improve diagnostics for unterminated nested block comment

compiler/rustc_parse/src/lexer/mod.rs

@@ -182,16 +182,7 @@ impl<'a> StringReader<'a> {
             }
             rustc_lexer::TokenKind::BlockComment { doc_style, terminated } => {
                 if !terminated {
-                    let msg = match doc_style {
-                        Some(_) => "unterminated block doc-comment",
-                        None => "unterminated block comment",
-                    };
-                    let last_bpos = self.pos;
-                    self.sess.span_diagnostic.span_fatal_with_code(
-                        self.mk_sp(start, last_bpos),
-                        msg,
-                        error_code!(E0758),
-                    );
+                    self.report_unterminated_block_comment(start, doc_style);
                 }
 
                 // Skip non-doc comments
@@ -553,6 +544,58 @@ impl<'a> StringReader<'a> {
         err.emit()
     }
 
+    fn report_unterminated_block_comment(&self, start: BytePos, doc_style: Option<DocStyle>) {
+        let msg = match doc_style {
+            Some(_) => "unterminated block doc-comment",
+            None => "unterminated block comment",
+        };
+        let last_bpos = self.pos;
+        let mut err = self.sess.span_diagnostic.struct_span_fatal_with_code(
+            self.mk_sp(start, last_bpos),
+            msg,
+            error_code!(E0758),
+        );
+        let mut nested_block_comment_open_idxs = vec![];
+        let mut last_nested_block_comment_idxs = None;
+        let mut content_chars = self.str_from(start).char_indices();
+
+        if let Some((_, mut last_char)) = content_chars.next() {
+            while let Some((idx, c)) = content_chars.next() {
+                match c {
+                    '*' if last_char == '/' => {
+                        nested_block_comment_open_idxs.push(idx);
+                    }
+                    '/' if last_char == '*' => {
+                        last_nested_block_comment_idxs =
+                            nested_block_comment_open_idxs.pop().map(|open_idx| (open_idx, idx));
+                    }
+                    _ => {}
+                };
+                last_char = c;
+            }
+        }
+
+        if let Some((nested_open_idx, nested_close_idx)) = last_nested_block_comment_idxs {
+            err.span_label(self.mk_sp(start, start + BytePos(2)), msg)
+                .span_label(
+                    self.mk_sp(
+                        start + BytePos(nested_open_idx as u32 - 1),
+                        start + BytePos(nested_open_idx as u32 + 1),
+                    ),
+                    "...as last nested comment starts here, maybe you want to close this instead?",
+                )
+                .span_label(
+                    self.mk_sp(
+                        start + BytePos(nested_close_idx as u32 - 1),
+                        start + BytePos(nested_close_idx as u32 + 1),
+                    ),
+                    "...and last nested comment terminates here",
+                );
+        }
+
+        err.emit();
+    }
+
     // RFC 3101 introduced the idea of (reserved) prefixes. As of Rust 2021,
     // using a (unknown) prefix is an error. In earlier editions, however, they
     // only result in a (allowed by default) lint, and are treated as regular

src/test/ui/unterminated-nested-comment.rs

@@ -0,0 +1,4 @@
+/* //~ ERROR E0758
+/* */
+/*
+*/

src/test/ui/unterminated-nested-comment.stderr

@@ -0,0 +1,21 @@
+error[E0758]: unterminated block comment
+  --> $DIR/unterminated-nested-comment.rs:1:1
+   |
+LL |   /*
+   |   ^-
+   |   |
+   |  _unterminated block comment
+   | |
+LL | | /* */
+LL | | /*
+   | | --
+   | | |
+   | | ...as last nested comment starts here, maybe you want to close this instead?
+LL | | */
+   | |_--^
+   |   |
+   |   ...and last nested comment terminates here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0758`.