[Bug rust/31431] New: Sub-par output for partially optimized-out slice

[Bug rust/31431] New: Sub-par output for partially optimized-out slice

[tromey at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31431

            Bug ID: 31431
           Summary: Sub-par output for partially optimized-out slice
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: rust
          Assignee: unassigned at sourceware dot org
          Reporter: tromey at sourceware dot org
  Target Milestone: ---

This comes from https://github.com/rust-lang/rust/issues/46698

The test case is:

use std::env;

#[inline(never)]
fn foo(x: &str, y: &str) -> usize {
    x.len() + y.len()
}

fn main() {
    let mut args = env::args();

    let x = args.next().unwrap();
    let y = args.next().unwrap();
    let z = foo(&x, &y);
    println!("z={:?}", z);
}

Compile it with "rustc -O -g q.rs".

In gdb, do:

(gdb) break 3
(gdb) run a b
... stop
(gdb) info args

I get:

(gdb) info args
x = <error reading variable: value has been optimized out>
y = <error reading variable: value has been optimized out>

This isn't really great.  The "error" text here is somewhat
misleading -- no error has actually occurred.

What's happening here is that the slice is partially optimized
out by the compiler.  This is more obvious in C mode:

(gdb) set lang c
Warning: the current language does not match this frame.
(gdb) p x
$4 = {data_ptr = <optimized out>, length = 6}


What's happening is that rust-lang.c is unconditionally trying
to read the data from the slice:

#2  0x0000000000d1c0e3 in value::require_not_optimized_out (this=0x2b442b0) at
../../binutils-gdb/gdb/value.c:1130
#3  0x0000000000d1cad5 in value::contents (this=0x2b442b0) at
../../binutils-gdb/gdb/value.c:1310
#4  0x0000000000d1fcc7 in value_as_address (val=0x2b442b0) at
../../binutils-gdb/gdb/value.c:2779
#5  0x0000000000b2f42e in convert_slice (val=0x2b9e840) at
../../binutils-gdb/gdb/rust-lang.c:365
#6  0x0000000000b2f68a in rust_language::val_print_slice (this=0x2653180
<rust_language_defn>, val=0x2b9e840, stream=0x26c8850, 
    recurse=0, options=0x7fffffffd620) at
../../binutils-gdb/gdb/rust-lang.c:461
#7  0x0000000000b2f801 in rust_language::val_print_struct (this=0x2653180
<rust_language_defn>, val=0x2b9e840, stream=0x26c8850, 
    recurse=0, options=0x7fffffffd620) at
../../binutils-gdb/gdb/rust-lang.c:495


It's not totally clear what is ideal here.
I'm thinking just:

(gdb) print x
$999 = <optimized out>

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