Set the dwarf linkage_name to the mangled name

[m4b]

ref #46453

@michaelwoerister or anyone else who knows, i'm not sure if this is the correct instance to pass here (or how to get the correct one precisely): https://github.com//m4b/rust/blob/5a94a48678ec0a20ea6a63a783e63546bf9459b1/src/librustc_trans/debuginfo/namespace.rs#L36

So don't merge this yet, I'd like to learn about correct instance first; however, I think this already fixes a bunch of weirdness i'm seeing debugging from time to time, not to mention backtraces in gdb via bt are now readable meaningful 🎉

E.g.:

new:

(gdb) bt
#0  <inline::Foo as core::convert::From<()>>::from () at /home/m4b/tmp/bad_debug/inline.rs:11
#1  0x000055555555a35d in inline::deadbeef () at /home/m4b/tmp/bad_debug/inline.rs:16
#2  0x000055555555a380 in inline::main () at /home/m4b/tmp/bad_debug/inline.rs:20

old:

(gdb) bt
#0  inline::{{impl}}::from () at /home/m4b/tmp/bad_debug/inline.rs:11
#1  0x000055555555b0ed in inline::deadbeef () at /home/m4b/tmp/bad_debug/inline.rs:16
#2  0x000055555555b120 in inline::main () at /home/m4b/tmp/bad_debug/inline.rs:20

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @estebank (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[estebank]

r? @michaelwoerister

[michaelwoerister]

Thanks, @m4b! Yes, that is the correct instance (it's the same that is also used for translating to LLVM IR).

@bors r+

[bors]

📌 Commit 990a5cc has been approved by michaelwoerister

[m4b]

@michaelwoerister np, and thanks for clarifying its correct!

So, one last component I thought should be remedied here is the following addition:

diff --git a/src/librustc_trans/debuginfo/namespace.rs b/src/librustc_trans/debuginfo/namespace.rs
index 47e2b8c461..8324077e29 100644
--- a/src/librustc_trans/debuginfo/namespace.rs
+++ b/src/librustc_trans/debuginfo/namespace.rs
@@ -58,7 +58,8 @@ pub fn item_namespace(ccx: &CrateContext, def_id: DefId) -> DIScope {
 
     let namespace_name = match def_key.disambiguated_data.data {
         DefPathData::CrateRoot => ccx.tcx().crate_name(def_id.krate).as_str(),
-        data => data.as_interned_str()
+        DefPathData::Module(module) => module,
+        _ => return parent_scope,
     };
 
     let namespace_name = CString::new(namespace_name.as_bytes()).unwrap();

What this does is drop the {{impl}} in crate::mod::{{impl}}::fn in the dwarf namespaces.

So, in our example above, this would yield inline as the namespace, instead of inline::{{impl}}.

I believe we should do this. One immediate benefit is it won't show up in autocomplete (which you can't use anyway, since the current parser won't accept {{ and it isn't any symbol or namespace anywhere anyway).

However, this had the side effect of sending me down a rabbit hole w.r.t. namespaces, debugging info, gdb, and rusts "universal method call syntax". I wrote up a large thing here, but I don't think it's the time or place at the moment to paste here, probably another issue is better.

But it basically it has to do with apparently a known issue with the symbol names for trait impls and generics, e.g., one of these is the symbol table name for a impl Debug for Foo<u32>, and the other for Foo<u64>:

<foo::Foo<T> as core::fmt::Debug>::fmt::h0a79f028de68abf6
<foo::Foo<T> as core::fmt::Debug>::fmt::ha10b9f27c105328e  

E.g., they only differ in hash, but really we'd like to see the type parameter reified/instantiated with the appropriate u32/u64 (or whatever it was impl'd for)

The solution here will be probably complicated, and may involve drastically changing rusts symbol name output, but I think in the future its really important to get right.

Also would like to thank @eddyb on irc for long and fruitful discussion about some of the complexities involved here, was very interesting :)

[bors]

⌛ Testing commit 990a5cc with merge a834b86...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: michaelwoerister
Pushing a834b86 to master...