docs(db): add info about populating Auto fields

[m4tx]

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / cleanup
• Performance improvement
• Other (describe above)

[Copilot]

Copilot review overview

🟡 Changes recommended

Clarify that the documented requirement applies only to Auto-typed fields.

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Review effort: Lite
Findings: 1

Open (1)

• Clarify Auto::Fixed requirement applies only to Auto-typed fields · New

What changed in this PR

This documentation PR explains how Auto fields are populated and handled during database operations.

Changes:

• Added Auto population documentation and examples.
• Documented update behavior and panic conditions.
• Added backend trait documentation.

File	Summary
cot/​src/​db.rs	Expanded database API documentation and examples.

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[github-actions]

Bencher Report

Project	cot
Branch	db-auto
Testbed	github-ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	Benchmark Result milliseconds (ms) (Result Δ%)	Upper Boundary milliseconds (ms) (Limit %)
empty_router/empty_router	📈 view plot 🚷 view threshold	14.25 ms (+40.00%) Baseline: 10.18 ms	18.68 ms (76.28%)
json_api/json_api	📈 view plot 🚷 view threshold	1.16 ms (+12.73%) Baseline: 1.03 ms	1.37 ms (84.76%)
nested_routers/nested_routers	📈 view plot 🚷 view threshold	1.06 ms (+9.98%) Baseline: 0.96 ms	1.26 ms (83.99%)
single_root_route/single_root_route	📈 view plot 🚷 view threshold	1.03 ms (+11.29%) Baseline: 0.93 ms	1.22 ms (84.33%)
single_root_route_burst/single_root_route_burst	📈 view plot 🚷 view threshold	19.68 ms (+16.99%) Baseline: 16.82 ms	21.97 ms (89.56%)

🐰 View full continuous benchmarking report in Bencher

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
rust	90.04% <ø> (ø)

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Files with missing lines	Coverage Δ
cot/src/db.rs	84.06% <ø> (ø)

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[seqre]

Overall I think it's good, I just wonder if we need to copy&paste it everywhere 🤔

[m4tx]

Overall I think it's good, I just wonder if we need to copy&paste it everywhere 🤔

Yeah; my thoughts, too, but sadly there isn't much we can do about this cleanly considering these are doctests.

Maybe it's worth extracting this into some internal common place, but this isn't easy, as doctests are only allowed to test public API, so either doc(hidden) or putting this in cot-test could make sense.

In any case, I think we have more stuff like this, so I'd argue this is out of scope for this PR anyway. I've created #678 to track this.