From 51ba14b167e2e222fbc866485b8ea0d7f8fe2bfd Mon Sep 17 00:00:00 2001
From: Peter Waller <p@pwaller.net>
Date: Sun, 10 Nov 2024 00:23:24 +0000
Subject: [PATCH] llvmPackages: Make targetLlvmLibraries overridable

Pull #320261 introduced the possibility to consistently override
dependencies within an llvm package set. This is useful for development
and testing exotic configurations.

Go one step further and enable overriding targetLlvmLibraries.

This makes it possible to write an overlay such as:

```nix
overlays = [
  (self: super: {
    llvmPackages = super.llvmPackages.override (prev: {
      targetLlvmLibraries = super.targetPackages.llvmPackages.libraries // {
        compiler-rt = super.targetPackages.llvmPackages.libraries.compiler-rt.override {
          ...
        }
      };
    });
  })
];
```

... where the overridden compiler-rt will be used in a pkgsLLVM build.

As a straw man, I've done the minimally invasive thing to the code
structure: `targetLlvmLibraries` is not an explicitly named parameter
for llvmPackages; but it is available in `packageSetArgs` if passed.
This makes it slightly less discoverable, but this seems like a
reasonable tradeoff considered that modifying this would be a fairly
advanced/esoteric thing to need to do.

In some ways it would be better to have as an explicit parameter with a
default, but the obvious thing won't work because the default needs to
be a non-trivial expression. Potentially we could instead have it as a
defaulted parameter with the value of 'null', and if it's null, then
compute the current thing.

Signed-off-by: Peter Waller <p@pwaller.net>
---
 pkgs/development/compilers/llvm/default.nix | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/pkgs/development/compilers/llvm/default.nix b/pkgs/development/compilers/llvm/default.nix
index 4d52e48d8775..774190ef0c0c 100644
--- a/pkgs/development/compilers/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/default.nix
@@ -63,7 +63,9 @@ let
             inherit (stdenvAdapters) overrideCC;
             buildLlvmTools = buildPackages."llvmPackages_${attrName}".tools;
             targetLlvmLibraries =
-              targetPackages."llvmPackages_${attrName}".libraries or llvmPackages."${attrName}".libraries;
+              # Allow overriding targetLlvmLibraries; this enables custom runtime builds.
+              packageSetArgs.targetLlvmLibraries or targetPackages."llvmPackages_${attrName}".libraries
+                or llvmPackages."${attrName}".libraries;
             targetLlvm = targetPackages."llvmPackages_${attrName}".llvm or llvmPackages."${attrName}".llvm;
             inherit
               officialRelease