Refactor: store wasm error code in-line (no pointer)

I want to avoid storing the "E1234" string in memory permanently. This
means the lifetime of qljs_web_demo_diagnostic::code will be temporary.

Simplify lifetimes by having qljs_web_demo_diagnostic::code store a copy
of the error code string instead of storing a pointer. Copying also
fixes a TODO regarding no guarantee of null termination.

This commit should not change behavior.

Author: strager

src/c-api-error-reporter.cpp

@@ -1,6 +1,7 @@
 // Copyright (C) 2020  Matthew "strager" Glazar
 // See end of file for extended copyright information.
 
+#include <algorithm>
 #include <memory>
 #include <quick-lint-js/c-api-error-reporter.h>
 #include <quick-lint-js/c-api.h>
@@ -111,8 +112,10 @@ void c_api_error_formatter<Diagnostic, Locator>::write_after_message(
     diag.begin_offset = narrow_cast<int>(r.begin);
     diag.end_offset = narrow_cast<int>(r.end);
   }
-  // FIXME(strager): code is not guaranteed to be null-terminated.
-  diag.code = code.data();
+
+  char *code_end = std::copy(code.begin(), code.end(), &diag.code[0]);
+  *code_end = '\0';
+
   diag.message = reinterpret_cast<const char *>(
       this->reporter_->allocate_c_string(this->current_message_));
   diag.severity = diag_severity;

src/quick-lint-js/c-api.h

@@ -18,7 +18,7 @@ typedef enum qljs_severity {
 typedef struct qljs_web_demo_document qljs_web_demo_document;
 struct qljs_web_demo_diagnostic {
   const char* message;
-  const char* code;
+  char code[6];  // null-terminated
   qljs_severity severity;
   // Offsets count UTF-16 code units.
   int begin_offset;

test/test-c-api.cpp

@@ -22,7 +22,7 @@ TEST(test_c_api_web_demo, lint_error_after_text_insertion) {
   const qljs_web_demo_diagnostic* diagnostics = qljs_web_demo_lint(p);
   EXPECT_NE(diagnostics[0].message, nullptr);
   EXPECT_EQ(diagnostics[1].message, nullptr);
-  EXPECT_EQ(diagnostics[1].code, nullptr);
+  EXPECT_STREQ(diagnostics[1].code, "");
 
   EXPECT_STREQ(diagnostics[0].message, "redeclaration of variable: x");
   EXPECT_STREQ(diagnostics[0].code, "E0034");
@@ -45,7 +45,7 @@ TEST(test_c_api_web_demo, lint_new_error_after_second_text_insertion) {
   diagnostics = qljs_web_demo_lint(p);
   EXPECT_NE(diagnostics[0].message, nullptr);
   EXPECT_EQ(diagnostics[1].message, nullptr);
-  EXPECT_EQ(diagnostics[1].code, nullptr);
+  EXPECT_STREQ(diagnostics[1].code, "");
 
   EXPECT_STREQ(diagnostics[0].message, "redeclaration of variable: x");
   EXPECT_STREQ(diagnostics[0].code, "E0034");
@@ -66,7 +66,7 @@ TEST(test_c_api_web_demo, linting_uses_config) {
 
   const qljs_web_demo_diagnostic* diagnostics = qljs_web_demo_lint(p);
   EXPECT_EQ(diagnostics[0].message, nullptr);
-  EXPECT_EQ(diagnostics[0].code, nullptr);
+  EXPECT_STREQ(diagnostics[0].code, "");
 
   qljs_web_demo_destroy_document(p);
 }

website/wasm/quick-lint-js.js

@@ -288,6 +288,10 @@ class DocumentForWebDemo {
   }
 
   _parseDiagnostics(diagnosticsPointer) {
+    let rawDiagnostics = new Uint8Array(
+      this._process._heap,
+      diagnosticsPointer
+    );
     let rawDiagnosticsU32 = new Uint32Array(
       this._process._heap,
       diagnosticsPointer
@@ -298,30 +302,32 @@ class DocumentForWebDemo {
     );
     // struct qljs_web_demo_diagnostic {
     //   const char* message;
-    //   const char* code;
+    //   char code[6];
     //   qljs_vscode_severity severity;
     //   int begin_offset;
     //   int end_offset;
     // };
     let ERROR = {
       message: 0,
-      code: 1,
-      severity: 2,
-      begin_offset: 3,
-      end_offset: 4,
-
-      _ptr_size: 5,
-      _u32_size: 5,
+      code: 4,
+      severity: 3,
+      begin_offset: 4,
+      end_offset: 5,
+
+      _byte_size: 6 * 4,
+      _ptr_size: 6,
+      _u32_size: 6,
     };
     let diagnostics = [];
     for (let i = 0; ; ++i) {
       let messagePtr = rawDiagnosticsPtr[i * ERROR._ptr_size + ERROR.message];
       if (messagePtr === 0) {
         break;
       }
-      let codePtr = rawDiagnosticsPtr[i * ERROR._ptr_size + ERROR.code];
       diagnostics.push({
-        code: decodeUTF8CString(new Uint8Array(this._process._heap, codePtr)),
+        code: decodeUTF8CString(
+          rawDiagnostics.slice(i * ERROR._byte_size + ERROR.code)
+        ),
         message: decodeUTF8CString(
           new Uint8Array(this._process._heap, messagePtr)
         ),