format!() generates code without line number debug info

[vadimcn]

Compile the following with -g flag, then try stepping line-by-line in gdb:

fn main() {
    let a = 42
    println!("The answer = {}", a);
    println!("The end!");
}

After each printlt!() statement the execution point jumps to the first line of the function.
Examination of LLVM IR shows that much of the fmt::Arguments construction code has no line debug metadata attached to it (or at least that's what I think that code does).

[huonw]

The debuginfo of macros still seems to be problematic, e.g. this is a session with the example in the issue, first using next and then using step.

(gdb) break debug_println.rs:2
Breakpoint 1 at 0x544b: file debug_println.rs, line 2.
(gdb) r
...

Breakpoint 1, debug_println::main () at debug_println.rs:2
2       let a = 42;
(gdb) n
3       println!("The answer = {}", a);
(gdb) n
2   <std macros>: No such file or directory.
(gdb) n
5   }
(gdb) n
0x0000555555561b55 in sys_common::unwind::try::try_fn::h4848098439110500489 ()
(gdb) 
(gdb)
(gdb) r
...

Breakpoint 1, debug_println::main () at debug_println.rs:2
2       let a = 42;
(gdb) s
3       println!("The answer = {}", a);
(gdb) s
2   <std macros>: No such file or directory.
(gdb) s
debug_println::fmt::ArgumentV1<'a>::new<i32> (x=0x7fffffffdb18, f=0x55555558a570 <fmt::num::i32.fmt..Display::fmt::h05897f40d7b1bb4buzU>) at ../src/libcore/fmt/mod.rs:192
192 ../src/libcore/fmt/mod.rs: No such file or directory.
(gdb) s
193 in ../src/libcore/fmt/mod.rs
(gdb) s
189 in ../src/libcore/fmt/mod.rs
(gdb) s
debug_println::fmt::Arguments<'a>::new_v1 (pieces=&[&str](len: 2) = {...}, args=&[core::fmt::ArgumentV1](len: 1) = {...}) at ../src/libcore/fmt/mod.rs:226
226 in ../src/libcore/fmt/mod.rs
(gdb) s
debug_println::fmt::Arguments<'a>::new_v1 (pieces=&[&str](len: 1) = {...}, args=&[core::fmt::ArgumentV1](len: 0)) at ../src/libcore/fmt/mod.rs:226
226 in ../src/libcore/fmt/mod.rs
(gdb) s
debug_println::main () at debug_println.rs:5
5   }
(gdb) s
0x0000555555561b55 in sys_common::unwind::try::try_fn::h4848098439110500489 ()
(gdb) 

cc #9232, #17234, #18285

[vadimcn]

I've hit this issue again today. This got me thinking: most of the time I don't want to step through a macro. It should be possible to step over them just like any regular function.

We could probably accomplish this by generating the same kind of debug info that LLVM itself creates when it inlines a function. The only issue might be recovering the location of macro invocation from expanded AST nodes. IIRC, the AST currently does not store this information,does it?