Feature Request: Option to enforce a consistent quote style

[vivodi]

pyproject-fmt currently provides an opinionated format but lacks an option to enforce a consistent quote style for strings. While TOML's syntax allows for both single and double quotes, development teams often prefer a single, consistent style across their project configuration.

This feature request proposes adding a configuration option to automatically format TOML strings to a preferred quote style, while correctly handling the semantic differences between them to ensure the string's value is never altered.

Proposed Configuration and Behavior

A new quote_style option should be added to the [tool.pyproject-fmt] table.

Configuration Example:

[tool.pyproject-fmt]
quote_style = "single"  # or "double"

The formatting logic must correctly handle the two types of TOML strings:

• Literal Strings ('...'): All content is taken as-is. Backslashes have no special meaning.
• Basic Strings ("..."): Support escape sequences (e.g., \n for a newline, \\ for a literal backslash).

The conversion rules should be as follows:

1. When quote_style = "double":

This option will convert all single-quoted literal strings to double-quoted basic strings. This transformation is always possible, provided that special characters are correctly escaped.

• Rule: When converting from '...' to "...", any backslashes (\) and double quotes (") within the string must be escaped.
• Example 1: 'hello' becomes "hello".
• Example 2: 'path\to\file' must become "path\\to\\file" to preserve the literal backslashes.
• Example 3: 'this contains "quotes"' must become "this contains \"quotes\"".

2. When quote_style = "single":

This option will convert double-quoted basic strings to single-quoted literal strings, but only when it is safe to do so without changing the string's value.

• Rule: Convert from "..." to '...' if, and only if, the string's content meets both of these conditions:
  1. It does not contain any single quotes (').
  2. It does not contain any escape sequences (like \n, \t, \uXXXX) that would be interpreted differently in a literal string. A string like "a\\b" (representing a\b) can be safely converted, but "a\nb" (representing a newline) cannot.
• If a string cannot be safely converted, it must remain double-quoted to preserve its semantic value.
• Example 1: "hello" becomes 'hello'.
• Example 2: "don't change" remains "don't change" because it contains a single quote.
• Example 3: "line1\nline2" remains "line1\nline2" because its value depends on interpreting the \n escape sequence.

Implementing this feature would significantly improve pyproject-fmt's utility for projects with strict style guides, allowing for stylistic consistency without compromising the correctness of the TOML file.