Relax warning 41 inside package:installed filters

Warning 41 is never triggered for the use of package:installed. Extend
this so that the warning is not triggered for any uses of package:foo
_underneath_ package:installed, i.e.
  "%{package:foo}% {package:installed}
can no longer cause warning 41 on package.

master_changes.md

@@ -57,6 +57,7 @@ users)
   * Fix extraction of tarballs on Windows which contain symlinks both when those symlinks can't be created or if they point to files which don't exist [#5953 @dra27]
 
 ## Lint
+  * Relax warning 41 not to trigger on uses of package variables which are guarded by a package:installed filter [#5927 @dra27]
 
 ## Repository
   * Fix download URLs containing invalid characters on Windows (e.g. the ? character in `?full_index=1`) [#5921 @dra27]

src/format/opamFilter.mli

@@ -155,6 +155,9 @@ val commands: env -> command list -> string list list
 (** Process a simpler command, without filters *)
 val single_command: env -> arg list -> string list
 
+(** Extracts the list of variables from an argument *)
+val simple_arg_variables: simple_arg -> full_variable list
+
 (** Extracts variables appearing in a list of commands *)
 val commands_variables: command list -> full_variable list
 

src/state/opamFileTools.ml

@@ -119,6 +119,71 @@ let map_all_filters f t =
   with_deprecated_build_test (map_commands t.deprecated_build_test) |>
   with_deprecated_build_doc (map_commands t.deprecated_build_doc)
 
+(* unguarded_commands_variables is an alternative implementation of
+   OpamFilter.commands_variables which excludes package variables which are
+   guarded by an unambiguous {package:installed} filter. That is, at each level,
+   if assuming !package:installed reduces the filter to false, then the uses of
+   package:variable are not returned. This allows expressions like:
+   ["--with-foo=%{foo:share}%" {foo:installed}] or even
+   ["--with-foo"] {foo:installed & foo:bar != "baz"} not to trigger warning 41
+   if the package is not explicitly depended on. *)
+
+let is_installed_variable filter guarded_packages v =
+  match OpamVariable.Full.package v with
+  | None -> guarded_packages
+  | (Some name) as package ->
+    if OpamVariable.to_string (OpamVariable.Full.variable v) = "installed" then
+      let env v =
+        if OpamVariable.Full.package v = package &&
+           OpamVariable.(to_string (Full.variable v)) = "installed" then
+          Some (B false)
+        else
+          None
+      in
+      if OpamFilter.partial_eval env filter = FBool false then
+        OpamPackage.Name.Set.add name guarded_packages
+      else
+        guarded_packages
+    else
+      guarded_packages
+
+let filter_guarded variables guarded_packages =
+  let is_unguarded v =
+    match OpamVariable.Full.package v with
+    | Some package ->
+      not (OpamPackage.Name.Set.mem package guarded_packages)
+    | None ->
+      true
+  in
+  List.filter is_unguarded variables
+
+let unguarded_packages_from_filter guarded_packages = function
+| None -> [], guarded_packages
+| Some f ->
+  let filter_variables = OpamFilter.variables f in
+  let guarded_packages =
+    List.fold_left (is_installed_variable f) guarded_packages filter_variables
+  in
+  filter_guarded filter_variables guarded_packages, guarded_packages
+
+let unguarded_argument_variables guarded_packages (argument, filter) =
+  let filter_variables, guarded_packages =
+    unguarded_packages_from_filter guarded_packages filter
+  in
+  (filter_guarded (OpamFilter.simple_arg_variables argument) guarded_packages) @ filter_variables
+
+let unguarded_command_variables (command, filter) =
+  let filter_variables, guarded_packages =
+    unguarded_packages_from_filter OpamPackage.Name.Set.empty filter
+  in
+  let add_argument acc argument =
+    unguarded_argument_variables guarded_packages argument @ acc
+  in
+  List.fold_left add_argument filter_variables command
+
+let unguarded_commands_variables commands =
+  List.fold_left (fun acc c -> unguarded_command_variables c @ acc) [] commands
+
 (* Returns all variables from all commands (or on given [command]) and all filters *)
 let all_variables ?exclude_post ?command t =
   let commands =
@@ -130,6 +195,14 @@ let all_variables ?exclude_post ?command t =
   List.fold_left (fun acc f -> OpamFilter.variables f @ acc)
     [] (all_filters ?exclude_post t)
 
+(* As all_variables, but any commands or arguments which are fully guarded by
+   package:installed are excluded; used for Warning 41 so that
+   ["%{foo:share}%" {foo:installed}] doesn't trigger a warning on foo *)
+let all_unguarded_variables ?exclude_post t =
+  unguarded_commands_variables (all_commands t) @
+  List.fold_left (fun acc f -> OpamFilter.variables f @ acc)
+    [] (all_filters ?exclude_post t)
+
 let map_all_variables f t =
   let map_fld (x, flt) = x, OpamFilter.map_variables f flt in
   let map_optfld = function
@@ -467,7 +540,7 @@ let t_lint ?check_extra_files ?(check_upstream=false) ?(all=false) t =
               ->
               OpamPackage.Name.Set.add n acc
             | _ -> acc)
-         OpamPackage.Name.Set.empty (all_variables ~exclude_post:true t)
+         OpamPackage.Name.Set.empty (all_unguarded_variables ~exclude_post:true t)
      in
      cond 41 `Warning
        "Some packages are mentioned in package scripts or features, but \

tests/reftests/lint.test

@@ -423,7 +423,7 @@ build: [
 ]
 ### opam lint ./lint.opam
 ${BASEDIR}/lint.opam: Warnings.
-           warning 41: Some packages are mentioned in package scripts or features, but there is no dependency or depopt toward them: "bar", "baz", "foo"
+           warning 41: Some packages are mentioned in package scripts or features, but there is no dependency or depopt toward them: "baz"
 ### : E42: The 'dev-repo:' field doesn't use version control. You should use URLs of the form "git://", "git+https://", "hg+https://"...
 ### <lint.opam>
 opam-version: "2.0"