first draft of "forbid with"

Author: 6543

rfcs/0190-forbid-with.md

@@ -0,0 +1,123 @@
+---
+feature: ban-with-expressions
+start-date: 2025-10-07
+author: 6543
+co-authors:
+shepherd-team:
+shepherd-leader:
+related-issues:
+  - https://github.com/NixOS/rfcs/pull/120
+  - https://github.com/NixOS/nixpkgs/pull/413393
+---
+
+# Summary
+[summary]: #summary
+
+Completely prohibit the usage of `with` expressions in nixpkgs to improve code clarity, maintainability, and enable better static analysis. Instead using explicit `inherit` statements should be used if bringing attributes into scope is desired.
+
+# Motivation
+[motivation]: #motivation
+
+The `with` expression in Nix creates several problems that impact code quality and tooling:
+
+- **Unclear variable origins**: When reading code with `with`, it's unclear where variables come from without evaluating the expression
+- **Difficult static analysis**: Tools cannot determine variables without running full nix evaluation
+- **Debugging difficulties**: Error messages become less helpful when variable sources are ambiguous
+- **Code review challenges**: Reviewers can not simply relay on a diff view and must read the full code and not miss any scope
+
+The `inherit` statement provides a better alternative that is:
+
+- **Explicit**: Clearly shows which attributes are being brought into scope
+- **Traceable**: Easy to see where each variable comes from
+- **Analyzable**: Static analysis tools can easily understand the code structure
+- **Debuggable**: Error messages can point to specific inherit statements
+
+# Detailed design
+[design]: #detailed-design
+
+1. **Immediate prohibition**: New code in Nixpkgs must not use `with` expressions
+2. **Gradual migration**: Existing `with` expressions should be migrated to `inherit` over time
+3. **CI enforcement**: Add automated checks to prevent new `with` expressions from being merged
+4. **Documentation updates**: Update Nixpkgs contributor guidelines to reflect this policy
+
+# Examples and Interactions
+[examples-and-interactions]: #examples-and-interactions
+
+## Simple attribute access
+
+```nix
+# Before
+with pkgs; [
+ git
+ vim
+ curl
+]
+
+# After  
+let
+  inherit (pkgs) git vim curl;
+in [
+ git
+ vim 
+ curl
+]
+```
+
+## Function arguments
+
+```nix
+# Before
+{ pkgs, ... }:
+with pkgs; {
+  buildInputs = [ git vim ];
+}
+
+# After
+{ pkgs, ... }:
+let
+  inherit (pkgs) git vim;
+in {
+  buildInputs = [ git vim ];
+}
+```
+
+## Nested scopes
+
+```nix
+# Before
+with pkgs; {
+  meta = with lib; {
+    license = with licenses; [ mit ];
+  };
+}
+
+# After
+let
+  inherit (pkgs.lib.licenses) mit;
+in {
+  meta = {
+    license = [ mit ];
+  };
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- **Increased verbosity**: Code may become slightly longer due to explicit prefixes or inherit statements
+- **Migration effort**: Existing codebase requires changes
+- **Learning curve**: Contributors familiar with `with` need to adapt to new patterns
+- **Backward incompatible**: requires changing Nix code used "in the wild"
+
+However, these drawbacks are outweighed by the benefits of clearer, more maintainable code.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Just discourage usage of `with`, but in this case most things still can not be statically checked.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+# Future work
+[future]: #future-work