Add constructor for Diagnostic that takes Vec<(DiagnosticMessage, Style)>

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1787,8 +1787,7 @@ pub fn check(&self, sess: &Session, blocking: bool) {
             match message {
                 Ok(SharedEmitterMessage::Diagnostic(diag)) => {
                     let handler = sess.diagnostic();
-                    let mut d = rustc_errors::Diagnostic::new(diag.lvl, String::new());
+                    let mut d = rustc_errors::Diagnostic::new_with_messages(diag.lvl, diag.msg);
-                    d.message = diag.msg;
                     if let Some(code) = diag.code {
                         d.code(code);
                     }

compiler/rustc_errors/src/diagnostic.rs

@@ -213,6 +213,21 @@ pub fn new<M: Into<DiagnosticMessage>>(level: Level, message: M) -> Self {
         Diagnostic::new_with_code(level, None, message)
     }
 
+    #[track_caller]
+    pub fn new_with_messages(level: Level, messages: Vec<(DiagnosticMessage, Style)>) -> Self {
+        Diagnostic {
+            level,
+            message: messages,
+            code: None,
+            span: MultiSpan::new(),
+            children: vec![],
+            suggestions: Ok(vec![]),
+            args: Default::default(),
+            sort_span: DUMMY_SP,
+            is_lint: false,
+        }
+    }
+
     #[track_caller]
     pub fn new_with_code<M: Into<DiagnosticMessage>>(
         level: Level,