fix(argv): reject duplicate flags

[jdx]

Summary

• reject repeated non-repeatable scalar and boolean flags
• preserve repeatability for counters, collections, variadic values, and var fields
• add Clap-style diagnostics and conformance coverage
• keep parsing allocation-free

This closes a behavior gap exposed by migrating the usage CLI itself: a scalar flag such as --format can no longer silently take its last repeated value.

Stack

Targets the command-alias migration PR. The missing-subcommand diagnostic PR follows this one, then #936.

Checks

• cargo test -p usage-conformance --test post_binding
• cargo test -p usage-argv --features diagnostics
• cargo test -p usage-derive
• full stacked cargo test --all --all-features
• full stacked Clippy with warnings denied

This PR was generated by Codex.

---

Note

Medium Risk
Changes core CLI parsing semantics for duplicate flags across all derived commands; behavior is well-tested but could surface new errors for invocations that previously succeeded silently.

Overview
Non-repeatable flags (scalar booleans and value-taking flags that are not var, counters, or collections) now fail parse with a new Error::DuplicateFlag instead of silently keeping the last occurrence—matching clap-style behavior for cases like repeated --format.

The derive tracks __duplicated_* (and __negated_* for negate pairs) during binding; post-binding checks emit the error. Positive/negative spellings (--color / --no-color) override each other like overrides, but repeating the same spelling still errors. Overrides still win over earlier duplicates (e.g. --file twice then --stdin). Displaced flags clear duplicate state so override flows stay correct.

Diagnostics render: the argument '--jobs' cannot be used multiple times. Conformance tests cover duplicates, repeatability, and negate/override interactions.

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[greptile-apps]

Greptile Summary

The PR rejects repeated non-repeatable scalar and boolean flags while preserving repetition for counters, collections, variadic values, and var fields.

• Adds duplicate-state tracking and post-binding validation to generated parsers.
• Clears duplicate state when an overriding flag displaces the duplicated flag.
• Distinguishes repeated boolean spellings from positive/negative spellings that override each other.
• Adds diagnostics and conformance coverage for duplicate and displacement behavior.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
derive/src/codegen.rs	Generates duplicate and negation state, clears duplicate state during displacement, and reports duplicate flags during post-binding validation; the previously reported stale-state issue is fixed.
conformance/tests/post_binding.rs	Adds regression coverage for duplicate scalar and boolean flags, repeatable flags, displaced duplicates, and positive/negative boolean spellings.
argv/src/lib.rs	Adds the borrowed DuplicateFlag error variant used by generated post-binding checks.
argv/src/diagnostic.rs	Renders the new duplicate-flag error with a Clap-style message and usage information.

_{Reviews (2): Last reviewed commit: "fix(derive): reset duplicate state for o..." | Re-trigger Greptile}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[github-actions]

Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	▁████████	180,301,068 → 180,363,699	+0.03%	17.50 → 17.32ms	-1.03%
startup	▁▆▂▂▅█▆██	1,226,319 → 1,226,289	-0.00%	1.00 → 0.99ms	-0.70%

No instruction-count regression above 1%.

_{Only instruction counts gate. Wall clock is shown for context — on identical hardware it moves 4-20% run to run.}

_{Measured by tak — instruction-counted CLI benchmarks, stored in this repository's git notes.}

Shadow comparison

Parsing mise use -g node@20 against a shadow of mise's committed spec.
Reported, not gated: the shadow grows as the derive learns to express more, so
what to watch is the ratio rather than either column.

	usage	clap	ratio
instructions, cold parse	64525	5895248	91x

usage: argv -> struct                            1052 ns      1.05 µs
clap: build tree + parse -> struct             498957 ns    498.96 µs
clap: parse -> struct, tree reused              23234 ns     23.23 µs
clap: build tree only                          310826 ns    310.83 µs

_{407362e095c3 vs 02e459bc675b · measured on the runner, not pushed to the history.}