Trait PreWriteHook 

pub trait PreWriteHook {
    // Required method
    fn before_write(
        &mut self,
        cell: StateCellId,
        loc: SlotLocation,
        old: &FourStateValue,
        new_incoming: &FourStateValue,
        at: SimTime,
    );
}

The ST-091/ST-031 “hook present in the calling convention from the first freeze” requirement made concrete now: commit_write takes hook: &mut dyn PreWriteHook as a required parameter, not an optional add-on, so a later real undo-log PreWriteHook implementer slots in without touching this signature or any of its call sites.

Deviation from the plan’s literal signature (binding per this task’s own unresolved-review-findings list): before_write also receives cell: StateCellId, not just loc/old/new_incoming — commit_write already has cell in scope at the call site, and a later undo-log implementer needs the StateCellId (not merely a SlotLocation) to replay a rollback via commit_write_no_hook(cell, ...); no SlotLocation -> StateCellId reverse lookup exists anywhere in this crate, so cell must be threaded through here directly.

Second deviation (Task 16): before_write also receives at: SimTime. Task 16’s UndoLog (state::rollback) implements this trait and must tag every entry it records with the SimTime the write happened at — fossil_collect(gvt)’s own acceptance text (“trims entries the caller asserts are provably before gvt”) is only meaningful if the log’s entries actually carry a timestamp to compare against gvt, and nothing else in this calling convention threads one to the hook. The one call site (commit_write_at, below) already reads vc.current_time() for its own VcRecord.at field, so passing that same value to the hook costs nothing extra and keeps both timestamps (the VC record’s and the undo entry’s) identically sourced.

Required Methods

fn before_write( &mut self, cell: StateCellId, loc: SlotLocation, old: &FourStateValue, new_incoming: &FourStateValue, at: SimTime, )

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementations on Foreign Types

impl PreWriteHook for ()

The no-op default: v1 (LOOKAHEAD_CYCLES = 0) never exercises rollback, so every call site that doesn’t care passes &mut ().

fn before_write( &mut self, _cell: StateCellId, _loc: SlotLocation, _old: &FourStateValue, _new_incoming: &FourStateValue, _at: SimTime, )

Implementors

impl PreWriteHook for UndoLog