String_with_vars: represent quoted vars differently from unquoted ones

Define the representation for quoted variables, adapt the test of
strings made of a single variable, and add a constructor.
[String_with_vars.t] is not yet able to use that representation
because the necessary information is not available from the parser.

src/action.ml

@@ -363,7 +363,7 @@ module Unexpanded = struct
       let expand ~generic ~special ~map ~dir ~f = function
         | Inl x -> map x
         | Inr template as x ->
-          match SW.just_a_var template with
+          match SW.unquoted_var template with
           | None -> generic ~dir (string ~dir ~f x)
           | Some var ->
             match f (SW.loc template) var with
@@ -452,7 +452,7 @@ module Unexpanded = struct
         | Some e -> Some (VE.to_string ~dir e))
 
     let expand ~generic ~special ~dir ~f template =
-      match SW.just_a_var template with
+      match SW.unquoted_var template with
       | None -> begin
           match string ~dir ~f template with
           | Inl x -> Inl (generic ~dir x)

src/string_with_vars.ml

@@ -6,6 +6,8 @@ type item =
   | Text of string
   | Var of var_syntax * string
 
+(* A single unquoted variable is encoded as the list [Var v].  A
+   quoted variable is encoded as [Var v; Text ""]. *)
 type t =
   { items : item list
   ; loc   : Loc.t
@@ -67,9 +69,11 @@ let loc t = t.loc
 
 let virt pos s = of_string ~loc:(Loc.of_pos pos) s
 let virt_var  pos s = { loc = Loc.of_pos pos; items = [Var (Braces, s)] }
+let virt_quoted_var pos s = { loc = Loc.of_pos pos;
+                              items = [Var (Braces, s); Text ""] }
 let virt_text pos s = { loc = Loc.of_pos pos; items = [Text s] }
 
-let just_a_var t =
+let unquoted_var t =
   match t.items with
   | [Var (_, s)] -> Some s
   | _ -> None

src/string_with_vars.mli

@@ -21,12 +21,13 @@ val to_string : t -> string
 (** [t] generated by the OCaml code. The first argument should be
    [__POS__]. The second is either a string to parse, a variable name
    or plain text. *)
-val virt      : (string * int * int * int) -> string -> t
-val virt_var  : (string * int * int * int) -> string -> t
-val virt_text : (string * int * int * int) -> string -> t
+val virt             : (string * int * int * int) -> string -> t
+val virt_var         : (string * int * int * int) -> string -> t
+val virt_quoted_var  : (string * int * int * int) -> string -> t
+val virt_text        : (string * int * int * int) -> string -> t
 
-val just_a_var : t -> string option
-(** [just_a_var t] return the [Some name] where [name] is the name of
+val unquoted_var : t -> string option
+(** [unquoted_var t] return the [Some name] where [name] is the name of
    the variable if [t] is solely made of a variable and [None] otherwise. *)
 
 val vars : t -> String_set.t