`diff` action directive isn't working as expected with a non-existant file

[shym]

Expected Behavior

The documentation of the diff action directive states that (promotion.rst):

• If <file1> doesn't exist, it will compare with the empty file.

A message about a missing file could make sense (but it would be less convenient).

Actual Behavior

I actually get the following error message:

Error: Unable to resolve symlink _build/default/file1

When trying to reproduce it inside a cram test in Dune’s source, I get a different error message, though.

Reproduction

cat > dune-project << EOF
(lang dune 3.0)
EOF

cat > dune << EOF
(rule
 (alias runtest)
 (action
  (diff a b)))

(rule
 (write-file b b))
EOF

I get:

$ dune runtest
File "dune", lines 1-4, characters 0-46:
1 | (rule
2 |  (alias runtest)
3 |  (action
4 |   (diff a b)))
Error: Unable to resolve symlink _build/default/a
[1]

Specifications

$ dune --version
3.21.0-748-g98b7042