Missing function coverage when transform prepends code right before the function

[hi-ogawa]

Describe the bug

EDIT: I made a simpler reproduction with user land transform #7130 (comment).

related:

• fix(ssrTransform): preserve line offset when transforming imports vitejs/vite#19004
• fix(ssr): hoist export to handle cyclic import better vitejs/vite#18983

After vitejs/vite#19004, a following code is transformed in this way (source map vis)

input

export function f(a: number, b: number) {
	return a + b;
}

import "node:path"

output

const __vite_ssr_import_0__ = await __vite_ssr_import__("node:path");function f(a, b) {
  return a + b;
}
Object.defineProperty(__vite_ssr_exports__, "f", { enumerable: true, configurable: true, get(){ return f }});;

This seems to cause function f to be considered not covered even when f is executed. I confirmed Vite commit right before vitejs/vite#19004 didn't have this issue.

---

Independently from vitejs/vite#19004, I'm trying to hoist Object.defineProperty in vitejs/vite#18983 and that seems to cause a similar missing coverage.

Transform works like this (source map vis)

input

export function f(a: number, b: number) {
	return a + b;
}

output

Object.defineProperty(__vite_ssr_exports__, "f", { enumerable: true, configurable: true, get(){ return f }});function f(a, b) {
  return a + b;
}

---

People usually don't write import after the export like the first case and we didn't have a test case for that, so I didn't notice it when reviewing vitejs/vite#19004. While testing my PR #7096, this just got caught because it broke the coverage test, which has this pattern (export function at the first line):

vitest/test/coverage-test/fixtures/src/math.ts

Line 1 in 178f515

export function sum(a: number, b: number) {

Reproduction

https://github.com/hi-ogawa/reproductions/tree/main/vitest-function-coverage-vite-19004
https://github.com/hi-ogawa/reproductions/tree/main/vitest-function-coverage-prepend-transform

System Info

System:
    OS: Linux 6.12 Arch Linux
    CPU: (16) x64 12th Gen Intel(R) Core(TM) i7-12650H
    Memory: 10.09 GB / 31.05 GB
    Container: Yes
    Shell: 5.2.37 - /usr/bin/bash
  Binaries:
    Node: 20.18.1 - ~/.volta/tools/image/node/20.18.1/bin/node
    npm: 10.8.2 - ~/.volta/tools/image/node/20.18.1/bin/npm
    pnpm: 9.15.1 - ~/.volta/bin/pnpm
    bun: 1.1.29 - ~/.volta/bin/bun
  Browsers:
    Chromium: 131.0.6778.85
  npmPackages:
    @vitest/coverage-v8: 3.0.0-beta.3 => 3.0.0-beta.3 
    vitest: 3.0.0-beta.3 => 3.0.0-beta.3

Used Package Manager

pnpm

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

The 2nd one is because the lines with __vite_ssr_exports__ is stripped away for v8 coverage.

vitest/packages/coverage-v8/src/provider.ts

Lines 34 to 35 in 178f515

	const VITE_EXPORTS_LINE_PATTERN
	= /Object\.defineProperty\(__vite_ssr_exports__.*\n/g

vitest/packages/coverage-v8/src/provider.ts

Line 402 in 178f515

trimmed.replaceAll(VITE_EXPORTS_LINE_PATTERN, '\n')

[hi-ogawa]

I think this is a bug in coverage conversion and probably we can fix it. I made a simpler reproduction with user land transform plugin. https://github.com/hi-ogawa/reproductions/tree/main/vitest-7130-prepended-function-coverage

Probably the issue is that istanbul fails to filter out "injected function" from the coverage when it lives in the same line which has mappings. https://github.com/istanbuljs/v8-to-istanbul/blob/46b972458b40ea0d628c80dd60223cc909f81cd0/lib/v8-to-istanbul.js#L144-L149

For example, for the following generated code, istanbul shows prepended in fnMap while appended is removed.

// source
console.log("x");

// generated
function prepended(){};console.log("x");
function appended(){};

---

Hmm, actually the reason why appended is filtered properly is because Vitest got this patch?

• feat: exclude lines that are missing from source maps istanbuljs/v8-to-istanbul#244

[hi-ogawa]

The issue might be actually same for istanbul provider based on the same simple reproduction of #7130 (comment).
I thought I only saw issues with v8 for Vite SSR transform, but I need to look into that again.

---

Probably relevant coverage filtering logic on istanbul is here https://github.com/istanbuljs/istanbuljs/blob/06eec782dc8a248f0516cdba06b280c410515890/packages/istanbul-lib-source-maps/lib/transformer.js#L34-L62.

[AriPerkkio]

Hmm, actually the reason why appended is filtered properly is because Vitest got this patch?

istanbuljs/v8-to-istanbul#244

This patch only affects line coverage of source files. Lines of transformed files do not affect that patch.