test: repro case for #8075

Signed-off-by: Ali Caglayan <[email protected]>

Author: Alizter

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 "dune", lines 4-7, characters 0-55:
+  4 | (rule
+  5 |  (alias bench)
+  6 |  (action
+  7 |   (diff promoted x.gen)))
+  Error: Unable to resolve symlink _build/default/promoted
+  [1]
+
+Promotion should work
+  $ dune promote
+  Promoting _build/default/x.gen to promoted.
+
+  $ cat promoted
+  toto
+