fix: promotion over non-existent file

doc/changes/fixed/12615.md

@@ -0,0 +1,2 @@
+- Fix a bug where dune was unable to promote over non-existant files in certain
+  cases (#12615, fixes #8075, @Alizter)

src/promote/print_diff.ml

@@ -13,8 +13,12 @@ let resolve_link_for_git path =
           "Unable to resolve symlink %s. Max recursion depth exceeded"
           (Path.to_string path)
       ]
-  | Error (Unix_error _) ->
-    User_error.raise [ Pp.textf "Unable to resolve symlink %s" (Path.to_string path) ]
+  | Error (Unix_error (ENOENT, _, _)) -> path
+  | Error (Unix_error err) ->
+    User_error.raise
+      [ Pp.textf "Unable to resolve symlink %s" (Path.to_string path)
+      ; Unix_error.Detailed.pp err
+      ]
 ;;
 
 module Diff = struct

test/blackbox-tests/test-cases/promote/dune

@@ -0,0 +1,3 @@
+(cram
+ (applies_to symlink-to-nonexistent)
+ (deps %{bin:git} ../git-helpers.sh))

test/blackbox-tests/test-cases/promote/symlink-to-nonexistent.t

@@ -0,0 +1,44 @@
+Tests for promoting with symlink to non-existent file.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.21)
+  > EOF
+  $ cp dune-project dune-workspace
+
+  $ cat > dune <<EOF
+  > (rule
+  >  (with-stdout-to "x.gen"
+  >   (echo "toto")))
+  > (rule
+  >  (alias bench)
+  >  (action
+  >   (diff promoted x.gen)))
+  > EOF
+
+Unset INSIDE_DUNE in order to choose the "git diff" diff algorithm. Root
+detection will now be automatic so a dune-workspace file was included above.
+  $ unset INSIDE_DUNE
+
+The "git diff" command that dune will run will unfortunately go looking for a
+`.git` root. We therefore initialise a git repo to avoid this from escaping the
+test.
+  $ . ../git-helpers.sh
+  $ git init -q
+
+This should fail initially but not with the "Unable to resolve symlink" error.
+  $ dune build @bench
+  File "promoted", line 1, characters 0-0:
+  ------ /dev/null
+  ++++++ x.gen
+  File "/dev/null", line 1, characters 0-1:
+  +|toto
+  No newline at the end of x.gen
+  [1]
+
+Promotion should work
+  $ dune promote
+  Promoting _build/default/x.gen to promoted.
+
+  $ cat promoted
+  toto
+