Field Mutation v1

Audience: language users, tool authors, and compiler contributors.

Status: implemented bounded profile; HOSTED GREEN under the v0.4.0 release baseline. Historical local, authoring-time, ignored, device/simulator, or separately provisioned evidence below retains its narrower scope; public promotion, registry publication and broader product completion remain separately gated.

Objective

Records and classes are value-semantics aggregates. This tranche admits whole-field replacement of one direct checked Copy scalar field of a let mut record- or class-typed local:

let mut point = Point { x: 1, y: 2, enabled: false };
point.x = point.x + 41;

Everything else stays closed: nested place chains (a.b.c = ...), aggregate or String/resource fields, mutation through parameters or immutable bindings (including method receivers), arrays (none exist), and any new expression forms. Simple <binding> = <expr>; assignment keeps its exact previous behavior, diagnostics, and serialization.

Semantics

1. Evaluation order matches Explicit Mutation v1 exactly: the assigned value is evaluated completely before the store; checked-arithmetic failure statuses propagate exactly as from the same expression in an initializer position. The left-hand side is a pure place (one local binding plus one field offset), so RHS-first lowering is observationally identical to left-to-right evaluation.
2. The target binding must be a let mut local whose type is a nominal record or class; the named field must exist and be a scalar Copy type (i64, i32, u8, char, f32, f64, bool) after generic substitution. Whole-field replacement only; no partial or nested stores.
3. Types must match the field type exactly; there is no conversion. The store creates no new value identity: the statement reuses the original binding's ValueId and names the targeted field's stable id.

Diagnostics (family SPX-U1xx, continuing after SPX-U106)

Code	Meaning
SPX-U107	Field assignment targets an immutable binding (declare it let mut).
SPX-U108	The target record/class has no field with the given name.
SPX-U109	The targeted field is not a direct scalar Copy field.
SPX-U110	Assigned value type does not exactly match the field type.
SPX-U111	Nested place chains (a.b.c = ...) are outside this slice (parse-time).
SPX-U112	Field mutation base is not a record/class value.

Unknown assignment names keep the established unknown-value diagnostic (SPX-T202); contract expressions still reject every assignment via SPX-U106.

Layer behavior

• Parser/AST: Statement::Assign carries field: Option<FieldTarget>; lookahead recognizes <binding> (. <field>)+ = so deeper chains enter the assignment path and fail with SPX-U111. Programs without the new syntax parse identically.
• Formatter: renders {name}.{field} = {value};; all pre-existing programs format byte-for-byte identically.
• HIR/resolver: ResolvedStatement::Assign gained field: Option<DeclarationId>; both the iterative resolver and its recursive oracle twin resolve the field before the assigned value and agree on U107–U112 fail-closed checks. Source-level verification implements the same checks for functions; method bodies remain covered by resolution itself.
• Graph: statement_json emits an additive "field":"<stable-id>" attribute on assign nodes only; programs without field mutation serialize byte-for-byte identically (pinned by the pre-feature digest in tests/language/explicit_mutation.rs and the meaning.spx revision pin).
• Cleanup: Copy-scalar field stores lower their RHS exactly like an initializer and never transfer into cleanup slots; straight-line field mutation produces CleanupPlan v2 output structurally identical to the initializer-only equivalent (asserted modulo function names in tests). Note that extending ResolvedStatement legitimately grows the deterministic HIR_EXPR_FIXED_BUNDLE budget-model term; affected workspace budget KATs were re-pinned accordingly in tests/workspace/semantic_graph.rs.
• Native C11: plain struct-member store local.field = value; after full RHS evaluation; O0/O2 identical including failure statuses.
• Wasm: aggregate lane computes the field pointer (binding frame slot + layout offset) and copies the fully evaluated scalar into it; i32 overflow traps/fails identically to initializer positions.

Evidence

tests/language/field_mutation.rs pins canonical round-trips, all six new U-family regressions, additive deterministic Graph JSON (schema stays v10), CleanupPlan structural equality, native O0/O2 probes over records and classes including branch-local mutation, Node/Wasm equivalence (134 corpus result, overflow trap), and native failure-status selection. examples/field_mutation.spx exercises the feature under the example check/format/run gates.

Non-claims

No nested-place stores, no aggregate/String/resource field stores, no method receiver mutation, no mutation through parameters, no arrays, no references, no concurrency claims, and no change to simple-assignment behavior.