prometeu-studio/docs/pbs/pull-requests/PR-016-pbs-assignment-target-and-field-access-semantics.md

PR-016 - PBS Assignment Target and Field Access Semantics

Briefing

Assignment validation currently does not enforce target existence, assignability, mutability, or write-access constraints. This PR closes those gaps.

Motivation

Unchecked assignment targets allow invalid programs through static semantics.

Target

• Assignment statement semantic analysis.
• Struct field read/write access validation (private, pub, pub mut).

Scope

• Resolve LValue path against known symbols.
• Validate assignment compatibility and write permissions.
• Emit deterministic diagnostics for invalid target/write access.

Method

• Add LValue resolver in flow semantic analyzer.
• Use struct metadata (isPublic, isMutable) for access checks.
• Distinguish read-access and write-access diagnostics.

Acceptance Criteria

• Missing assignment target is rejected deterministically.
• Writes to private fields from invalid contexts are rejected.
• Writes to non-pub mut external fields are rejected.
• Type mismatch in assignment is reported deterministically.

Tests

• Add assignment tests for local, field, and nested member targets.
• Add negative access tests (pub read-only vs pub mut).
• Add type-mismatch assignment tests.

Non-Goals

• New mutation syntax.
• Runtime memory model changes.