[RFC 0004] Replace Unicode Quotes (#4)

* Add Replace Unicode Quotes draft.

* Replace Unicode Quotes: various fixes.

rfcs/0004-replace-unicode-quotes.md

@@ -0,0 +1,104 @@
+---
+feature: replace-unicode-quotes
+start-date: 2017-03-19
+author: layus
+co-authors: zimbatm
+related-issues:
+    - https://github.com/NixOS/nix/pull/1140
+    - https://github.com/NixOS/nix/issues/915
+    - https://github.com/NixOS/nix/pull/910
+---
+
+# Summary
+[summary]: #summary
+
+Nix uses unicode glyphs to quote strings and paths in its output.
+This RFC proposes to use only ASCII `"` and `'` for quoting purposes in strings
+printed during evaluation.
+
+# Motivation
+[motivation]: #motivation
+
+There are three main reasons for this change.
+
+1. _Correctness_: By removing preventively these characters, we will not have
+   to track the triggered issues separately. Unicode interact badly with
+   variable interpolation in bash and will create more issues if we keep them. 
+   See "nix installer produces broken output on Darwin"
+   https://github.com/NixOS/nix/issues/915.
+   and the detailed explanation in https://github.com/NixOS/nix/issues/910.
+
+   As an example, on the following code the shell is treating the 1st UTF-8
+   byte of `’` as part of the variable name (which is undefined, thus "").
+   This results in `echo ""$'\x80\x99'`
+
+       $ x=y
+       $ echo "$x"
+       y
+       $ echo "$x’" # WTF is going on here!?
+       ??
+
+   Also, such quotes should be removed from code snippets in the documentation.
+   Otherwise, they cannot be used as is. See
+   http://lists.science.uu.nl/pipermail/nix-dev/2010-April/004286.html
+
+2. _Compatibility_: Most terminal emulators do not recognise unicode quotes as
+   string delimiters. This makes string copy/paste from the terminal clumsy.
+
+   For example, with a double-click in the following text, gnome-terminal will
+   correctly select the derivation path without the quotes, while rxvt-unicode
+   and st will select the string with the quotes. The string without quotes
+   needs to be tediously edited to be reused anywhere.
+
+   > building path(s) ‘/nix/store/hdlkn4pnc7l79jbawlkvssx1hc7gqmj8-gnum4-1.4.18’
+
+   Some terminals like Eterm seem unable to print these characters correctly.
+
+3. _Consistency_: As some quotes were replaced for compatibility with shells and
+   terminal emulators, we end up with a mix of both styles.
+
+See also
+ - https://github.com/NixOS/nix/pull/947#r71710959
+ - https://github.com/NixOS/nix/pull/1140 (Get rid of unicode quotes)
+ - https://github.com/NixOS/nix/commit/b3fc0160618d89bf63ce87ccad27fc68360c9731
+
+# Detailed design
+[design]: #detailed-design
+
+Implementing this requires to replace every unicode quote glyph by an ASCII
+character. This change needs only happen in strings intended to be part of
+build logs or otherwise printed in the console.
+
+The automated change should not alter comments nor documentation, except for
+code snippets within that documentation. Neither should it alter derivations
+outputs by changing input variables.
+
+After the change, using ASCII quotes should be enforced to maintain consistency.
+
+The change mainly needs to happen in Nix, but nixpkgs should follow for consistency.
+For example, https://github.com/NixOS/nixpkgs/blob/c86f05e7ce13e64238960ebf3ee9706142db961b/nixos/modules/tasks/filesystems.nix#L236 should be updated.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Snippets of nix output in documentation and blogs will be out of sync (their
+quotes would not match the real printed output).
+Also, ASCII quotes are less aesthetic.
+
+# Alternatives
+[alternatives]: #alternatives
+
+As a decision needs to be take, we could also prefer to keep these nice unicode
+glyphs and fix issues as we encounter them.
+This also requires to push patches upstream in terminal emulators, and provide
+documentation as how to use them safely in shell scripts.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+Should we also force ASCII quotes in pkgs meta fields and nixos options description ?
+These are displayed on the web (see https://nixos.org/nixos/options.html) and in the console
+with nixos-option.
+It would be simpler to consider these as documentation, leaving them as-is.
+
+