Add split:"false" tag to keep string array values whole

[evanphx]

[]string flags split every value on commas. That is right for lists like --tags a,b but wrong for values that contain commas themselves, such as -e KEY=a,b,c or an HTTP header value, and there was no way to opt out.

This adds:

• StringArrayNoSplitVar, which registers a string array flag that contributes exactly one element per flag occurrence.
• A split:"false" struct tag that makes FromStruct use it. A default on such a field is kept whole too. Any value other than "true" or "false" is rejected when the struct is bound.

Existing comma-splitting behavior is unchanged for everything else, and the existing string array tests still pass unmodified.

Motivation: mirendev/runtime MIR-1168, where miren env set -e KEY=a,b,c was setting three variables.

[coderabbitai]

📝 Walkthrough

Walkthrough

The change adds configurable splitting for []string flags. String-array values split comma-separated input by default. The split:"false" tag and StringArrayNoSplitVar preserve each flag occurrence as one value. Struct-tag validation rejects invalid split values. Documentation and tests cover parsing, defaults, repeated values, nil initialization, and tag behavior.

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to bda34

Comma-containing string-array values are now preserved when configured, but split:"" is still accepted instead of rejected and falls back to comma splitting. This is a limited configuration issue suitable for bounded follow-up.

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[miren-code-agent]

🍪 biscuit: ✅ ready to merge — auto-review, non-blocking

I read through all five changed files and traced the full execution path from the struct tag down through fromstruct.go, mflags.go, and values.go. The implementation is clean and correct.

What's right:

• The noSplit flag is a one-line boolean added to stringArrayValue. The existing Set path is split cleanly — when noSplit is true, the value is appended as-is; otherwise the existing comma-split path runs unchanged. No existing behaviour is touched.
• StringArrayNoSplitVar mirrors StringArrayVar exactly in its nil-vs-empty-default semantics, so the two behave consistently.
• fromstruct.go handles the three valid tag values ("", "true", "false") and returns a descriptive error for anything else — that's the right design; silent ignoring would be worse.
• Type() returning "value" (instead of "value,...") is a useful help-text improvement; it accurately signals that the flag doesn't accept comma-separated lists.
• The test file (stringarray_nosplit_test.go) covers the meaningful cases: single comma-containing value, multiple occurrences with mixed syntax (-e, -eVAL, --env=VAL), default-replacement-on-first-set, nil default, split:"false" and split:"true" through FromStruct, and the invalid tag value rejection.

One small gap worth noting:

There is a non-Var convenience form StringArray(…) *[]string for the splitting variant, but no StringArrayNoSplit(…) *[]string equivalent. This isn't a blocker — FromStruct and StringArrayNoSplitVar are both sufficient — but callers who build flag sets imperatively rather than via structs would have to use the Var form. Worth adding in a follow-up if the API surface is important for consistency.

Everything else looks solid. No correctness bugs, no regressions to existing behaviour, tests are meaningful and passing. Ready to merge.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@fromstruct.go`:
- Around line 428-429: Update the split-tag handling around the field tag switch
to use field.Tag.Lookup("split"), distinguishing an absent tag from an
explicitly empty value; accept only present values "true" and "false", and
reject split:"" while preserving the default behavior when the tag is absent.
Add a regression test covering an explicit empty split tag.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 1a9c108 and bda3448.

📒 Files selected for processing (5)

• README.md
• fromstruct.go
• mflags.go
• stringarray_nosplit_test.go
• values.go

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