fix(autofmt): stop dropping comments on attributes, spreads, control flow and empty macros

[nicoburns]

Summary

dx fmt deleted // comments in a number of positions, and in a few of them wrote output that no longer parses. This fixes those and adds a comment test matrix for the block syntax. It is the bottom of a stack: #5730 and #5731 build on the refactor in the first commit.

Output that no longer parsed (the comment swallows the closing brace):

// before                            // on main
div {                                div { class: "a", // note "child" }
    class: "a", // note
    "child"
}

Comments that were deleted

• after an attribute of an element short enough to collapse onto one line, and after the last attribute of an element with no children
• above or after a ..spread
• above a node or attribute when the line in between has trailing whitespace
• in for / if / else bodies: after the opening brace, before the closing brace, in a body containing nothing else, and after the closing brace of an else if chain
• between the closing brace of a branch and its else
• the only thing in an rsx! call, or after its opening delimiter (also for an rsx! nested in an expression)
• after anything on a line containing multi-byte characters (columns were used as byte offsets)
• in the middle of a construct: between the name of an element or component and its {, between the name of an attribute and its value, inside the branches of an if attribute value, and inside the header of a for or an if

Smaller changes in behaviour

• A comment-only body keeps one blank line between its comments, and indents them with the configured indentation rather than four spaces.
• An if with empty bodies is written if a {} else {} rather than with a blank line in each body.
• div {..attrs, (spread only, children below) gets its missing space.
• A long if attribute value in an element with one attribute per line starts on the line of its name (class: if a {). It was written below the name at the same indentation.

How

• First commit, no behaviour change: the comment helpers took a &Brace but only used the positions of its two tokens. They now take a BodyDelimiters { open, close }, so they work for any delimited body.

• for / if bodies go through the same open-comment, comment-only-body and closing-comment helpers as elements (write_block_body, write_comment_only_body, write_closing_line).

• Attributes with comments are written one per line. The exception is a comment after the last attribute when children follow, which keeps the existing div { class: "a", // note layout.

• A comment can't share a line with else, so comments before one are left in place and the else goes on its own line (write_else):

  if a {
      "a"
  } // after the brace
  // above the else
  else {
      "b"
  }

• A nested rsx! with a comment after its opening brace is always written across several lines, so the existing expression comment pass finds the line to attach it to.

• text_before / text_after convert a column to a byte offset before slicing a source line.

• has_leading_comments replaces three copies of the "are there comments above this" loop, which stopped at whitespace-only lines.

• Comments in the middle of a construct (last commit). These constructs are rebuilt from their tokens, so each needs a place to put a comment:

  div { // was between `div` and `{`
      class:
          // was between `class:` and its value; the value is still formatted
          "a",
      id: if a {
          // inside a branch; the chain is still formatted
          "x"
      } else {
          "y" // after the value of a branch
      },
  }
  for item in items
      // inside a header: the header is written as it is in the source, re-indented
      .iter()
  {
      div {}
  }

  A header or condition is only taken from the source if line_comments, a small scanner that skips strings, characters and block comments, finds a comment in it.

Tests

• Idempotency samples: comments_attributes, comments_control_flow, comments_positions.
• Idempotency sample comments_constructs, and messy input comments-constructs-4sp / comments-constructs-tab, for comments in the middle of a construct.
• Messy input, with spaces and with tabs: comments-messy-4sp, comments-messy-tab.

All of these fail on main.

Not fixed here

• /* */ comments are still deleted everywhere (dx fmt eats block comments #2751).
• Comments inside the arguments of a call in an attribute value (format!(a, // note …) are deleted when the expression is reflowed.
• With tabs or split_line_attributes, expressions that contain comments and a nested rsx! are not idempotent, and with split_line_attributes can lose comments.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/99eedb36d44345dc8ce0f3ed5c460bd6
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/99eedb36d44345dc8ce0f3ed5c460bd6?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring