Include InstanceDef's discriminant in the symbol hash.

Author: qnighy

src/librustc_codegen_utils/symbol_names.rs

@@ -114,6 +114,7 @@ use rustc_mir::monomorphize::Instance;
 use syntax_pos::symbol::Symbol;
 
 use std::fmt::Write;
+use std::mem::discriminant;
 
 pub fn provide(providers: &mut Providers) {
     *providers = Providers {
@@ -220,8 +221,9 @@ fn get_symbol_hash<'a, 'tcx>(
             (&tcx.crate_disambiguator(instantiating_crate)).hash_stable(&mut hcx, &mut hasher);
         }
 
-        let is_vtable_shim = instance.is_vtable_shim();
-        is_vtable_shim.hash_stable(&mut hcx, &mut hasher);
+        // We want to avoid accidental collision between different types of instances.
+        // Especially, VtableShim may overlap with its original instance without this.
+        discriminant(&instance.def).hash_stable(&mut hcx, &mut hasher);
     });
 
     // 64 bits should be enough to avoid collisions.

src/test/ui/symbol-names/basic.stderr

@@ -1,4 +1,4 @@
-error: symbol-name(_ZN5basic4main17h6ab1850bb0b9f417E)
+error: symbol-name(_ZN5basic4main17h08bcaf310214ed52E)
   --> $DIR/basic.rs:13:1
    |
 LL | #[rustc_symbol_name] //~ ERROR _ZN5basic4main

src/test/ui/symbol-names/impl1.stderr

@@ -1,4 +1,4 @@
-error: symbol-name(_ZN5impl13foo3Foo3bar17h99c48478d64a0eb0E)
+error: symbol-name(_ZN5impl13foo3Foo3bar17hc487d6ec13fe9124E)
   --> $DIR/impl1.rs:18:9
    |
 LL |         #[rustc_symbol_name] //~ ERROR _ZN5impl13foo3Foo3bar
@@ -10,7 +10,7 @@ error: item-path(foo::Foo::bar)
 LL |         #[rustc_item_path] //~ ERROR item-path(foo::Foo::bar)
    |         ^^^^^^^^^^^^^^^^^^
 
-error: symbol-name(_ZN5impl13bar33_$LT$impl$u20$impl1..foo..Foo$GT$3baz17hebf13830acf865d9E)
+error: symbol-name(_ZN5impl13bar33_$LT$impl$u20$impl1..foo..Foo$GT$3baz17h38577281258e1527E)
   --> $DIR/impl1.rs:28:9
    |
 LL |         #[rustc_symbol_name] //~ ERROR _ZN5impl13bar33_$LT$impl$u20$impl1..foo..Foo$GT$3baz