Add support for project-local importation

An import without a project name or with the same name as the importing
project's is now treated as importation from the same project.

For example, given the libhello project that exports the lib{hello} target, a
buildfile for an executable in the same project instead of doing something
like this:

include ../libhello/
exe{hello}: ../libhello/lib{hello}

Can now do this:

import lib = libhello%lib{hello}

Or:

import lib = lib{hello}

And then:

exe{hello}: $lib

Note that a target in project-local importation must still be exported in
the project's export stub. In other words, project-local importation goes
through the same mechanisms as normal import.

2 files changed, 47 insertions, 11 deletions

diff --git a/doc/manual.cli b/doc/manual.cli
index ba0aa63..483c61d 100644
--- a/doc/manual.cli
+++ b/doc/manual.cli
@@ -2601,6 +2601,34 @@ If no \c{export} directive is executed in an export stub then the build system
 assumes that the target is not exported by the project and issues appropriate
 diagnostics.|
 
+Let's revisit the executable \c{buildfile} with which we started this section.
+Recall that it is for an executable that depends on a library which resides
+in the same project:
+
+\
+include ../libhello/ # Include lib{hello}.
+
+exe{hello}: {hxx cxx}{**} ../libhello/lib{hello}
+\
+
+If \c{lib{hello\}} is exported by this project, then instead of manually
+including its \c{buildfile} we can use \i{project-local importation}:
+
+\
+import lib = lib{hello}
+
+exe{hello}: {hxx cxx}{**} $lib
+\
+
+The main advantage of project-local importation over inclusion is the ability
+to move things around without having to adjust locations in multiple places
+(the only place we need to do it is the export stub). This advantage becomes
+noticeable in more complex projects with a large number of components.
+
+\N|An import is project-local if the target being imported has no project
+name. Note that the target must still be exported in the project's export
+stub. In other words, project-local importation use the same mechanism as the
+normal import.|
 
 \h#intro-lib|Library Exportation and Versioning|
 
diff --git a/libbuild2/file.cxx b/libbuild2/file.cxx
index 7ca8a91..f91dbbd 100644
--- a/libbuild2/file.cxx
+++ b/libbuild2/file.cxx
@@ -1838,6 +1838,9 @@ namespace build2
   {
     tracer trace ("import_search");
 
+    context& ctx (ibase.ctx);
+    scope& iroot (*ibase.root_scope ());
+
     // Depending on the target, we have four cases:
     //
     // 1. Ad hoc import: target is unqualified and is either absolute or is a
@@ -1847,9 +1850,8 @@ namespace build2
     //    (e.g., because they don't know where it is), then they will have to
     //    specify it with an explicit dir{} target type.
     //
-    // 2. Project-local import: target is unqualified.
-    //
-    //    Note: this is still a TODO.
+    // 2. Project-local import: target is unqualified or the project name is
+    //    the same as the importing project's.
     //
     // 3. Project-less import: target is empty-qualified.
     //
@@ -1869,9 +1871,19 @@ namespace build2
         return make_pair (move (tgt), optional<dir_path> (tgt.dir));
       }
       else
-        fail (loc) << "importation of an unqualified target " << tgt <<
-          info     << "use empty project qualification to import a target "
-                   << "without a project";
+      {
+        // Project-local import.
+        //
+        const project_name& pn (project (iroot));
+
+        if (pn.empty ())
+          fail (loc) << "project-local importation of target " << tgt
+                     << " from an unnamed project";
+
+        tgt.proj = pn;
+
+        return make_pair (move (tgt), optional<dir_path> (iroot.out_path ()));
+      }
     }
 
     // If the project name is empty then we simply return it as is to let
@@ -1886,15 +1898,11 @@ namespace build2
     if (tgt.absolute ())
       fail (loc) << "absolute directory in imported target " << tgt;
 
-    context& ctx (ibase.ctx);
-
-    // Otherwise, get the project name and convert the target to unqualified.
+    // Get the project name and convert the target to unqualified.
     //
     project_name proj (move (*tgt.proj));
     tgt.proj = nullopt;
 
-    scope& iroot (*ibase.root_scope ());
-
     // Figure out the imported project's out_root.
     //
     optional<dir_path> out_root;