Support $<condition> in unnamed conditions

docs/cmake-toml.md

@@ -88,7 +88,7 @@ sources = ["src/main.cpp"]
 ptr64.sources = ["src/ptr64_only.cpp"]
 ```
 
-Instead of a named condition you can also specify a [CMake expression](https://cmake.org/cmake/help/latest/command/if.html#condition-syntax) directly.
+Instead of a named condition you can also specify a [CMake expression](https://cmake.org/cmake/help/latest/command/if.html#condition-syntax) in quotes. Instances of `$<name>` are replaced with the corresponding condition. For example: `"CONDITIONS_BUILD_TESTS AND $<linux>"` becomes `CONDITIONS_BUILD_TESTS AND (CMAKE_SYSTEM_NAME MATCHES "Linux")` in the final `CMakeLists.txt` file.
 
 ### Predefined conditions
 

src/cmake_generator.cpp

@@ -547,12 +547,57 @@ struct Generator {
                 // NOTE: this should have been caught by the parser already
                 throw std::runtime_error("Condition '" + condition + "' is not defined");
             }
-            cmd("if", "NOTE: unnamed condition")(RawArg(condition));
+            cmd("if", "NOTE: unnamed condition")(RawArg(cmake_condition(condition)));
         } else {
             cmd("if", condition)(RawArg(found->second));
         }
         return true;
     }
+
+  private:
+    std::string cmake_condition(const std::string &condition) {
+        // HACK: this replaces '$<name>' with the value of the 'name' condition. We can safely
+        // reuse the generator expression syntax, because it is not valid in CMake conditions.
+        // TODO: properly handle quoted arguments (using a simple state machine):
+        // https://cmake.org/cmake/help/latest/manual/cmake-language.7.html#quoted-argument
+        std::string result = "";
+        bool in_replacement = false;
+        std::string temp;
+        for (size_t i = 0; i < condition.length(); i++) {
+            if (in_replacement) {
+                if (condition[i] == '>') {
+                    in_replacement = false;
+                    if (temp.empty()) {
+                        throw std::runtime_error("Empty replacement in condition '" + condition + "'");
+                    }
+                    auto found = project.conditions.find(temp);
+                    if (found == project.conditions.end()) {
+                        throw std::runtime_error("Unknown condition '" + temp + "' in replacement");
+                    }
+                    auto has_space = found->second.find(' ') != std::string::npos;
+                    if (has_space) {
+                        result += '(';
+                    }
+                    result += found->second;
+                    if (has_space) {
+                        result += ')';
+                    }
+                    temp.clear();
+                } else {
+                    temp += condition[i];
+                }
+            } else if (condition[i] == '$' && i + 1 < condition.length() && condition[i + 1] == '<') {
+                i++;
+                in_replacement = true;
+            } else {
+                result += condition[i];
+            }
+        }
+        if (!temp.empty()) {
+            throw std::runtime_error("Unterminated replacement in condition '" + condition + "'");
+        }
+        return result;
+    }
 };
 
 struct ConditionScope {

tests/conditions/cmake.toml

@@ -3,7 +3,7 @@ name = "conditions"
 cmake-after = "set(CUSTOM ON)"
 
 [options]
-CONDITIONS_BUILD_TESTS = false
+CONDITIONS_BUILD_TESTS = "root"
 
 [conditions]
 custom = "CUSTOM"
@@ -19,6 +19,7 @@ linux.cmake-after = "message(STATUS linux-after)"
 unix.cmake-after = "message(STATUS unix-after)"
 custom.cmake-after = "message(STATUS custom-after)"
 build-tests.cmake-after = "message(STATUS build-tests)"
+"CONDITIONS_BUILD_TESTS AND $<linux>".cmake-after = "message(STATUS linux-tests)"
 
 [target.example.properties]
 AUTOMOC = false