Add support for specifying minimum required build2 version

The syntax is:

using build@0.1.0-a1

The idea is that we will later also use it for modules and 'build' is
a special, the "build system itself" module.

Also fix a problem with peeking and lexer mode switching.

1 files changed, 12 insertions, 1 deletions

diff --git a/build/lexer b/build/lexer
index 6880d39..4a50e2a 100644
--- a/build/lexer
+++ b/build/lexer
@@ -15,6 +15,9 @@
 
 #include <butl/char-scanner>
 
+#include <build/types>
+#include <build/utility>
+
 #include <build/token>
 #include <build/diagnostics>
 
@@ -43,7 +46,7 @@ namespace build
     lexer (std::istream& is,
            const std::string& name,
            void (*processor) (token&, const lexer&) = nullptr)
-        : char_scanner (is), fail (name), processor_ (processor)
+        : char_scanner (is), fail (name), processor_ (processor), sep_ (false)
     {
       mode_.push (lexer_mode::normal);
     }
@@ -77,6 +80,13 @@ namespace build
     token
     next ();
 
+    // Peek at the first character of the next token. Return the character
+    // or 0 if the next token will be eos. Also return an indicator of
+    // whether the next token will be separated.
+    //
+    pair<char, bool>
+    peek_char ();
+
   private:
     token
     next_impl ();
@@ -121,6 +131,7 @@ namespace build
 
     std::stack<lexer_mode> mode_;
     char pair_separator_;
+    bool sep_; // True if we skipped spaces in peek().
   };
 }