[tor-bugs] #28077 [Core Tor/Tor]: remove unsafe block from cstr! macro

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#28077: remove unsafe block from cstr! macro
--------------------------+------------------------------------
 Reporter:  cyberpunks    |          Owner:  (none)
     Type:  defect        |         Status:  needs_review
 Priority:  Medium        |      Milestone:  Tor: 0.3.6.x-final
Component:  Core Tor/Tor  |        Version:  Tor: 0.3.4.1-alpha
 Severity:  Normal        |     Resolution:
 Keywords:  rust          |  Actual Points:
Parent ID:                |         Points:
 Reviewer:  ahf           |        Sponsor:
--------------------------+------------------------------------

Comment (by ahf):

 The way I read the `default()` implementation for `CStr` is that it is:

 {{{
 #[stable(feature = "cstr_default", since = "1.10.0")]
 impl<'a> Default for &'a CStr {
     fn default() -> &'a CStr {
         const SLICE: &'static [c_char] = &[0];
         unsafe { CStr::from_ptr(SLICE.as_ptr()) }
     }
 }
 }}}

 Which seems to be what we are doing too, which means that to me it seems
 sensible to use the `unwrap_or_default()` method.

 Opened a PR to see what Travis says:
 https://github.com/torproject/tor/pull/466

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28077#comment:8>
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