writeTextFile,doc/build-helpers: assert destination without a leading slash (#343595)

doc/build-helpers/trivial-build-helpers.chapter.md

@@ -501,7 +501,7 @@ writeTextFile {
   text = ''
     Contents of File
   '';
-  destination = "share/my-file";
+  destination = "/share/my-file";
 }
 ```
 
@@ -586,7 +586,7 @@ writeTextFile {
     echo "hi"
   '';
   executable = true;
-  destination = "bin/my-script";
+  destination = "/bin/my-script";
 }
 ```
 
@@ -674,7 +674,7 @@ writeTextFile {
     echo "hi"
   '';
   executable = true;
-  destination = "bin/my-script";
+  destination = "/bin/my-script";
 }
 ```
 

pkgs/build-support/trivial-builders/default.nix

@@ -86,6 +86,13 @@ rec {
     , preferLocalBuild ? true
     , derivationArgs ? { }
     }:
+    assert lib.assertMsg (destination != "" -> (lib.hasPrefix "/" destination && destination != "/")) ''
+      destination must be an absolute path, relative to the derivation's out path,
+      got '${destination}' instead.
+
+      Ensure that the path starts with a / and specifies at least the filename.
+    '';
+
     let
       matches = builtins.match "/bin/([^/]+)" destination;
     in