface: the entry\n",
            "              keeps its number forever with the word `retired`, so the\n",
            "              number is never allocated again (lock design 4 and 7, rsdl\n",
            "              decision D-7). A number gone with no retired entry is\n",
            "              decl_removed, breaking, and `ridl baseline` refuses to publish\n",
            "              it (RIDL-412)"
        ),
        Category::InterfaceFrozen => concat!(
            "An interface matched by name whose number is the same on both sides and\n",
            "whose provisional flag went from true to false: `ridl lock` froze the\n",
            "number in place.\n",
            "  compatible  always — the number is the routing key and it did not\n",
            "              change, so nothing moves on the wire. The catalog hash\n",
            "              changes, because it covers each interface's number and its\n",
            "              provisional flag, and a generated face refuses a port whose\n",
            "              catalog hash is another one (ADR-0023 decision 8): a peer\n",
            "              built before the lock and a peer built after it refuse each\n",
            "              other. This is why the text report lists it under the\n",
            "              heading \"compatible on the wire, may change the catalog\n",
            "              hash\".\n",
            "              The detail carries the number before and after, marked\n",
            "              (provisional) on the old side\n",
            "  note        a baseline never holds a provisional number (`ridl baseline`\n",
            "              refuses one, RIDL-411), so this change is seen between two\n",
            "              builds, or in a reversed comparison whose old side is a\n",
            "              build. A number 0 on either side, from a snapshot written\n",
            "              before the lock existed, is not compared"
        ),
        Category::InterfaceNumberChanged => concat!(
            "An interface matched across the two sides whose number differs.\n",
            "  breaking    always — the number is the interface's routing key, so a\n",
            "              peer built against the old number cannot reach the\n",
            "              interface at the new one. Seen between two provisional\n",
            "              builds when a sibling interface added before this one in\n",
            "              byte order moves its provisional number, and when `ridl\n",
            "              lock` freezes a provisional number as another number. The\n",
            "              detail carries the old and the new number, each marked\n",
            "              (provisional) when it is\n",
            "  note        two frozen numbers are matched by number, so they never\n",
            "              differ here: a frozen number changed by hand is\n",
            "              decl_removed plus decl_added. A number 0 on either side,\n",
            "              from a snapshot written before the lock existed, is not\n",
            "              compared.\n",
            "              Run `ridl lock` to freeze the numbers, so that a later\n",
            "              sibling cannot move them"
        ),
        Category::MemberReordered => concat!(
            "A surviving composite member whose slot in the body changed.\n",
            "  breaking    always — a struct field or union arm takes its wire\n",
            "              identity from its ordinal, its 1-based place in the body\n",
            "              counting tombstones (typl 7.4), so a member whose ordinal\n",
            "              changed has a new wire identity; the detail carries the old\n",
            "              and new ordinal. An enum value or enum-set bit takes its\n",
            "              identity from the explicit number it declares (typl 8, 9),\n",
            "              so one whose number changed has a new wire identity; the\n",
            "              detail carries the old and new value or bit. An enum or\n",
            "              enum-set body reordered with no number changed is\n",
            "              enum_reordered, compatible, instead\n",
            "  note        reported only when both bodies hold the same member names:\n",
            "              a reorder in the same edit as an addition or a removal is\n",
            "              reported through decl_added or decl_removed alone. A reorder\n",
            "              in the same edit as an in-place change to the body is\n",
            "              reported with constraint_changed on the container as well.\n",
            "              An enum or enum-set body whose only differences are changed\n",
            "              numbers, in any order, reports member_reordered alone"
        ),
        Category::EnumReordered => concat!(
            "An enum or enum-set body whose difference is only order: its values or\n",
            "bits reordered with no number changed, the enum's reserved list\n",
            "reordered with no entry added, removed or changed, or both.\n",
            "  compatible  always — a value or bit takes its identity from the\n",
            "              explicit number it declares (typl 8, 9), so nothing moves\n",
            "              on the wire. The catalog hash covers only the declarations\n",
            "              an interface reaches. When an interface reaches the enum or\n",
            "              enum set, the order of its values or bits, and of an enum's\n",
            "              reserved list, is part of the hash, so the hash changes\n",
            "              then, and a generated face refuses a port whose catalog hash\n",
            "              is another one (ADR-0023 decision 8). This is why the text\n",
            "              report lists it under the heading \"compatible on the wire,\n",
            "              may change the catalog hash\". The path is the container's;\n",
            "              there is no detail\n",
            "  note        a value or bit whose number changed is member_reordered,\n",
            "              breaking, not enum_reordered"
        ),
        Category::InteractionAppended => concat!(
            "An interaction added after every slot that existed before.\n",
            "  compatible  the slot it takes was never occupied\n",
            "  breaking    the slot was freed by an untombstoned removal and is now\n",
            "              reused by a new name — a reused wire identity (ADR-0008 d14)"
        ),
        Category::InteractionInserted => concat!(
            "An interaction added before the end of the interface body.\n",
            "  breaking    always — every later ordinal shifts, and the ordinal is the\n",
            "              transport identity (ridl 11)"
        ),
        Category::InteractionReordered => concat!(
            "A surviving interaction whose relative order in the body changed.\n",
            "  breaking    always — a reorder shifts wire identities (ridl 11)"
        ),
        Category::InteractionRemoved => concat!(
            "An interaction removed without a `reserved` tombstone holding its slot.\n",
            "  breaking    always — the freed ordinal is reusable, so the wire identity\n",
            "              is no longer permanent (ridl 11)"
        ),
        Category::InteractionRetired => concat!(
            "An interaction retired to a `reserved` tombstone in its own ordinal slot.\n",
            "  compatible  always — the sanctioned retirement: the slot stays occupied\n",
            "              and every later ordinal holds (ridl 11)"
        ),
        Category::KindChanged => concat!(
            "An interaction whose kind changed (signal, event, command, query, fixed).\n",
            "  breaking    any direction — the kind selects the transport shape"
        ),
        Category::PayloadChanged => concat!(
            "A signal, event, or fixed payload type changed.\n",
            "  breaking    any direction, a stream added or removed included"
        ),
        Category::ReturnChanged => concat!(
            "A query return shape changed.\n",
            "  breaking    any direction — an ok-arm change, an error arm added, removed,\n",
            "              or retyped, a stream added or removed, or any other change to\n",
            "              the synthesized inline `T | E` transport identity (ADR-0008 d4:\n",
            "              interface + interaction ordinal + ordered arm types)"
        ),
        Category::ParamsChanged => concat!(
            "A command or query parameter list changed.\n",
            "  breaking    any direction — a parameter added, removed, renamed, retyped,\n",
            "              or a stream added or removed on one"
        ),
        Category::TimingChanged => concat!(
            "A signal or event resolved timing changed (ADR-0008 d12).\n",
            "  compatible  min raised (a higher rate floor) or max lowered (a tighter\n",
            "              staleness bound) with the mode unchanged; default_applied\n",
            "              flipped over identical resolved bounds — a default made\n",
            "              explicit\n",
            "  breaking    min lowered, max raised, a bound added where none was, a bound\n",
            "              removed, or the strict-periodic/range mode flipped\n",
            "  note        editing `[defaults].timing` needs no special rule: diff\n",
            "              compares resolved bounds, so it surfaces here on every\n",
            "              defaulted interaction (ridl 9.1)"
        ),
        Category::RpcBoundChanged => concat!(
            "A command or query declared RPC bound changed (ADR-0015 d8).\n",
            "  compatible  min lowered (the caller may call more often) or max lowered\n",
            "              (a stronger provider promise), with the mode unchanged;\n",
            "              default_applied flipped over identical resolved bounds — a\n",
            "              default made explicit\n",
            "  breaking    min raised — on an RPC, min is the call throttle and\n",
            "              constrains the caller, so raising it withdraws a call rate\n",
            "              the caller was entitled to use; max raised (a weaker provider\n",
            "              promise); a bound added or removed, the whole annotation\n",
            "              included\n",
            "  note        the min direction is the inverse of timing_changed's, which\n",
            "              is why this is a category of its own rather than a branch:\n",
            "              a missed branch would inherit the signal rule and call a\n",
            "              raised RPC min compatible (ADR-0012 d9, fail closed).\n",
            "              an RPC bound may come from a default, and diff compares\n",
            "              resolved bounds, so editing a default surfaces here on\n",
            "              every defaulted command or query (ridl 9.1)"
        ),
        Category::ContractChanged => concat!(
            "A command or query require/ensure clause set changed (ridl 13).\n",
            "  compatible  a require removed, or an ensure added\n",
            "  breaking    a require added or its text changed, or an ensure removed or\n",
            "              its text changed. Clause text is compared verbatim: the\n",
            "              classifier does not prove that one clause implies another\n",
            "              (ADR-0008 d14)"
        ),
        Category::WidthChanged => concat!(
            "A derived wire width or scalar backing changed.\n",
            "  breaking    any IntWidth or FloatWidth change, uint64 versus int64\n",
            "              included — the resolved width is part of the contract\n",
            "              (typl 4.2, 5.6)"
        ),
        Category::ConstraintChanged => concat!(
            "A scalar constraint changed, or a composite body changed in place.\n",
            "  compatible  widened — min lowered, max raised, a length bound loosened,\n",
            "              a step removed, a match pattern removed (by literal or by\n",
            "              named constant), or the whole constraint dropped so the\n",
            "              value is unbounded again\n",
            "  breaking    narrowed — min raised, max lowered, a length bound\n",
            "              tightened, a step added or changed (divisibility is never\n",
            "              proved), a match pattern added or rewritten (by literal or\n",
            "              by named constant), or a constraint appearing where there\n",
            "              was none, which bounds a previously unbounded value. A\n",
            "              composite body changed in place is breaking: the walk\n",
            "              does not say which member changed inside it\n",
            "  note        each facet is judged on its own and any one narrowing\n",
            "              decides the change, so a mixed edit is breaking on the half\n",
            "              that narrows. A widening that flips the resolved wire width\n",
            "              is separately reported as width_changed, which is always\n",
            "              breaking (typl 5.6)"
        ),
        Category::InitChanged => concat!(
            "A declared or resolved init value changed.\n",
            "  breaking    any direction — the init is the value a consumer reads before\n",
            "              the first publish, so it is part of the contract (ridl 9.1)"
        ),
        Category::ReservedNameRedeclared => concat!(
            "A name retired by a `reserved` tombstone is live again — an interaction\n",
            "inside an interface body.\n",
            "  breaking    always — a retired identity is never reused (ridl 11,\n",
            "              RIDL-401)"
        ),
        Category::ServiceChanged => concat!(
            "A service switched between the named list and an inline shape.\n",
            "  breaking    always — extraction rewrites the transport identity of every\n",
            "              fallible query in the shape: an inline shape derives it from\n",
            "              the service's dotted name, a named interface from its own name\n",
            "              (ADR-0008 d4, ADR-0015 d15). A changed list is not this\n",
            "              category: it is read as a set by the service_interface_*\n",
            "              rows (ADR-0015 d19, as amended 2026-09-15)"
        ),
        Category::ServiceInterfaceAdded => concat!(
            "An interface joined a service's set of interfaces.\n",
            "  compatible  always — nothing that existed moved: an interface's number\n",
            "              comes from its unit's interfaces.lock, not from its place\n",
            "              in the list, and the routing key does not contain the service\n",
            "              (ADR-0015 d19, as amended 2026-09-15)"
        ),
        Category::ServiceInterfaceRemoved => concat!(
            "An interface left a service's set of interfaces.\n",
            "  compatible  always — on the wire: the routing key does not contain the\n",
            "              service, so no identity moves, and a split into two services\n",
            "              is a removal plus an addition. Visible in source: the\n",
            "              service.member addresses of that interface stop resolving\n",
            "              under this service, which is why the text report lists it\n",
            "              under the heading \"compatible on the wire, visible in\n",
            "              source\". A consumer that loses its only provider is a\n",
            "              wiring error for rsdl, not a package diff (ADR-0015 d19, as\n",
            "              amended 2026-09-15)"
        ),
        Category::DocOnly => concat!(
            "Only doc comment, labels, or deprecation metadata changed.\n",
            "  compatible  always — none of it reaches a consumer's build.\n",
            "              Visibility is NOT in this category: see\n",
            "              visibility_changed"
        ),
        Category::VisibilityChanged => concat!(
            "The visibility a declaration is published at changed.\n",
            "  compatible  internal -> public: the declaration is offered to more\n",
            "              consumers than before\n",
            "  breaking    public -> internal: `internal` maps to the target's\n",
            "              package-private mechanism — Rust `pub(crate)`, a\n",
            "              non-exported TypeScript member (ADR-0002 8) — so the\n",
            "              declaration disappears from every out-of-package\n",
            "              consumer's build. The wire layout does not move, but the\n",
            "              consumer-visible guarantee narrows (ADR-0008 d14). Any\n",
            "              direction involving an unset visibility is breaking"
        ),
    }
}