Add support for %file{...} expansions

This should make the use case exposed in #2836 implementable.

doc/pages/expansions.asciidoc

@@ -167,6 +167,11 @@ TIP: These environment variables are also available in other contexts where
 Kakoune uses a shell command, such as the `|`, `!` or `$` normal mode commands
 (See <<keys#,`:doc keys`>>).
 
+== File expansions
+
+Expansions with the type `file` will expand to the content of the filename
+given in argument as read from the host filesystem.
+
 == Value expansions
 
 Expansions with the type `val` give access to Kakoune internal data that is

src/command_manager.cc

@@ -5,6 +5,7 @@
 #include "buffer_utils.hh"
 #include "context.hh"
 #include "flags.hh"
+#include "file.hh"
 #include "optional.hh"
 #include "option_types.hh"
 #include "ranges.hh"
@@ -168,6 +169,8 @@ Token::Type token_type(StringView type_name, bool throw_on_invalid)
         return Token::Type::ValExpand;
     else if (type_name == "arg")
         return Token::Type::ArgExpand;
+    else if (type_name == "file")
+        return Token::Type::FileExpand;
     else if (throw_on_invalid)
         throw parse_error{format("unknown expand '{}'", type_name)};
     else
@@ -327,6 +330,8 @@ expand_token(const Token& token, const Context& context, const ShellContext& she
             throw runtime_error("invalid argument index");
         return {arg < params.size() ? params[arg] : String{}};
     }
+    case Token::Type::FileExpand:
+        return {read_file(content)};
     case Token::Type::RawEval:
         return {expand(content, context, shell_context)};
     case Token::Type::Raw:

src/command_manager.hh

@@ -51,6 +51,7 @@ struct Token
         OptionExpand,
         ValExpand,
         ArgExpand,
+        FileExpand,
         CommandSeparator
     };
 

test/compose/file-expansion/cmd

@@ -0,0 +1 @@
+:echo -to-file commands 'foo bar'<ret>:execute-keys i %file{commands} <lt>esc><ret>

test/compose/file-expansion/in

@@ -0,0 +1 @@
+

test/compose/file-expansion/out

@@ -0,0 +1 @@
+foo bar