Auto merge of #54183 - qnighy:by-value-object-safety, r=oli-obk

Implement by-value object safety

This PR implements **by-value object safety**, which is part of unsized rvalues #48055. That means, with `#![feature(unsized_locals)]`, you can call a method `fn foo(self, ...)` on trait objects. One aim of this is to enable `Box<FnOnce>`  in the near future.

The difficulty here is this: when constructing a vtable for a trait `Foo`, we can't just put the function `<T as Foo>::foo` into the table. If `T` is no larger than `usize`, `self` is usually passed directly. However, as the caller of the vtable doesn't know the concrete `Self` type, we want a variant of `<T as Foo>::foo` where `self` is always passed by reference.

Therefore, when the compiler encounters such a method to be generated as a vtable entry, it produces a newly introduced instance called `InstanceDef::VtableShim(def_id)` (that wraps the original instance). the shim just derefs the receiver and calls the original method. We give different symbol names for the shims by appending `::{{vtable-shim}}` to the symbol path (and also adding vtable-shimness as an ingredient to the symbol hash).

r? @eddyb

1 files changed, 12 insertions, 1 deletions

diff --git a/src/librustc_codegen_utils/symbol_names.rs b/src/librustc_codegen_utils/symbol_names.rs
index c1e80234a77..0d95b0c7bbc 100644
--- a/src/librustc_codegen_utils/symbol_names.rs
+++ b/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 {
@@ -219,6 +220,10 @@ fn get_symbol_hash<'a, 'tcx>(
                 .hash_stable(&mut hcx, &mut hasher);
             (&tcx.crate_disambiguator(instantiating_crate)).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.
@@ -322,7 +327,13 @@ fn compute_symbol_name<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, instance: Instance
 
     let hash = get_symbol_hash(tcx, def_id, instance, instance_ty, substs);
 
-    SymbolPathBuffer::from_interned(tcx.def_symbol_name(def_id)).finish(hash)
+    let mut buf = SymbolPathBuffer::from_interned(tcx.def_symbol_name(def_id));
+
+    if instance.is_vtable_shim() {
+        buf.push("{{vtable-shim}}");
+    }
+
+    buf.finish(hash)
 }
 
 // Follow C++ namespace-mangling style, see