Add ability to specify recipes in separate files

This can now be achieved with the new `recipe` directive:

recipe <language> <file>

Note that similar to the use of if-else and switch directives with recipes,
this directive requires explicit % recipe header. For example, instead of:

file{foo.output}:
{{
   echo 'hello' >$path($>)
}}

We can now write:

file{foo.output}:
%
recipe buildscript hello.buildscript

With hello.buildscript containing:

echo 'hello' >$path($>)

Similarly, for C++ recipes (this time for a pattern), instead of:

[rule_name=hello] file{~'/(.+)\.output/'}:
% update clean
{{ c++ 1 --

--

...

}}

We can now write:

[rule_name=hello] file{~'/(.+)\.output/'}:
% update clean
recipe c++ hello.cxx

With hello.cxx containing:

// c++ 1 --

--

...

Relative <file> paths are resolved using the buildfile directory that contains
the `recipe` directive as a base.

Note also that this mechanism can be used in exported buildfiles with recipe
files placed into build/export/ together with buildfiles.

1 files changed, 11 insertions, 4 deletions

diff --git a/libbuild2/parser.hxx b/libbuild2/parser.hxx
index b756984..0645b5a 100644
--- a/libbuild2/parser.hxx
+++ b/libbuild2/parser.hxx
@@ -265,7 +265,9 @@ namespace build2
     parse_if_else (token&, token_type&,
                    bool,
                    const function<void (
-                     token&, token_type&, bool, const string&)>&);
+                     token&, token_type&, bool, const string&)>&,
+                   const function<void (
+                     token&, token_type&, const string&)>&);
 
     void
     parse_switch (token&, token_type&);
@@ -274,7 +276,9 @@ namespace build2
     parse_switch (token&, token_type&,
                   bool,
                   const function<void (
-                    token&, token_type&, bool, const string&)>&);
+                    token&, token_type&, bool, const string&)>&,
+                  const function<void (
+                    token&, token_type&, const string&)>&);
 
     void
     parse_for (token&, token_type&);
@@ -617,9 +621,12 @@ namespace build2
     void
     process_default_target (token&, const buildfile*);
 
-    // Enter buildfile as a target.
+  private:
+    // Enter buildfile or buildfile-file like file (e.g., a recipe file) as a
+    // target.
     //
-    const buildfile&
+    template <typename T>
+    const T&
     enter_buildfile (const path&, optional<dir_path> out = nullopt);
 
     // Lexer.