diff --git a/pkgs/development/compilers/llvm/13/default.nix b/pkgs/development/compilers/llvm/13/default.nix
index 0469b77fe425..0a1b90fc9fd4 100644
--- a/pkgs/development/compilers/llvm/13/default.nix
+++ b/pkgs/development/compilers/llvm/13/default.nix
@@ -16,25 +16,51 @@
     then null
     else pkgs.bintools
 , darwin
+# LLVM release information; specify one of these but not both:
+, gitRelease ? null
+  # i.e.:
+  # {
+  #   version = /* i.e. "15.0.0" */;
+  #   rev = /* commit SHA */;
+  #   rev-version = /* human readable version; i.e. "unstable-2022-26-07" */;
+  #   sha256 = /* checksum for this release, can omit if specifying your own `monorepoSrc` */;
+  # }
+, officialRelease ? { version = "13.0.1"; sha256 = "06dv6h5dmvzdxbif2s8njki6h32796v368dyb5945x8gjj72xh7k"; }
+  # i.e.:
+  # {
+  #   version = /* i.e. "15.0.0" */;
+  #   candidate = /* optional; if specified, should be: "rcN" */
+  #   sha256 = /* checksum for this release, can omit if specifying your own `monorepoSrc` */;
+  # }
+# By default, we'll try to fetch a release from `github:llvm/llvm-project`
+# corresponding to the `gitRelease` or `officialRelease` specified.
+#
+# You can provide your own LLVM source by specifying this arg but then it's up
+# to you to make sure that the LLVM repo given matches the release configuration
+# specified.
+, monorepoSrc ? null
+
 }:
 
+assert let
+  int = a: if a then 1 else 0;
+  xor = a: b: ((builtins.bitXor (int a) (int b)) == 1);
+in
+  lib.assertMsg
+    (xor
+      (gitRelease != null)
+      (officialRelease != null))
+    ("must specify `gitRelease` or `officialRelease`" +
+      (lib.optionalString (gitRelease != null) " — not both"));
 let
-  release_version = "13.0.1";
-  candidate = ""; # empty or "rcN"
-  dash-candidate = lib.optionalString (candidate != "") "-${candidate}";
-  rev = ""; # When using a Git commit
-  rev-version = ""; # When using a Git commit
-  version = if rev != "" then rev-version else "${release_version}${dash-candidate}";
-  targetConfig = stdenv.targetPlatform.config;
+  monorepoSrc' = monorepoSrc;
+in let
+  # Import releaseInfo separately to avoid infinite recursion
+  inherit (import ../common/common-let.nix { inherit lib gitRelease officialRelease; }) releaseInfo;
+  inherit (releaseInfo) release_version version;
+  inherit (import ../common/common-let.nix { inherit lib fetchFromGitHub release_version gitRelease officialRelease monorepoSrc'; }) llvm_meta monorepoSrc;
 
-  src = fetchFromGitHub {
-    owner = "llvm";
-    repo = "llvm-project";
-    rev = if rev != "" then rev else "llvmorg-${version}";
-    sha256 = "06dv6h5dmvzdxbif2s8njki6h32796v368dyb5945x8gjj72xh7k";
-  };
-
-  inherit (import ../common/common-let.nix { inherit lib release_version; }) llvm_meta;
+  src = monorepoSrc;
 
   tools = lib.makeExtensible (tools: let
     callPackage = newScope (tools // { inherit stdenv cmake libxml2 python3 isl release_version version src buildLlvmTools; });
diff --git a/pkgs/development/compilers/llvm/14/default.nix b/pkgs/development/compilers/llvm/14/default.nix
index 442b0f731579..da37ce0ef1e2 100644
--- a/pkgs/development/compilers/llvm/14/default.nix
+++ b/pkgs/development/compilers/llvm/14/default.nix
@@ -16,25 +16,48 @@
     then null
     else pkgs.bintools
 , darwin
+# LLVM release information; specify one of these but not both:
+, gitRelease ? null
+  # i.e.:
+  # {
+  #   version = /* i.e. "15.0.0" */;
+  #   rev = /* commit SHA */;
+  #   rev-version = /* human readable version; i.e. "unstable-2022-26-07" */;
+  #   sha256 = /* checksum for this release, can omit if specifying your own `monorepoSrc` */;
+  # }
+, officialRelease ? { version = "14.0.6"; sha256 = "sha256-vffu4HilvYwtzwgq+NlS26m65DGbp6OSSne2aje1yJE="; }
+  # i.e.:
+  # {
+  #   version = /* i.e. "15.0.0" */;
+  #   candidate = /* optional; if specified, should be: "rcN" */
+  #   sha256 = /* checksum for this release, can omit if specifying your own `monorepoSrc` */;
+  # }
+# By default, we'll try to fetch a release from `github:llvm/llvm-project`
+# corresponding to the `gitRelease` or `officialRelease` specified.
+#
+# You can provide your own LLVM source by specifying this arg but then it's up
+# to you to make sure that the LLVM repo given matches the release configuration
+# specified.
+, monorepoSrc ? null
 }:
 
+assert let
+  int = a: if a then 1 else 0;
+  xor = a: b: ((builtins.bitXor (int a) (int b)) == 1);
+in
+  lib.assertMsg
+    (xor
+      (gitRelease != null)
+      (officialRelease != null))
+    ("must specify `gitRelease` or `officialRelease`" +
+      (lib.optionalString (gitRelease != null) " — not both"));
 let
-  release_version = "14.0.6";
-  candidate = ""; # empty or "rcN"
-  dash-candidate = lib.optionalString (candidate != "") "-${candidate}";
-  rev = ""; # When using a Git commit
-  rev-version = ""; # When using a Git commit
-  version = if rev != "" then rev-version else "${release_version}${dash-candidate}";
-  targetConfig = stdenv.targetPlatform.config;
-
-  monorepoSrc = fetchFromGitHub {
-    owner = "llvm";
-    repo = "llvm-project";
-    rev = if rev != "" then rev else "llvmorg-${version}";
-    sha256 = "sha256-vffu4HilvYwtzwgq+NlS26m65DGbp6OSSne2aje1yJE=";
-  };
-
-  inherit (import ../common/common-let.nix { inherit lib; }) llvm_meta;
+  monorepoSrc' = monorepoSrc;
+in let
+  # Import releaseInfo separately to avoid infinite recursion
+  inherit (import ../common/common-let.nix { inherit lib gitRelease officialRelease; }) releaseInfo;
+  inherit (releaseInfo) release_version version;
+  inherit (import ../common/common-let.nix { inherit lib fetchFromGitHub release_version gitRelease officialRelease monorepoSrc'; }) llvm_meta monorepoSrc;
 
   tools = lib.makeExtensible (tools: let
     callPackage = newScope (tools // { inherit stdenv cmake libxml2 python3 release_version version monorepoSrc buildLlvmTools; });