Auto merge of rust-lang#46899 - m4b:linkage_name_equals_symbol_name, r=michaelwoerister

Set the dwarf linkage_name to the mangled name

ref rust-lang#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
rust-lang#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
rust-lang#2  0x000055555555b120 in inline::main () at /home/m4b/tmp/bad_debug/inline.rs:20
```

Author: bors

src/librustc_trans/debuginfo/metadata.rs

@@ -1665,8 +1665,8 @@ pub fn create_global_var_metadata(cx: &CrateContext,
     let linkage_name = if no_mangle {
         None
     } else {
-        let linkage_name = mangled_name_of_item(cx, node_def_id, "");
-        Some(CString::new(linkage_name).unwrap())
+        let linkage_name = mangled_name_of_item(cx, node_id);
+        Some(CString::new(linkage_name.to_string()).unwrap())
     };
 
     let global_align = cx.align_of(variable_type);

src/librustc_trans/debuginfo/mod.rs

@@ -15,7 +15,7 @@ use self::VariableAccess::*;
 use self::VariableKind::*;
 
 use self::utils::{DIB, span_start, create_DIArray, is_node_local_to_unit};
-use self::namespace::mangled_name_of_item;
+use self::namespace::mangled_name_of_instance;
 use self::type_names::compute_debuginfo_type_name;
 use self::metadata::{type_metadata, file_metadata, TypeMap};
 use self::source_loc::InternalDebugLocation::{self, UnknownLocation};
@@ -237,7 +237,6 @@ pub fn create_function_debug_context<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
     // Find the enclosing function, in case this is a closure.
     let def_key = cx.tcx().def_key(def_id);
     let mut name = def_key.disambiguated_data.data.to_string();
-    let name_len = name.len();
 
     let enclosing_fn_def_id = cx.tcx().closure_base_def_id(def_id);
 
@@ -251,16 +250,16 @@ pub fn create_function_debug_context<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
                                                       file_metadata,
                                                       &mut name);
 
-    // Build the linkage_name out of the item path and "template" parameters.
-    let linkage_name = mangled_name_of_item(cx, instance.def_id(), &name[name_len..]);
+    // Get the linkage_name, which is just the symbol name
+    let linkage_name = mangled_name_of_instance(cx, instance);
 
     let scope_line = span_start(cx, span).line;
 
     let local_id = cx.tcx().hir.as_local_node_id(instance.def_id());
     let is_local_to_unit = local_id.map_or(false, |id| is_node_local_to_unit(cx, id));
 
     let function_name = CString::new(name).unwrap();
-    let linkage_name = CString::new(linkage_name).unwrap();
+    let linkage_name = CString::new(linkage_name.to_string()).unwrap();
 
     let mut flags = DIFlags::FlagPrototyped;
     match *cx.sess().entry_fn.borrow() {

src/librustc_trans/debuginfo/namespace.rs

@@ -12,6 +12,9 @@
 
 use super::metadata::{unknown_file_metadata, UNKNOWN_LINE_NUMBER};
 use super::utils::{DIB, debug_context};
+use monomorphize::Instance;
+use rustc::ty;
+use syntax::ast;
 
 use llvm;
 use llvm::debuginfo::DIScope;
@@ -22,30 +25,22 @@ use common::CrateContext;
 use std::ffi::CString;
 use std::ptr;
 
-pub fn mangled_name_of_item(ccx: &CrateContext, def_id: DefId, extra: &str) -> String {
-    fn fill_nested(ccx: &CrateContext, def_id: DefId, extra: &str, output: &mut String) {
-        let def_key = ccx.tcx().def_key(def_id);
-        if let Some(parent) = def_key.parent {
-            fill_nested(ccx, DefId {
-                krate: def_id.krate,
-                index: parent
-            }, "", output);
-        }
-
-        let name = match def_key.disambiguated_data.data {
-            DefPathData::CrateRoot => ccx.tcx().crate_name(def_id.krate).as_str(),
-            data => data.as_interned_str()
-        };
-
-        output.push_str(&(name.len() + extra.len()).to_string());
-        output.push_str(&name);
-        output.push_str(extra);
-    }
+pub fn mangled_name_of_instance<'a, 'tcx>(
+    ccx: &CrateContext<'a, 'tcx>,
+    instance: Instance<'tcx>,
+) -> ty::SymbolName {
+     let tcx = ccx.tcx();
+     tcx.symbol_name(instance)
+}
 
-    let mut name = String::from("_ZN");
-    fill_nested(ccx, def_id, extra, &mut name);
-    name.push('E');
-    name
+pub fn mangled_name_of_item<'a, 'tcx>(
+    ccx: &CrateContext<'a, 'tcx>,
+    node_id: ast::NodeId,
+) -> ty::SymbolName {
+    let tcx = ccx.tcx();
+    let node_def_id = tcx.hir.local_def_id(node_id);
+    let instance = Instance::mono(tcx, node_def_id);
+    tcx.symbol_name(instance)
 }
 
 pub fn item_namespace(ccx: &CrateContext, def_id: DefId) -> DIScope {