Added a better Boost Build diagnostic error message when attempting to reference a target using an explicitly specified project id but without a leading slash ('/') character, e.g. as in:

use-project xxx : ../external/some-project ;
  exe a : xxx//some-obj ;

as opposed to:

  use-project xxx : ../external/some-project ;
  exe a : /xxx//some-obj ;

[SVN r79931]

src/build/project.jam

@@ -575,6 +575,14 @@ rule inherit-attributes ( project-module : parent-module )
 }
 
 
+# Returns whether the given string is a valid registered project id.
+#
+rule is-registered-id ( id )
+{
+    return $($(id).jamfile-module) ;
+}
+
+
 # Associate the given id with the given project module. Returns the possibly
 # corrected project id.
 #

src/build/targets.jam

@@ -374,6 +374,15 @@ class project-target : abstract-target
             {
                 extra-error-message = could not resolve project reference
                     '$(project-part)' ;
+                if ! [ path.is-rooted $(project-part) ]
+                {
+                    local rooted = [ path.root $(project-part) / ] ;
+                    if [ project.is-registered-id $(rooted) ]
+                    {
+                        extra-error-message += - possibly missing a leading
+                            slash ('/') character. ;
+                    }
+                }
             }
         }
         else