Env stanza: warn if some rules are ignored

Since matching is sequential, anything after `_` cannot be reached.

Closes ocaml#5886

Signed-off-by: Etienne Millon <[email protected]>

CHANGES.md

@@ -4,6 +4,9 @@
 - [ctypes] Add support for the `errno` parameter using the `errno_policy` field
   in the ctypes settings. (#5827, @droyo)
 
+- env stanza: warn if some rules are ignored because they appear after a
+  wildcard rule. (#...., fix #5886, @emillon)
+
 3.3.1 (19-06-2022)
 ------------------
 

src/dune_rules/dune_env.ml

@@ -226,10 +226,27 @@ module Stanza = struct
        and+ configs = fields config in
        (pat, configs))
 
+  let check_rules ~version ~loc rules =
+    let has_after_any l =
+      let is_any = function
+        | Any, _ -> true
+        | Profile _, _ -> false
+      in
+      List.split_while l ~f:is_any |> snd |> List.is_non_empty
+    in
+    if version >= (3, 4) && has_after_any rules then
+      User_warning.emit ~loc
+        [ Pp.text
+            "This stanza contains rules after a wildcard rule. These are going \
+             to be ignored."
+        ]
+
   let decode =
     let+ () = Dune_lang.Syntax.since Stanza.syntax (1, 0)
     and+ loc = loc
-    and+ rules = repeat rule in
+    and+ rules = repeat rule
+    and+ version = Dune_lang.Syntax.get_exn Stanza.syntax in
+    check_rules ~version ~loc rules;
     { loc; rules }
 
   let empty = { loc = Loc.none; rules = [] }

test/blackbox-tests/test-cases/env/env-unused.t

@@ -0,0 +1,32 @@
+  $ cat > dune-project << EOF
+  > (lang dune 3.3)
+  > EOF
+
+(env) is evaluated sequentially:
+
+  $ cat > dune << 'EOF'
+  > (rule
+  >  (alias runtest)
+  >  (action (system "echo $VAR")))
+  > 
+  > (env
+  >  (_   (env-vars (VAR default )))
+  >  (dev (env-vars (VAR specific))))
+  > EOF
+
+  $ dune runtest
+  default
+
+In 3.4, a warning is added for that case:
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.4)
+  > EOF
+
+  $ dune runtest
+  File "dune", line 5, characters 0-71:
+  5 | (env
+  6 |  (_   (env-vars (VAR default )))
+  7 |  (dev (env-vars (VAR specific))))
+  Warning: This stanza contains rules after a wildcard rule. These are going to
+  be ignored.