Reject mutations in condition expressions

[LunaStev]

Summary

• reject assignments, compound assignments, and increment/decrement expressions inside if, else if, while, and for conditions
• detect mutations even when they are nested inside grouped, binary, call, index, aggregate, or other expression forms
• preserve ordinary assignment statements and for increment expressions
• provide actionable diagnostics that distinguish comparison mistakes from intentional mutations

Root cause

Assignment expressions return the assigned value, and Wave permits scalar values such as char in truthy conditions. As a result, code such as if (command[0] = 'h') mutated the value and then entered the branch instead of being rejected as a likely comparison mistake.

Impact

Condition evaluation can no longer silently mutate program state through assignment-like expressions. Programs must perform mutations before the condition and use an explicit comparison or value test inside the condition.

Validation

• cargo fmt --all --check
• cargo test --locked --all-targets --verbose
• cargo clippy --locked --all-targets -- -D warnings
• manual wavec check reproduction for if (command[0] = 'h')