Provide mode to ignore all environment variables

[SimplyDanny]

Description

Working on Swift projects in VS Code has an annoying side effect, which is that switching between building within VS Code to building on the command-line basically leads to a full rebuild of the whole projects. That's presumably because environment variables are slightly different in both environments. That's what I learned from a lengthy thread in the Swift Open Source Slack.

Working without a manual --scratch-path in one or the other environment is therefore impossible (or very annoying). This, however, leads to 3 build locations (VS Code build, command-line build, index build) which host more or less the same build artifacts independently.

Expected behavior

One possible solution brought up by @MaxDesiatov is a mode for swift build (and perhaps other commands) that basically says "ignore all environment variables". Given that the same toolchain, compiler and compile options are in use, the same build folder could be used no matter from which environment the build got triggered.

This could also be helpful to make a package independent from the build context. Accessing environment variables from within manifest files would be an error or at least it would be in the user's responsibility to make sure there happens to be no access in the package's manifest and the manifests of all dependencies.

Actual behavior

Environment variables are the most volatile and uncontrollable influence on a build and cause regular rebuilds. Environments variables altering the build outcome is probably generally not a good pattern. The option could shield the build from outside influences.

Steps to reproduce

1. Make sure to use the same build toolchain in VS Code and the command-line.
2. Build in VS Code.
3. Build from the command-line.
4. Notice how a complete rebuild is performed.

Swift Package Manager version/commit hash

6.2.1

Swift & OS version (output of swift --version && uname -a)

Apple Swift version 6.2.1 (swift-6.2.1-RELEASE)
Darwin Kernel Version 25.1.0, macOS 26.1

[SimplyDanny]

@MaxDesiatov: Please correct/extend if there's anything incorrect or missing.

[MaxDesiatov]

This makes sense to me, thanks! The exact implementation is TBC. I don't know if we can remove absolutely all environment variables for a child process on POSIX-ish OSes. But even if there are "special" ones that we'll have to keep, maybe that's ok at a first approximation.

[plemarquand]

Are there specific environment variables that affect whether or not a rebuild happens? I tried a simple test with swift build then FOO=BAR swift build and the second build completed without rebuilding anything unnecessarily.

[MaxDesiatov]

Any environment variable that's not on this pretty short list will cause a package manifest reload and likely a rebuild as a consequence of that reload:

swift-package-manager/Sources/Basics/Environment/EnvironmentKey.swift

Line 38 in 4f5a634

package static let nonCachable: Set<Self> = [