fix(runtime): serialize BigInt attribute values instead of throwing

[chatman-media]

Problem

Fixes #3437.

Passing a BigInt as an attribute value throws instead of rendering it. For example:

pug.render("input(type='hidden' value=whatIsIt)", { whatIsIt: BigInt(10) });

On the current master this throws:

TypeError: Do not know how to serialize a BigInt

(On 3.0.2, as reported in #3437, it produced value=""10"".) The expected output is value="10", exactly as it already works for a Number.

Cause

In pug_attr (packages/pug-runtime/index.js), every non-string value is serialized with JSON.stringify:

if (typeof val !== 'string') {
  val = JSON.stringify(val);
  ...
}

typeof someBigInt === 'bigint', so a BigInt reaches JSON.stringify, which throws for BigInt values. Number works only because JSON.stringify(10) === "10".

This affects both:

• runtime values (value=local where the local is a BigInt), and
• compile-time constant attributes, since pug-attrs folds constants through runtime.attr(...) too.

Fix

Coerce bigint to its decimal string before the JSON.stringify branch, mirroring how a Number is rendered and preserving precision beyond Number.MAX_SAFE_INTEGER:

if (typeof val === 'bigint') {
  val = val.toString();
}

Tests

Added BigInt cases to the existing attr test in packages/pug-runtime/test/index.test.js (all escaped/terse combinations, plus a value above Number.MAX_SAFE_INTEGER to confirm precision is preserved). The test fails without the fix (TypeError: Do not know how to serialize a BigInt) and passes with it.

The two failing tests in packages/pug/test/error.reporting.test.js are pre-existing on master (outdated V8 error-message wording, see #3466) and are unrelated to this change.

[rollingversions]

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