Implement by-value object safety #54183

qnighy · Sep 13, 2018

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

TimNN · Sep 25, 2018

Ping from triage @eddyb: This PR requires your review.

eddyb · Sep 26, 2018

eddyb reviewed Sep 26, 2018

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eddyb · Sep 26, 2018

src/librustc_mir/shim.rs

+            assert!(arg_ty.is_self());
+            local_decls[rcvr_arg].ty = tcx.mk_mut_ptr(arg_ty);
+
+            Operand::Copy(rcvr_l.deref())


This doesn't seem right, should probably be Operand::Move. cc @nikomatsakis

Changed to Operand::Move.

eddyb · Sep 26, 2018

eddyb reviewed Sep 26, 2018

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eddyb · Sep 26, 2018

src/librustc_codegen_utils/symbol_names.rs

+
+    if instance.is_vtable_shim() {
+        buf.push("{{vtable-shim}}");
+    }


Is this needed?

It's unnecessary for symbol distinction. It's useful when reading the generated LLVM IR or assembly.

eddyb · Sep 26, 2018

eddyb reviewed Sep 26, 2018

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eddyb · Sep 26, 2018

src/librustc_codegen_utils/symbol_names.rs

@@ -219,6 +219,9 @@ fn get_symbol_hash<'a, 'tcx>(
                .hash_stable(&mut hcx, &mut hasher);
            (&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);


Shouldn't we be hashing something from the Instance anyway? I thought we already had overlaps in (DefId, Substs) between possible Instances, did I imagine that?

cc @michaelwoerister

My understanding is that, aside from VtableShim that I've just introduced, there are no overlaps in (DefId, Substs). Intrinsics just differs from Item in its ABI. FnPtrShim, Virtual, ClosureOnceShim, and CloneShim all refers to trait methods, and these instances do not have manual implementations. DropGlue refers to core::ptr::drop_in_palce, but for that function DropGlue is specifically generated, so it won't collide with Item.

With that said, hashing InstanceDef's discriminator seems good to me.

eddyb · Sep 26, 2018

eddyb reviewed Sep 26, 2018

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src/librustc_codegen_llvm/mir/block.rs

eddyb · Sep 26, 2018

eddyb reviewed Sep 26, 2018

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eddyb · Sep 26, 2018

src/librustc_codegen_llvm/meth.rs

@@ -98,7 +98,7 @@ pub fn get_vtable(
        let methods = tcx.vtable_methods(trait_ref);
        let methods = methods.iter().cloned().map(|opt_mth| {
            opt_mth.map_or(nullptr, |(def_id, substs)| {
-                callee::resolve_and_get_fn(cx, def_id, substs)
+                callee::resolve_and_get_fn_for_vtable(cx, def_id, substs)


You probably need to do something similar in miri, but it might be harder.

eddyb · Sep 26, 2018

eddyb reviewed Sep 26, 2018

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eddyb · Sep 26, 2018

src/librustc_codegen_llvm/common.rs

@@ -456,3 +456,22 @@ pub fn ty_fn_sig<'a, 'tcx>(cx: &CodegenCx<'a, 'tcx>,
        _ => bug!("unexpected type {:?} to ty_fn_sig", ty)
    }
 }
+
+pub fn ty_fn_sig_vtable<'a, 'tcx>(


I would expect this to be on Instance itself. Same for ty_fn_sig above, I guess. Feel free to leave FIXME comments here instead.

Should I introduce InstanceExt in codegen_llvm, or just move them into librustc?

This feels like it would fit in librustc as a function of instance. It would also allow us to get rid of the ugly is_vtable_shim boolean passed around.

Moved ty_fn_sig(_vtable) under Instance as Instance::fn_sig. Now is_vtable_shim is seen in only a few places.

eddyb · Sep 28, 2018

r? @michaelwoerister or @arielb1 on the instance/symbol changes.

michaelwoerister · Oct 1, 2018

@eddyb The changes to the symbol names and hashes look good to me.

arielb1 · Oct 1, 2018

arielb1 reviewed Oct 1, 2018

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arielb1 · Oct 1, 2018

src/librustc_codegen_llvm/declare.rs

-    let sig = common::ty_fn_sig(cx, fn_type);
+    debug!("declare_rust_fn(name={:?}, fn_type={:?}, is_vtable_shim={:?})",
+           name, fn_type, is_vtable_shim);
+    let sig = common::ty_fn_sig_vtable(cx, fn_type, is_vtable_shim);


Could is_vtable_shim become some sort of enum?

As a result of the ty_fn_sig refactoring, we now have only a reasonable amount of occurrences of is_vtable_shim.

arielb1 · Oct 1, 2018

arielb1 requested changes Oct 1, 2018

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arielb1 · Oct 1, 2018

src/librustc_codegen_llvm/common.rs

@@ -456,3 +456,22 @@ pub fn ty_fn_sig<'a, 'tcx>(cx: &CodegenCx<'a, 'tcx>,
        _ => bug!("unexpected type {:?} to ty_fn_sig", ty)
    }
 }
+
+pub fn ty_fn_sig_vtable<'a, 'tcx>(


This feels like it would fit in librustc as a function of instance. It would also allow us to get rid of the ugly is_vtable_shim boolean passed around.

michaelwoerister · Oct 4, 2018

r? @arielb1

bors · Oct 5, 2018

☔️ The latest upstream changes (presumably #54743) made this pull request unmergeable. Please resolve the merge conflicts.

bors · Oct 11, 2018

☔️ The latest upstream changes (presumably #54911) made this pull request unmergeable. Please resolve the merge conflicts.

bors · Oct 14, 2018

☔️ The latest upstream changes (presumably #55032) made this pull request unmergeable. Please resolve the merge conflicts.

TimNN · Oct 23, 2018

Ping from triage @arielb1 / @rust-lang/compiler: This PR requires your review.

oli-obk · Oct 23, 2018

oli-obk requested changes Oct 23, 2018

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oli-obk · Oct 23, 2018

src/test/compile-fail/unsized-locals/by-value-trait-object-safety.rs

@@ -0,0 +1,30 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT


remove the copyright header and move the test to src/test/ui

Sure, done.

oli-obk · Oct 23, 2018

src/test/run-pass-valgrind/unsized-locals/by-value-trait-objects-rust-call.rs

@@ -0,0 +1,65 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT


remove the copyright header here and in all other new tests

Sure, removed these headers for all unsized-locals tests including those from #51131.

oli-obk · Oct 23, 2018

src/test/run-pass/unsized-locals/by-value-trait-object-safety.rs

+
+
+fn main() {
+    let x = *(Box::new(A) as Box<dyn Foo>);


Are there preexisting tests for calling such a method directly on a Box<dyn Foo> without first dereferencing the box? If not, please add a ui test for that.

I've not tested that, and it turned out to compile! I added this as a run-pass test (just below here).

awesome!

The RFC isn't very clear on this. But I would see that as a natural consequence.

Can you add more tests (also failure tests ensuring that we can't use the box anymore after having called a method on it?)

Sure, added three tests.

oli-obk · Oct 27, 2018

oli-obk approved these changes Oct 27, 2018

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oli-obk · Oct 27, 2018

@bors r+

bors · Oct 27, 2018

📌 Commit 2f7ea4a has been approved by oli-obk

bors · Oct 27, 2018

⌛️ Testing commit 2f7ea4a with merge cae6efc...

bors · Oct 27, 2018

☀️ Test successful - status-appveyor, status-travis
Approved by: oli-obk
Pushing cae6efc to master...

rust-highfive assigned eddyb Sep 13, 2018

rust-highfive added the S-waiting-on-review label Sep 13, 2018

qnighy force-pushed the qnighy:by-value-object-safety branch 2 times, most recently from 7a494a3 to 74e4171 Sep 17, 2018

qnighy force-pushed the qnighy:by-value-object-safety branch from 74e4171 to 0d5e048 Sep 26, 2018

rust-highfive assigned michaelwoerister and unassigned eddyb Sep 28, 2018

rust-highfive assigned arielb1 and unassigned michaelwoerister Oct 4, 2018

qnighy force-pushed the qnighy:by-value-object-safety branch from 0d5e048 to 326e3c0 Oct 6, 2018

qnighy force-pushed the qnighy:by-value-object-safety branch from 326e3c0 to cd844f5 Oct 12, 2018

qnighy force-pushed the qnighy:by-value-object-safety branch from cd844f5 to 8e27255 Oct 15, 2018

Add InstanceDef::VtableShim.

0ad4c6f

qnighy force-pushed the qnighy:by-value-object-safety branch from 8e27255 to 1c48647 Oct 24, 2018

Add more tests on unsized locals autoderef and borrowck.

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bors added S-waiting-on-bors and removed S-waiting-on-review labels Oct 27, 2018

bors merged commit 2f7ea4a into rust-lang:master Oct 27, 2018
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Take 2: Implement object-safety and dynamic dispatch for arbitrary_self_types #54383

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Make `librustc_codegen_llvm` aware of LLVM address spaces. #51576

qnighy deleted the qnighy:by-value-object-safety branch Oct 27, 2018

qnighy referenced this pull request Oct 27, 2018
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Tracking issue for RFC #1909: Unsized Rvalues #48055
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Tracking issue for FnBox() #28796

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Fuchsia + lld page-faulting on FnOnce::call_once vtable shim #56089

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Teach miri about unsized locals #449

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qnighy added some commits Sep 11, 2018

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