Add mkGoEnv function

This creates an `mkGoEnv` function which takes care of adding the
correct Go package to your development environment and installs
development dependencies from tools.go in a Nix derivation.

The "normal" workflow around Go with tools.go just sticks development
dependencies in $GOBIN which isn't ideal since you have no separation
between projects.

builder/default.nix

@@ -33,6 +33,96 @@ let
       impureEnvVars = lib.fetchers.proxyImpureEnvVars ++ [ "GOPROXY" ];
     };
 
+  mkVendorEnv = { go, modulesStruct, localReplaceCommands ? [ ] }: runCommand "vendor-env"
+    {
+      nativeBuildInputs = [ go ];
+      json = builtins.toJSON modulesStruct;
+
+      sources = builtins.toJSON (
+        lib.mapAttrs
+          (goPackagePath: meta: fetchGoModule {
+            goPackagePath = meta.replaced or goPackagePath;
+            inherit (meta) version hash;
+            inherit go;
+          })
+          modulesStruct.mod
+      );
+
+      passAsFile = [ "json" "sources" ];
+    }
+    (
+      ''
+        mkdir vendor
+
+        export GOCACHE=$TMPDIR/go-cache
+        export GOPATH="$TMPDIR/go"
+
+        go run ${./symlink.go}
+        ${lib.concatStringsSep "\n" localReplaceCommands}
+
+        mv vendor $out
+      ''
+    );
+
+  mkGoEnv =
+    { pwd
+    }@attrs:
+    let
+      goMod = parseGoMod (builtins.readFile "${builtins.toString pwd}/go.mod");
+      modulesStruct = builtins.fromTOML (builtins.readFile "${builtins.toString pwd}/gomod2nix.toml");
+
+      go = attrs.go or (
+        let
+          goVersion = goMod.go;
+          goAttr = "go_" + (lib.replaceStrings [ "." ] [ "_" ] goVersion);
+        in
+        (
+          if builtins.hasAttr goAttr pkgs then pkgs.${goAttr}
+          else builtins.trace "go.mod specified Go version ${goVersion} but doesn't exist. Falling back to ${pkgs.go.version}." pkgs.go
+        )
+      );
+
+      vendorEnv = mkVendorEnv {
+        inherit go modulesStruct;
+      };
+
+    in
+    stdenv.mkDerivation {
+      name = "${builtins.baseNameOf goMod.module}-env";
+
+      dontUnpack = true;
+      dontConfigure = true;
+      dontInstall = true;
+
+      propagatedNativeBuildInputs = [ go ];
+
+      GO_NO_VENDOR_CHECKS = "1";
+
+      GO111MODULE = "on";
+      GOFLAGS = "-mod=vendor";
+
+      preferLocalBuild = true;
+
+      buildPhase = ''
+        mkdir $out
+
+        export GOCACHE=$TMPDIR/go-cache
+        export GOPATH="$out"
+        export GOSUMDB=off
+        export GOPROXY=off
+
+      '' + lib.optionalString (lib.pathExists (pwd + "/tools./.go")) ''
+        mkdir source
+        cp ${pwd + "/go.mod"} source/go.mod
+        cp ${pwd + "/go.sum"} source/go.sum
+        cp ${pwd + "/tools.go"} source/tools.go
+        cd source
+        ln -s ${vendorEnv} vendor
+
+        go run ${./install.go}
+      '';
+    };
+
   buildGoApplication =
     { modules
     , go ? pkgs.go
@@ -64,39 +154,12 @@ let
         in
         if pwd != null then commands else [ ];
 
-      vendorEnv = runCommand "vendor-env"
-        {
-          nativeBuildInputs = [ go ];
-          json = builtins.toJSON modulesStruct;
-
-          sources = builtins.toJSON (
-            lib.mapAttrs
-              (goPackagePath: meta: fetchGoModule {
-                goPackagePath = meta.replaced or goPackagePath;
-                inherit (meta) version hash;
-                inherit go;
-              })
-              modulesStruct.mod
-          );
-
-          passAsFile = [ "json" "sources" ];
-        }
-        (
-          ''
-            mkdir vendor
-
-            export GOCACHE=$TMPDIR/go-cache
-            export GOPATH="$TMPDIR/go"
-
-            go run ${./symlink.go}
-            ${lib.concatStringsSep "\n" localReplaceCommands}
-
-            mv vendor $out
-          ''
-        );
-
       removeReferences = [ ] ++ lib.optional (!allowGoReference) go;
 
+      vendorEnv = mkVendorEnv {
+        inherit go modulesStruct localReplaceCommands;
+      };
+
       package = stdenv.mkDerivation (attrs // {
         nativeBuildInputs = [ removeReferencesTo go ] ++ nativeBuildInputs;
 
@@ -230,4 +293,6 @@ let
     package;
 
 in
-buildGoApplication
+{
+  inherit buildGoApplication mkGoEnv;
+}

builder/install.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"fmt"
+	"go/parser"
+	"go/token"
+	"io"
+	"os"
+	"os/exec"
+	"strconv"
+)
+
+const filename = "tools.go"
+
+func main() {
+	fset := token.NewFileSet()
+
+	var src []byte
+	{
+		f, err := os.Open(filename)
+		if err != nil {
+			panic(err)
+		}
+
+		src, err = io.ReadAll(f)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	f, err := parser.ParseFile(fset, filename, src, parser.ImportsOnly)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	for _, s := range f.Imports {
+		path, err := strconv.Unquote(s.Path.Value)
+		if err != nil {
+			panic(err)
+		}
+
+		cmd := exec.Command("go", "install", path)
+
+		fmt.Printf("Executing '%s'\n", cmd)
+
+		err = cmd.Start()
+		if err != nil {
+			panic(err)
+		}
+
+		err = cmd.Wait()
+		if err != nil {
+			panic(err)
+		}
+	}
+}

go.mod

@@ -4,7 +4,7 @@ go 1.14
 
 require (
 	github.com/BurntSushi/toml v1.1.0
-	github.com/nix-community/go-nix v0.0.0-20220531154832-fb763dcb3ffc
+	github.com/nix-community/go-nix v0.0.0-20220528121639-b940fb0a12d8
 	github.com/sirupsen/logrus v1.8.1
 	golang.org/x/mod v0.5.1
 	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a // indirect

go.sum

@@ -15,8 +15,6 @@ github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/nix-community/go-nix v0.0.0-20220528121639-b940fb0a12d8 h1:IYr+3pldQtTwS55Xm2+3Ry81MpSKNVpMyYMNr7yQpts=
 github.com/nix-community/go-nix v0.0.0-20220528121639-b940fb0a12d8/go.mod h1:r5pCQAjHNSDWTsy6+UbacPN8EzMVJiCAXDWbRN2qfwU=
-github.com/nix-community/go-nix v0.0.0-20220531154832-fb763dcb3ffc h1:/uRl4vxjqF0N8m8ha+TouWZxsMRCA98Fft24WNIb0L8=
-github.com/nix-community/go-nix v0.0.0-20220531154832-fb763dcb3ffc/go.mod h1:r5pCQAjHNSDWTsy6+UbacPN8EzMVJiCAXDWbRN2qfwU=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=

gomod2nix.toml

@@ -29,8 +29,8 @@ schema = 1
     version = "v0.1.0"
     hash = "sha256-QT65kTrNypS5GPWGvgnCpGLPlVbQAL4IYvuqAKhepb4="
   [mod."github.com/nix-community/go-nix"]
-    version = "v0.0.0-20220531154832-fb763dcb3ffc"
+    version = "v0.0.0-20220528121639-b940fb0a12d8"
-    hash = "sha256-vCoRCe1DcyNXfz6gun9wGBu/o+j6WGcS8mbFmXZfyps="
+    hash = "sha256-r6fq1mu0gRFqdI6msuAZp3ZoncAw5L16oBFPUA9vPYk="
   [mod."github.com/pkg/errors"]
     version = "v0.9.1"
     hash = "sha256-mNfQtcrQmu3sNg/7IwiieKWOgFQOVVe2yXgKBpe/wZw="

overlay.nix

@@ -1,4 +1,4 @@
 final: prev: {
-  buildGoApplication = final.callPackage ./builder { };
+  inherit (final.callPackage ./builder { }) buildGoApplication mkGoEnv;
   gomod2nix = final.callPackage ./default.nix { };
 }