pass eval.nix as a file instead of expression

- passing it as expression gives large error messages
  which are not very readable
- this commits puts the file in nix-store
  and patches the final program to have access to
  the path to the file as env.
- We simply pass this file to nix-instantiate

pkgs/test/nixpkgs-check-by-name/default.nix

@@ -5,8 +5,10 @@
   rustfmt,
   clippy,
   mkShell,
+  makeWrapper,
 }:
 let
+  runtimeExprPath = "${./src/eval.nix}";
   package =
     rustPlatform.buildRustPackage {
       name = "nixpkgs-check-by-name";
@@ -16,7 +18,9 @@ let
         nix
         rustfmt
         clippy
+        makeWrapper
       ];
+      env.NIX_CHECK_BY_NAME_EXPR_PATH = runtimeExprPath;
       # Needed to make Nix evaluation work inside the nix build
       preCheck = ''
         export TEST_ROOT=$(pwd)/test-tmp
@@ -34,7 +38,12 @@ let
         cargo fmt --check
         cargo clippy -- -D warnings
       '';
+      postInstall = ''
+        wrapProgram $out/bin/nixpkgs-check-by-name \
+          --set NIX_CHECK_BY_NAME_EXPR_PATH "$NIX_CHECK_BY_NAME_EXPR_PATH"
+      '';
       passthru.shell = mkShell {
+        env.NIX_CHECK_BY_NAME_EXPR_PATH = runtimeExprPath;
         inputsFrom = [ package ];
       };
     };

pkgs/test/nixpkgs-check-by-name/src/eval.rs

@@ -35,8 +35,6 @@ enum AttributeVariant {
     Other,
 }
 
-const EXPR: &str = include_str!("eval.nix");
-
 /// Check that the Nixpkgs attribute values corresponding to the packages in pkgs/by-name are
 /// of the form `callPackage <package_file> { ... }`.
 /// See the `eval.nix` file for how this is achieved on the Nix side
@@ -60,9 +58,9 @@ pub fn check_values(
         attrs_file_path.display()
     ))?;
 
+    let expr_path = std::env::var("NIX_CHECK_BY_NAME_EXPR_PATH")?;
     // With restrict-eval, only paths in NIX_PATH can be accessed, so we explicitly specify the
     // ones needed needed
-
     let mut command = process::Command::new("nix-instantiate");
     command
         // Inherit stderr so that error messages always get shown
@@ -76,8 +74,6 @@ pub fn check_values(
             "--readonly-mode",
             "--restrict-eval",
             "--show-trace",
-            "--expr",
-            EXPR,
         ])
         // Pass the path to the attrs_file as an argument and add it to the NIX_PATH so it can be
         // accessed in restrict-eval mode
@@ -96,6 +92,8 @@ pub fn check_values(
         command.arg("-I");
         command.arg(path);
     }
+    command.args(["-I", &expr_path]);
+    command.arg(expr_path);
 
     let result = command
         .output()