Add ability to specify substitutions as key-value pairs

README.md

@@ -55,8 +55,8 @@ h{config}: in{config}
 ```
 
 Besides the built-in configuration options, custom substitutions can be
-specified as `buildfile` variables in the same way as with the
+specified as `buildfile` variables or key-value pairs in the same way as with
-[`in`][module-in] module. For example:
+the [`in`][module-in] module. For example, as `buildfile` variables:
 
 ```
 /* config.h.in */
@@ -76,7 +76,35 @@ h{config}: in{config}
 }
 ```
 
-This mechanism can also be used to override the built-in checks, for example:
+As key-value pairs in the `autoconf.substitutions` map (which is an alias for
+the `in.substitutions` variable; see the [`in`][module-in] module for
+details):
+
+```
+/* config.h.in */
+
+#undef _GNU_SOURCE
+#undef _POSIX_SOURCE
+```
+
+```
+gnu_source = ($c.stdlib == 'glibc')
+posix_source = ($c.target.class != 'windows' && !$gnu_source)
+
+h{config}: in{config}
+{
+  autoconf.substitutions  = _GNU_SOURCE@$gnu_source
+  autoconf.substitutions += _POSIX_SOURCE@$posix_source
+}
+```
+
+In particular, the `autoconf.substitutions` mechanism is the only way to have
+substitutions that cannot be specified as `buildfile` variables because they
+start with an underscore (and thus are reserved, as in the above example) or
+refer to one of the predefined variables.
+
+The custom substitutions can also be used to override the built-in checks, for
+example:
 
 ```
 h{config}: in{config}

libbuild2-autoconf-tests/manifest

@@ -7,5 +7,5 @@ license: MIT ; MIT License.
 description-file: README.md
 url: https://github.com/build2/libbuild2-autoconf
 email: users@build2.org
-depends: * build2 >= 0.15.0-
+depends: * build2 >= 0.16.0-
-depends: * bpkg >= 0.15.0-
+depends: * bpkg >= 0.16.0-

libbuild2-autoconf-tests/module/testscript

@@ -401,3 +401,26 @@ cat config.h >>EOO
   #  define PREFIX_zzz_TEST_DUMMY5_H PREFIX_zzz_TEST_DUMMY4_H
   #endif /*zzz_TEST_DUMMY3_H*/
   EOO
+
+: substitution-map
+:
+mkdir build;
+ln -s ../../bootstrap.build ../../root.build build/;
+cat <<EOI >=config.h.in;
+  #undef _GNU_SOURCE
+  #undef _POSIX_SOURCE
+  #undef FOO
+  EOI
+$* <<EOI &config.h &config.h.d;
+  gnu = false
+  ./: h{config}: in{config}
+  {
+    autoconf.substitutions = _GNU_SOURCE@$gnu _POSIX_SOURCE@true
+    FOO = true
+  }
+  EOI
+cat config.h >>EOO
+  #undef _GNU_SOURCE
+  #define _POSIX_SOURCE 1
+  #define FOO 1
+  EOO

libbuild2-autoconf/libbuild2/autoconf/init.cxx

@@ -43,6 +43,11 @@ namespace build2
         // Built-in checks prefix.
         //
         vp.insert<string> ("autoconf.prefix");
+
+        // Substitution map (an alias for in.substitutions).
+        //
+        vp.insert_alias (*vp.find ("in.substitutions"),
+                         "autoconf.substitutions");
       }
 
       // Register the rule.

libbuild2-autoconf/libbuild2/autoconf/rule.cxx

@@ -144,6 +144,7 @@ namespace build2
              const char* nl,
              char sym,
              bool strict,
+             const substitution_map* smap,
              const optional<string>& null) const
     {
       auto ws = [] (char c)
@@ -182,7 +183,7 @@ namespace build2
                                  &l,
                                  a, &t,
                                  &dd, &dd_skip,
-                                 nl, strict, &null,
+                                 nl, strict, smap, &null,
                                  &s] (const string& name,
                                       bool value = true) -> optional<bool>
       {
@@ -195,7 +196,7 @@ namespace build2
                                          a, t,
                                          dd, dd_skip,
                                          name, 1 /* flags */,
-                                         strict, null));
+                                         strict, smap, null));
 
         assert (ov); // C identifier is a valid variable name.
         string& v (*ov);
@@ -470,7 +471,8 @@ namespace build2
         }
       }
 
-      in::rule::process (l, a, t, dd, dd_skip, s, b, nl, sym, strict, null);
+      in::rule::process (
+        l, a, t, dd, dd_skip, s, b, nl, sym, strict, smap, null);
     }
 
     string rule::
@@ -478,6 +480,7 @@ namespace build2
             action a, const target& t,
             const string& n,
             optional<uint64_t> flags,
+            const substitution_map* smap,
             const optional<string>& null) const
     {
       if (flags)
@@ -508,7 +511,7 @@ namespace build2
         //    variable is unlikely, something like const or volatile is
         //    possible. Since there is no way to undefine a buildfile variable
         //    (becasue we could always see a value from the outer scope), we
-        //    will treat null as an indication to use the built-in check.
+        //    will treat null as an instruction to use the built-in check.
         //    While this clashes with the in.null semantics, it's just as
         //    easy to set the variable to the real default value as to null.
         //
@@ -554,11 +557,31 @@ namespace build2
                   (!up || strchr (i->modifier, '!') != nullptr)) ? i : nullptr;
         };
 
-        // Note: original name in variable lookup.
+        // Return true if there is a custom substitution for the name.
+        //
+        // Note: see above on the special NULL semantics.
+        //
+        auto custom = [&t, smap] (const string& n) -> bool
+        {
+          if (smap != nullptr)
+          {
+            auto i (smap->find (n));
+
+            // Note that we treat NULL as the "use built-in check" instruction
+            // (see above). So it's a return, not fall through.
+            //
+            if (i != smap->end ())
+              return i->second.has_value ();
+          }
+
+          return static_cast<bool> (t[n]);
+        };
+
+        // Note: original name in the custom substitution lookup.
         //
         const check* c (find (en, pn == nullptr));
 
-        if (c != nullptr && !t[n])
+        if (c != nullptr && !custom (n))
         {
           // The plan is as follows: keep adding base checks (suppressing
           // duplicates) followed by the main check while prefixing all the
@@ -616,9 +639,10 @@ namespace build2
           //          Buildscript).
           //
           auto base = [this,
-                       &l, &t, a, &null,
+                       &l, &t, a, smap, &null,
                        &md, &p, &ns,
-                       &find, &append, &prefix] (const string& n,
+                       &find, &custom,
+                       &append, &prefix] (const string& n,
                                           const char* bs,
                                           const auto& base) -> void
           {
@@ -666,9 +690,9 @@ namespace build2
 
               md.checks.emplace (pn, n);
 
-              if (t[n])
+              if (custom (n))
-                append (
+                append (this->in::rule::lookup (
-                  this->in::rule::lookup (l, a, t, n, nullopt, null).c_str ());
+                          l, a, t, n, nullopt, smap, null).c_str ());
               else
               {
                 if (*c->base != '\0')
@@ -706,7 +730,7 @@ namespace build2
         }
       }
 
-      return in::rule::lookup (l, a, t, n, nullopt, null);
+      return in::rule::lookup (l, a, t, n, nullopt, smap, null);
     }
   }
 }

libbuild2-autoconf/libbuild2/autoconf/rule.hxx

@@ -39,6 +39,7 @@ namespace build2
                const char*,
                char,
                bool,
+               const substitution_map*,
                const optional<string>&) const override;
 
       virtual string
@@ -46,6 +47,7 @@ namespace build2
               action, const target&,
               const string&,
               optional<uint64_t>,
+              const substitution_map*,
               const optional<string>&) const override;
     };
   }

libbuild2-autoconf/manifest

@@ -9,6 +9,6 @@ url: https://github.com/build2/libbuild2-autoconf
 email: users@build2.org
 build-warning-email: builds@build2.org
 builds: all
-depends: * build2 >= 0.15.0-
+depends: * build2 >= 0.16.0-
-depends: * bpkg >= 0.15.0-
+depends: * bpkg >= 0.16.0-
 tests: * libbuild2-autoconf-tests == $