Partial

Partial builds values incrementally. It has two frame-management modes:

• Strict (default): A frame must be fully initialized before you end() it. Best for inputs where nested data is contiguous (JSON/YAML objects).
• Deferred: Frames can be popped and stored, then re-entered later. Validation happens when you call finish_deferred(). Use this when nested fields arrive out of order or interleaved.

When to use deferred

• TOML dotted keys: inner.x = 1, later inner.y = 3.
• Flattened structs where child fields are mixed with parent fields.
• Any format that lets you jump in and out of nested paths.

API sketch

use facet_reflect::{Partial, Resolution};
use facet_solver::ResolutionBuilder; // or other way to obtain Resolution

let resolution: Resolution = /* describe the shape (fields, paths) */;

let partial = Partial::alloc::<Config>()?
    .begin_deferred(resolution)?   // enter deferred mode
    .begin_field("inner")?
        .set_field("x", 1i32)?
    .end()?                        // frame stored, not yet validated
    .set_field("count", 2u32)?
    .begin_field("inner")?         // re-enter; restores stored frame
        .set_field("y", 3i32)?
    .end()?
    .finish_deferred()?            // validates all stored frames
    .build()?;                     // produce the final value

Rules of thumb

• Call begin_deferred(resolution) once; it errors if already in deferred mode.
• Always call finish_deferred() before build(); missing fields are validated there.
• Paths are tracked relative to where deferred began; re-entering a path restores its stored frame and state.
• Strict mode remains the safest default; prefer deferred only when the input order demands it.

More examples coming soon (collections, enums, nested flatten).