Tuple struct field comments consume the field itself, only with doc comment

[danjl1100]

Describe the bug

When using the sequence /// doc comment, // regular comment, <field definition> within a tuple struct definition, rustfmt joins the field with the regular comment above, removing the field from the struct definition.

Input

struct MyTuple(
    /// Doc Comments
    // TODO note to add more to Doc Comments
    u32,
);

Output - correctness issue

struct MyTuple(
    /// Doc Comments
    // TODO note to add more to Doc Commentsu32,
);

The same applies for /* block comments */, but in this case the issue is purely cosmetic (does not change the struct definition).

Input (block comment)

struct MyTuple(
    /// Doc Comments
    /* TODO note to add more to Doc Comments */
    u32,
);

Output (block comment) - cosmetic issue

struct MyTuple(
    /// Doc Comments
    /* TODO note to add more to Doc Comments */u32,
);

At the very least, I expect there to be a separating space between the block comment and the u32, similar to the similar example with no doc comment

-    /* TODO note to add more to Doc Comments */u32,
+    /* TODO note to add more to Doc Comments */ u32,

Working cases

However, this issue doesn't seem to appear when the /// doc comment is removed, nor in named-field structs:

• No doc comment - works as expected

  struct MyTuple(
      // TODO note to add more to Doc Comments
      u32,
  );
  

• Struct with named fields (non-tuple) - works as expected

  struct MyStruct {
      /// Doc Comments
      // TODO note to add more to Doc Comments
      field: u32,
  }
  

To Reproduce

• Tuple struct with doc comment, single line comment (broken) - Playground
• Tuple struct with doc comment, block comment (broken, cosmetic only) - Playground
• Tuple struct with single line comment, only (not broken, just for comparison) - Playground
• Struct (named fields) with doc comment, single line comment (not broken, just for comparison) - Playground

Expected behavior

I expect rustfmt to never change the tuple struct definition (MyStruct(u32) --> MyStruct()), and also preserve cosmetic newlines in the block comment case.

Meta

• rustfmt version: rustfmt 1.4.38-nightly (2022-02-02 27f5d83)
• From where did you install rustfmt?: using playground only
• How do you run rustfmt: using playground only

Additional remarks

• I didn't find any existing issues matching this problem description, but corrupted output when using comments within generic type parameters of a trait #4643 may be similar.
• One of my first issues reported to rustfmt. Let me know if this is not actually a bug.

[ytmimi]

Thank you for the report!

There is a lot of great detail here! I appreciate how thorough the report is. I can confirm that I'm able to reproduce this on the current master rustfmt 1.4.38-nightly (8b0b213c 2022-01-29).

[ytmimi]

Did some digging into this. The issue is that we're making the assumption that the type, prefix, and attributes can all fit on a single line if the type is written on a single line. As demonstrated by this issue, we should also consider whether or not a comment was found between the attribute and the prefix.

If anyone's interested in taking this on this issue on they should look at rewrite_struct_field, but specifically these lines:

rustfmt/src/items.rs

Lines 1735 to 1739 in 368a9b7

	if let Some(ref ty) = orig_ty {
	if !ty.contains('\n') {
	return Some(attr_prefix + &spacing + ty);
	}
	}

[tharun208]

@ytmimi I like to work on this

[tharun208]

@rustbot claim