made gdb pretty-printing more robust when printing uninitialized vec

[pnkfelix]

made gdb pretty-printing more robust when printing uninitialized vec

I based this solution on my reading of:

https://rethinkdb.com/blog/make-debugging-easier-with-custom-pretty-printers#what-is-still-to-be-done

That post claims that there is no clean way to check for garbage pointers, and
so this PR adopts the same solution of tentatively attempting to convert a
dererence to a string, which throws a clean exception on garbage that we can
catch and recover from.

I only made the change to vec and not the other pretty printers because I wanted
to focus my effort on the simplest thing that would resolve issue #64343. In
particular, I considered generalizing this fix to work on the other datatypes
in the pretty-printing support library, but I don't want to invest effort in
that until after we resolve our overall debugging support strategy; see also
issues #60826 and #65564.

Fix #64343

[rust-highfive]

r? @alexcrichton

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[pnkfelix]

Oh, the specific behavior that occurs here is that when it hits garbage data, it terminates the loop over the children and prints inaccessible in place of the element of the last attempted access

I.e. for pretty-uninitialized-vec, you see this:

(gdb) b 'pretty-uninitialized-vec.rs':21
Breakpoint 1 at 0x5884: file ../src/test/debuginfo/pretty-uninitialized-vec.rs, line 21.
(gdb) r
Starting program: /home/pnkfelix/Dev/Mozilla/rust.git/objdir/pretty-uninitialized-vec
warning: Loadable section ".note.gnu.property" outside of ELF segments
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, pretty_uninitialized_vec::main () at ../src/test/debuginfo/pretty-uninitialized-vec.rs:21
21          zzz(); // #break
Missing separate debuginfos, use: dnf debuginfo-install libgcc-9.2.1-1.fc30.x86_64
(gdb) print vec
$1 = Vec<i32>(len: 93824992438960, cap: 140737478328320) = {inaccessible}
(gdb) 

[alexcrichton]

@bors: r+

[bors]

📌 Commit 9b40e0b has been approved by alexcrichton