feat(rust): port config simulate to native Rust (Phase 1.3b)

[jd]

Completes the config pilot. Second native command, flipped in
PORT_STATUS.toml from shimmed to native.

What ports natively

mergify config simulate PULL_REQUEST_URL [-f PATH] [-t TOKEN] [-u URL]:

1. Parses the PR URL into (owner/repo, number). Invalid URLs
   are rejected by clap's value_parser, which exits 2 (matching
   Python's click.BadParameter).
2. Resolves the config file (same three-location search as
   config validate — shared helper in paths.rs).
3. Reads the YAML as a raw string (no parsing — the simulator API
   accepts the text verbatim).
4. Resolves the bearer token: explicit --token → MERGIFY_TOKEN
   → GITHUB_TOKEN → error. Matches Python's precedence; the
   gh auth token subprocess fallback isn't ported yet.
5. Resolves the API URL: explicit --api-url → MERGIFY_API_URL
   → https://api.mergify.com default.
6. POSTs {"mergify_yml": <content>} to
   /v1/repos/<repo>/pulls/<number>/simulator via the 1.2b HTTP
   client (auth + retry + typed errors).
7. Prints the returned title + summary to stdout.

Rich Markdown rendering of summary is deliberately deferred —
we print raw Markdown today. Human output drift is allowed by the
compat contract; machine-readable paths aren't affected because
config simulate has no --json flag in Python either.

Dispatch

mergify-cli/src/main.rs now carries two clap subcommands
(validate + simulate). The dispatch logic got a light
reshape: on clap parse failure we distinguish "obvious native
intent" (argv contains config + one of validate/simulate)
from "unrecognized command, fall through". Native-intent parse
errors surface clap's formatted error and exit 2; anything else
falls through to the Python shim so unported commands keep working.

Refactor

The config-path resolver moved from validate.rs into a shared
paths.rs module so both commands share a single source of
truth for the [".mergify.yml", ".mergify/config.yml", ".github/mergify.yml"] search list.

Tests

24 tests in mergify-config (up from 11 in Phase 1.3):

• 3 for the shared path resolver (moved out of validate.rs)
• 6 for PR URL parsing (happy path + 5 rejection cases)
• 4 for token resolution (explicit, env fallback, error)
• 2 for API URL resolution
• 1 end-to-end wiremock test: POST + JSON body + auth header +
  response rendering

Covered by the port-inventory guard: PORT_STATUS.toml flips
config simulate from shimmed to native.

Binary size: 8.0 MB → 8.3 MB (small bump from simulate.rs).

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

Depends-On: #1297

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	ci(renovate): group cargo / github-actions updates and enable lockFileMaintenance	#1307
2	feat(rust): add mergify-core::http with retry + typed errors (Phase 1.2b)	#1281
3	feat(rust): port config validate to native Rust (Phase 1.3)	#1282
4	test: add port-inventory guard to catch un-ported Python commands	#1296
5	ci: add Rust release workflow targeting GitHub Release assets	#1297
6	feat(rust): port config simulate to native Rust (Phase 1.3b)	#1298	👈
7	feat(rust): port ci scopes-send to native Rust (Phase 1.4)	#1300
8	feat(rust): port queue pause and unpause to native Rust (Phase 1.5)	#1301
9	feat(rust): port ci git-refs and ci queue-info to native Rust (Phase 1.6)	#1302

[mergify]

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• depends-on = Mergifyio/mergify-cli#1281 [⛓️ feat(rust): add mergify-core::http with retry + typed errors (Phase 1.2b) #1281 [stack]]
• depends-on = Mergifyio/mergify-cli#1282 [⛓️ feat(rust): port config validate to native Rust (Phase 1.3) #1282 [stack]]
• depends-on = Mergifyio/mergify-cli#1296 [⛓️ test: add port-inventory guard to catch un-ported Python commands #1296 [stack]]
• depends-on = Mergifyio/mergify-cli#1297 [⛓️ ci: add Rust release workflow targeting GitHub Release assets #1297 [stack]]
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• depends-on = Mergifyio/mergify-cli#1282 [⛓️ feat(rust): port config validate to native Rust (Phase 1.3) #1282 [stack]]
• depends-on = Mergifyio/mergify-cli#1296 [⛓️ test: add port-inventory guard to catch un-ported Python commands #1296 [stack]]
• depends-on = Mergifyio/mergify-cli#1297 [⛓️ ci: add Rust release workflow targeting GitHub Release assets #1297 [stack]]
• depends-on = Mergifyio/mergify-cli#1307 [⛓️ ci(renovate): group cargo / github-actions updates and enable lockFileMaintenance #1307 [stack]]

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[jd]

Revision history

#	Type	Changes	Reason	Date
1	initial	2a502cd	2026-04-23 19:02 UTC
2	rebase	2a502cd → e4397c6	2026-04-23 19:02 UTC
3	rebase	e4397c6 → 6dee3f1	2026-04-24 13:41 UTC
4	rebase	cc09d55 → f5f004a		2026-04-27 07:03 UTC
5	rebase	f5f004a → d7398e1		2026-04-28 06:48 UTC