[M1] add util::Error, Result<T>, format()

src/main.cpp

@@ -1,5 +1,7 @@
 import cargoxx;
+import std;
 
 int main(int /*argc*/, char** /*argv*/) {
+    std::println("Hello, cargoxx!");
     return 0;
 }

src/util/error.cpp

@@ -0,0 +1,29 @@
+module cargoxx.util;
+
+import std;
+
+namespace cargoxx::util {
+
+auto format(const Error& e) -> std::string {
+    std::string out;
+    out += std::format("error[E{:04}]: {}\n", static_cast<int>(e.code), e.message);
+
+    if (e.location) {
+        if (e.line_col) {
+            out += std::format("  --> {}:{}:{}\n",
+                               e.location->string(),
+                               e.line_col->first,
+                               e.line_col->second);
+        } else {
+            out += std::format("  --> {}\n", e.location->string());
+        }
+    }
+
+    if (!e.hint.empty()) {
+        out += std::format("  hint: {}\n", e.hint);
+    }
+
+    return out;
+}
+
+} // namespace cargoxx::util

src/util/util.cppm

@@ -1 +1,50 @@
 export module cargoxx.util;
+
+import std;
+
+export namespace cargoxx::util {
+
+// Error code numbering follows TECH_SPEC.md §4.
+enum class ErrorCode {
+    ManifestNotFound = 1,
+    ManifestParseError,
+    ManifestInvalidField,
+    ManifestUnknownField,
+    ManifestVersionInvalid,
+
+    LayoutNoTarget = 20,
+    LayoutAmbiguousLib,
+    LayoutInvalidName,
+
+    ResolutionUnknownPackage = 40,
+    ResolutionNetworkError,
+    ResolutionUnsatisfiable,
+    ResolutionVersionNotFound,
+
+    LinkdbUnknownPackage = 60,
+    LinkdbCorrupt,
+    LinkdbComponentNotSupported,
+
+    ExecCommandFailed = 80,
+    ExecToolNotFound,
+    BuildCmakeFailed,
+    BuildNixFailed,
+
+    Internal = 100,
+    NotImplemented,
+};
+
+struct Error {
+    ErrorCode code;
+    std::string message;
+    std::string hint;
+    std::optional<std::filesystem::path> location;
+    std::optional<std::pair<int, int>> line_col;
+};
+
+template <typename T>
+using Result = std::expected<T, Error>;
+
+auto format(const Error& e) -> std::string;
+
+} // namespace cargoxx::util