da/type path resolver

modules/modules.nix

@@ -23,6 +23,7 @@ let
       _module.args.baseModules = modules;
       _module.args.pkgsPath = lib.mkDefault pkgs.path;
       _module.args.pkgs = lib.mkDefault pkgs;
+      _module.args._devshelltoml = config.lib._tomlfile or null; # set by importTOML
     };
   };
 in

nix/importTOML.nix

@@ -24,5 +24,5 @@ in
 {
   _file = file;
   imports = map importModule (data.imports or [ ]);
-  config = builtins.removeAttrs data [ "imports" ];
+  config = (builtins.removeAttrs data [ "imports" ]) // { lib._tomlfile = file; };
 }

nix/types.nix

@@ -0,0 +1,37 @@
+{ lib }:
+
+with lib.types; {
+
+  /**
+    Synopsis: maybeResolveRel <tomlfilepath> <string|path>
+
+    If it's a string, returns an absolute path transforming relative
+    paths with regard to the tomlfile attribute, first. Or
+    transparently passes through path types, otherwise.
+
+    Use for fields that can define relative paths in devshell TOML files.
+    **/
+  maybeResolveRel = tomlfile: let
+    tomldir = builtins.dirOf tomlfile;
+  in obj:
+    # Not a toml file: return as-is
+    if (file == null) then
+      obj
+    # It must be a string
+    else if (!(builtins.isString obj)) then
+      # Never happens untill nix gains some sort type
+      # caster for importTOML: prepare for day X.
+      builtins.throw "${obj} defined in ${tomlfile} is not a string."
+    # It looks like an absolute path: type cast into a path type
+    else if (lib.strings.hasPrefix "/" obj) then
+      /. + obj
+    # It looks like an explicit relpath: strip "." to conform to the builtin path type caster
+    else if (lib.strings.hasPrefix "./" obj) || then
+      /. + (tomldir + (lib.strings.removePrefix "." obj))
+    # It is treated as an implicit relpat: prefix with "/" to conform to ...
+    else
+      /. + (tomldir + "/" obj)
+  ;
+
+  pathType = tomlfile: o: coercedTo path maybeResolveRel tomlfile o;
+}