Markdown backtick

[roberth]

We're living in the age of the markdown backtick. Let's embrace it in Nix.

Proposal

Change all '%s' and '%1%' to `%s` and `%1%`

Effects

• (+) Clear, visually unambiguous stderr output
• (+) Renders into inline code spans when pasted into a markdown system such as GitHub, Discourse, etc
• (-) Slightly more typing when discussing the C++ code in markdown

  Try with ``"`%s`"``
  Use a backtick (`` ` ``)

  renders to
  Try with "`%s`"
  Use a backtick (`)

Background

Nix uses ' apostrophes to demarcate the start and end of code, identifiers, user input, etc, in error messages.
This is occasionally confusing:

• color information is not always available, including after copy/paste (i.e. when users need help)
• adds visual noise
• can be confused for the same ' that is allowed in identifiers, moderately obscuring the difference between 'mapAttrs' and 'mapAttrs''

Meanwhile, I have observed that over the past decade or so:

• ` has been largely phased out from bash scripts in favor of $( )
• markdown has won
• the markdown ` is starting to make its way into stderr

If we were to switch from 'foo' to `foo` in error messages, this will avoid any confusing with the ' used in the language, and it will even render nicely when pasted into markdown-based systems.

[edolstra]

Generally in favor of this. Ideally we would have the Error constructor making sure that string arguments are properly escaped so they can actually be rendered as Markdown in the terminal. E.g. instead of

        throw BadStorePath("path '%1%' is not in the Nix store", path);

do

        throw BadStorePath("path %s is not in the Nix store", path);

where the Error constructor would take care of adding backticks around string arguments and escaping any backtick characters in the string. (Worth noting that std::filesystem::path already automatically gets quoted in format strings.)

Also, while we're at it, I'd like to start capitalizing/punctuating errors properly, i.e.

        throw BadStorePath("Path %s is not in the Nix store.", path);

[edolstra]

Oh and we should probably switch to std::format as well, see #10294. So it would be

        throw BadStorePath("Path {} is not in the Nix store.", path);