NixOS/nix

[FEAT]: Introduce new error message for packages without `mainProgram`

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#15138 opened on Feb 3, 2026

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Description

Is your feature request related to a problem?

Some packages have multiple binaries with different purposes therefore setting mainProgram is not desirable.

When mainProgram is not set for these packages nix run errors out with

error: unable to execute '/nix/store/abc/bin/xyz': No such file or directory

Proposed solution

A new error for packages that don't have the mainProgram attribute set, something like

error: in 'abc': 'mainProgram' isn't set

Alternative solutions

An option to set mainProgram to null for packages that desire to do so, leaving the default behavior of mainProgram = pname; intact. An error like

error: 'abc' doesn't have a default binary

would be better in this case.

Additional context

https://github.com/NixOS/nixpkgs/issues/486543

Checklist

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Contributor guide

Tech stack
cpp
Domain
cli
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Basic understanding of NixFamiliarity with Nix error handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Investigate where the error 'unable to execute ... No such file or directory' is produced (likely in src/libstore or src/libexpr). Look for how mainProgram is handled in drv evaluation. The goal is to add a check before that error is thrown when mainProgram is not set, emitting a clearer message like 'mainProgram isn't set'. Refer to the linked issue https://github.com/NixOS/nixpkgs/issues/486543 for context. The implementation may involve modifying the Nix expression evaluator or the build execution code path.