feat(sprintf): support Go-compatible %q for strings by vitaliytv · Pull Request #809 · microsoft/regorus

Vitalii Tverdokhlib (vitaliytv) · 2026-09-06T11:22:53Z

Summary

Add support for Go-syntax %q formatting for Regorus string values. The implementation now matches Go 1.27.1 / OPA 1.20.2 for the supported string forms:

emits a double-quoted Go string literal;
uses Go's short escapes and \\xNN / \\uNNNN / \\UNNNNNNNN escapes;
preserves printable non-ASCII characters and does not HTML-escape <, >, or &;
applies the width and precision forms already recognized by the current Regorus sprintf parser (%Nq, %0Nq, and %.Nq);
appends directly to the sprintf output buffer and propagates allocator memory-limit errors.

Correctness changes

The initial implementation approximated Go's strconv.IsPrint with Rust character predicates. That differs for Unicode format characters, private-use characters, noncharacters, and unassigned code points. This revision ports the compact generated tables and lookup algorithm from Go 1.27.1's strconv/isprint.go (Unicode 17.0.0), so values such as U+00AD, U+200B, U+E000, U+FDD0, and U+0378 are escaped exactly like Go.

Go's %q applies string precision before quoting and field width after quoting, both measured in Unicode code points. The supported Regorus forms now do the same, including Unicode input and zero padding.

Verification

Compared directly with OPA 1.20.2 (built with Go 1.27.1) for regular strings, every escape class, printable Unicode, the Unicode edge cases above, width, zero padding, precision, and emoji width/precision.

Local checks:

cargo test --locked
cargo test --locked --no-default-features --lib
cargo check --locked --no-default-features
cargo xtask test-no-std
allocator memory-limit regression test for %q
cargo clippy --locked --lib --all-features -- -D warnings
repository pre-commit hook (all build, fmt, and clippy stages)
repository pre-push hook

Scope and full Go `%q`

This PR intentionally handles Value::String only and preserves the repository's existing partial sprintf parser. A complete implementation of Go's %q would also require:

combined width and precision (%8.3q);
+ for ASCII-only quoting, # for backquoted strings when allowed, and - for right padding;
dynamic width/precision and argument indexes (*, [n]);
integer/rune formatting as a single-quoted character literal.

OPA adds another compatibility layer before calling fmt.Sprintf: strings remain Go strings, fitting integers become Go integers, while booleans, nulls, arrays, objects, and sets are converted to their OPA string representation. Reproducing all of that is broader than a quoting helper and should be implemented with a proper format-spec parser and explicit OPA value-conversion rules rather than incrementally extending this string-only branch.

Go's `%q` verb (strconv.Quote) is used by OPA's sprintf, but regorus currently bails on it unconditionally. Add support for `Value::String` arguments only: wrap in double quotes, escape `"` and `\`, use the short escapes for the common control characters (\a \b \f \n \r \t \v), fall back to \xNN/\uNNNN/\UNNNNNNNN for other non-printable characters, and leave printable (including non-ASCII) characters untouched. Unlike json.marshal, %q does not HTML-escape < > &. %q on non-string values (numbers, bools, etc.) still bails as before - Go's %q on those produces different, single-quoted output that is out of scope for this change. Verified byte-for-byte against real OPA output (strconv.Quote semantics) for quotes, backslashes, control characters, DEL, non-ASCII printable text, and the < > & non-escaping case. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Vitalii Tverdokhlib (vitaliytv) · 2026-09-07T03:46:19Z

Follow-up for the complete format-spec parser and broader Go/OPA %q semantics: #810. It is intentionally stacked on this PR.

Vitalii Tverdokhlib (vitaliytv) · 2026-09-07T04:16:47Z

Would you prefer this change to be reviewed in two stages, or as one larger PR?

Keep feat(sprintf): support Go-compatible %q for strings #809 focused on correct Go-compatible string quoting (including width and precision), and review feat(sprintf): support full %q format specifications #810 separately for the complete format-spec parser, flags, dynamic/indexed operands, and broader OPA value semantics. This is my preference because it keeps the core fix smaller and makes the additional compatibility scope explicit.
Fold the feat(sprintf): support full %q format specifications #810 commit into feat(sprintf): support Go-compatible %q for strings #809 and close the follow-up, if a single complete implementation would be easier to review.

#810 is currently stacked on this PR, so its diff against main temporarily includes #809. I am happy to reorganize the commits according to whichever approach you prefer.

Anand Krishnamoorthi (anakrish) · 2026-09-09T10:08:13Z

Vitalii Tverdokhlib (@vitaliytv) Thank you for the contribution. Going to take a deeper look at this today/tomorrow.

Copilot

🟡 Changes recommended

The new behavior lacks an RVM regression, so execution-path parity is not verified.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds Go-compatible %q string formatting with Unicode-aware quoting and memory-limit propagation.

Changes:

Adds Go Unicode printability tables and escaping.
Integrates %q width and precision handling.
Adds interpreter, unit, and memory-limit tests.

File summaries

File	Description
`src/builtins/strings.rs`	Implements `%q` formatting.
`src/builtins/strings/go_is_print.rs`	Adds Unicode printability lookup.
`tests/interpreter/cases/builtins/strings/sprintf.yaml`	Tests interpreter behavior.
`tests/memory_limits.rs`	Tests memory-limit propagation.
`LICENSE`	Adds Go source attribution.

Review details

Files reviewed: 5/5 changed files
Comments generated: 2
Review effort level: Balanced

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Anand Krishnamoorthi (anakrish) · 2026-09-10T21:21:23Z

Hey Vitalii Tverdokhlib (@vitaliytv), Overall looks good to me.

Can you also lock down the following as a test case:

  - note: "%q must escape Unicode line and paragraph separators"
    data: {}
    modules:
      - |
        package test

        line_separator := sprintf("%q", ["a\u2028b"])
        paragraph_separator := sprintf("%q", ["a\u2029b"])
    query: data.test
    want_result:
      line_separator: "\"a\\u2028b\""
      paragraph_separator: "\"a\\u2029b\""

Vitalii Tverdokhlib (vitaliytv) · 2026-09-11T03:46:08Z

Hey Vitalii Tverdokhlib (Vitalii Tverdokhlib (@vitaliytv)), Overall looks good to me.

Can you also lock down the following as a test case:

  - note: "%q must escape Unicode line and paragraph separators"
    data: {}
    modules:
      - |
        package test

        line_separator := sprintf("%q", ["a\u2028b"])
        paragraph_separator := sprintf("%q", ["a\u2029b"])
    query: data.test
    want_result:
      line_separator: "\"a\\u2028b\""
      paragraph_separator: "\"a\\u2029b\""

done

Vitalii Tverdokhlib (vitaliytv) and others added 2 commits September 6, 2026 14:20

fix(sprintf): match Go string quoting semantics

2c31535

Vitalii Tverdokhlib (vitaliytv) changed the title ~~feat(sprintf): support %q format verb for strings~~ feat(sprintf): support Go-compatible %q for strings Sep 7, 2026

Vitalii Tverdokhlib (vitaliytv) mentioned this pull request Sep 7, 2026

feat(sprintf): support full %q format specifications #810

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Anand Krishnamoorthi (anakrish) requested a balanced review from Copilot September 10, 2026 20:49

Copilot started reviewing on behalf of Anand Krishnamoorthi (anakrish) September 10, 2026 20:49 View session

Copilot AI reviewed Sep 10, 2026

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Comment thread src/builtins/strings.rs

Comment thread src/builtins/strings/go_is_print.rs Outdated

test(sprintf): cover Unicode separators in q verb

9b64c5b

Vitalii Tverdokhlib (vitaliytv) added 2 commits September 11, 2026 06:48

test(sprintf): cover q verb through RVM

fd93906

docs(sprintf): correct Go print table size

02695ea

Anand Krishnamoorthi (anakrish) approved these changes Sep 11, 2026

View reviewed changes

Anand Krishnamoorthi (anakrish) merged commit 8f56e8f into microsoft:main Sep 11, 2026
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feat(sprintf): support Go-compatible %q for strings - #809

feat(sprintf): support Go-compatible %q for strings#809
Anand Krishnamoorthi (anakrish) merged 5 commits into
microsoft:mainfrom
vitaliytv:feat/sprintf-q-verb

Vitalii Tverdokhlib (vitaliytv) commented Sep 6, 2026 •

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Vitalii Tverdokhlib (vitaliytv) commented Sep 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Correctness changes

Verification

Scope and full Go %q

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Vitalii Tverdokhlib (vitaliytv) commented Sep 7, 2026

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Vitalii Tverdokhlib (vitaliytv) commented Sep 6, 2026 •

edited

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Scope and full Go `%q`