Improve the behaviour of promotion when files are missings

CHANGES.md

@@ -99,6 +99,9 @@ unreleased
   `.merlin` with `S`-directives pointed to original source locations and thus
   allowing merlin to see those.
 
+- Improve the behavior of `dune promote` when the files to be promoted have been
+  deleted. (#1775, fixes #1772, @diml)
+
 - unstable-fmt: preserve comments (#1766, @emillon)
 
 1.6.2 (05/12/2018)

src/promotion.ml

@@ -19,10 +19,17 @@ module File = struct
   let register t = db := t :: !db
 
   let promote { src; dst } =
-    Errors.print_to_console (Format.sprintf "Promoting %s to %s.@."
-      (Path.to_string_maybe_quoted src)
-      (Path.to_string_maybe_quoted dst));
-    Io.copy_file ~src ~dst ()
+    let src_exists = Path.exists src in
+    Errors.print_to_console
+      (Format.sprintf
+         (if src_exists then
+            "Promoting %s to %s.@."
+          else
+            "Skipping promotion of %s to %s as the file is missing.@.")
+         (Path.to_string_maybe_quoted src)
+         (Path.to_string_maybe_quoted dst));
+    if src_exists then
+      Io.copy_file ~src ~dst ()
 end
 
 let clear_cache () =

test/blackbox-tests/test-cases/promote/run.t

@@ -60,3 +60,19 @@ Test single file promotion
   $ dune promote x y
   Warning: Nothing to promote for x.
   Warning: Nothing to promote for y.
+
+Reproduction case for #1772
+---------------------------
+
+  $ printf a > x
+  $ printf a > y
+  $ dune build --display short @blah @blah2
+  File "x", line 1, characters 0-0:
+  Files _build/default/x and _build/default/x.gen differ.
+  File "y", line 1, characters 0-0:
+  Files _build/default/y and _build/default/y.gen differ.
+  [1]
+  $ rm -f _build/default/x.gen
+  $ dune promote
+  Skipping promotion of _build/default/x.gen to x as the file is missing.
+  Promoting _build/default/y.gen to y.