GDB cast raw pointer to &str

[TjeuKayim]

The &str type is not available in rust-gdb.

When debugging using rust-gdb, local variables with &str type are pretty printed.

(gdb) p my_string_local
$6 = "my string"
(gdb) ptype my_string_local
type = struct &str {
  data_ptr: *mut u8,
  length: usize,
}

I expect that a raw pointer referring to the same memory location on the stack can be casted to &str and printed. However, gdb throws an exception:

(gdb) p *(($rsp + 168) as *mut &[u8])
$15 = &[*gdb*](len: 14) = {109, 121, 32, 115, 116, 114, 105, 110, 103}
(gdb) p *(($rsp + 168) as *mut &str)
No typed name 'str' in current context

rust/src/etc/debugger_pretty_printers_common.py

Lines 149 to 151 in 8ee24f6

	# STR SLICE
	if unqualified_type_name == "&str":
	return TYPE_KIND_STR_SLICE

.

Workaround

You can use a helper function to cast and dereference the pointer:

unsafe fn print_str(ptr: *const &str) -> &str {
    return *ptr;
}

(gdb) p my_lib::print_str($rsp + 168)
$15 = "my string"

Meta

rustc --version --verbose
rustc 1.38.0 (625451e37 2019-09-23)
binary: rustc
commit-hash: 625451e376bb2e5283fc4741caa0a3e8a2ca4d54
commit-date: 2019-09-23
host: x86_64-unknown-linux-gnu
release: 1.38.0
LLVM version: 9.0

[Centril]

cc @mw @eddyb

[eddyb]

cc @michaelwoerister (not @mw)

[tromey]

I think this is a gdb bug. There's https://sourceware.org/bugzilla/show_bug.cgi?id=22251, but that concerns having &str as a built-in type... but in this case, I see it in the DWARF and I think gdb ought to pick it up.

[wesleywiser]

Visited during wg-debugging triage. Testing with gdb 12.1 shows the same issue (which makes sense as there hasn't been much activity on the upstream issue). A proper fix will need to be made in gdb before this can be closed.

[tromey]

I fixed this in gdb. The fix will be in gdb 14. I think you can close this.

[michaelwoerister]

Thanks, @tromey!