Undefined behavior in `test_negative_time_propagation`

[icmccorm]

I've been experimenting with a version of Miri that can execute foreign functions by interpreting the LLVM bytecode that is produced during a crate's build process. We're hoping that our results can assist with the Krabcake project.

Miri found the following error in the test case tests::test_negative_time_propagation

---- Foreign Error Trace ----

@ store i8 95, ptr %80, align 1, !dbg !639

.../sgp4-rs/src/sgp4/sgp4io.cpp:102:19
src/sgp4_sys.rs:463:9: 474:10
-----------------------------

error: Undefined Behavior: attempting a write access using <102969> at alloc26549[0xf], but that tag only grants SharedReadOnly permission for this location
    |
    = note: attempting a write access using <102969> at alloc26549[0xf], but that tag only grants SharedReadOnly permission for this location
    = note: this error occurs as part of an access at alloc26549[0xf..0x10]
    = note: (no span available)
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <102969> was created by a SharedReadOnly retag at offsets [0x0..0x46]
   --> src/sgp4_sys.rs:464:13
    |
464 |             l1.as_ptr(),
    |             ^^^^^^^^^^^

The pointer created by l1.as_ptr() has read-only permissions, but the C function twoline2rv mutates it.

  for (j = 10; j <= 15; j++)
    if (longstr1[j] == ' ')
      longstr1[j] = '_'

This seems like it would be non-trivial to fix, since l1 is an &str that's passed down through a constructor. Unless copying the string into a temporary variable would be a valid solution.