debuginfo: Generate debugging information for function level static variables

[cmacknz]

Support for crate level static variable debug info will be added once the patch for #9227 lands, but it does not include support for handling statics declared at the function level.

Fundamentally the new debuginfo::create_global_var_metadata() function needs to be updated to do the following:

1. Identify when the variable it was called with was declared within a function.
2. Emit scope metadata indicating that the variable belongs to the function, rather than the containing crate.

The "correct" way to approach this would be to update debuginfo::populate_scope_map::walk_decl() to handle the DeclItem case for ItemStatic items as is done for function local variables.

However, as a consequence of a bug in clang/llvm (see: http://llvm.org/bugs/show_bug.cgi?id=19238), passing the lexical scope metadata from a debuginfo::FunctionDebugContextData::scope_map member to llvm::LLVMDIBuilderCreateStaticVariable() results in a segfault when generating debug information.

The suggested work around is to use the function debug info as scope for the variable instead, with the consequence that the generated debug info will be not be entirely correct (similar to the clang case in the bug above).

[brson]

Neat!

cc @michaelwoerister

[huonw]

Hm, it seems this may've regressed, because I cannot see even module-level statics in GDB.

static OUTSIDE: i32 = 123;
fn main() {
    static INSIDE: i32 = 456;
}

(gdb) break debug_fn_static.rs:4
Breakpoint 1 at 0x5164: file debug_fn_static.rs, line 4.
(gdb) run
...

Breakpoint 1, debug_fn_static::main () at debug_fn_static.rs:4
4   }
(gdb) info variables .*SIDE.*
All variables matching regular expression ".*SIDE.*":
(gdb) 

[Mark-Simulacrum]

I believe that the static variables huonw notes above aren't being picked up because they're const folded away, but this appears to be working now. Both module-level and function-local statics are picked up.

pub static mut OUTSIDE: i32 = 123;

fn main() {
    static mut INSIDE: i32 = 456;

    unsafe { (OUTSIDE, INSIDE) };
}

generates

(gdb) info variables .*SIDE.*
All variables matching regular expression ".*SIDE.*":

File ./t.rs:
static i32 t::OUTSIDE;
static i32 t::main::INSIDE;

Non-debugging symbols:
0x000000000024e2a8  t::OUTSIDE::h631ab381ceee3286
0x000000000024e2ac  t::main::INSIDE::hcfa46a7f78bcb925