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Implement by-value object safety #54183

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merged 19 commits into from Oct 27, 2018

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@qnighy
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qnighy commented 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

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TimNN commented Sep 25, 2018

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

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

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@eddyb

eddyb Sep 26, 2018

Member

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

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@qnighy

qnighy Sep 26, 2018

Contributor

Changed to Operand::Move.

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

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@eddyb

eddyb Sep 26, 2018

Member

Is this needed?

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@qnighy

qnighy Sep 26, 2018

Contributor

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

@@ -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);

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@eddyb

eddyb Sep 26, 2018

Member

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

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@qnighy

qnighy Sep 26, 2018

Contributor

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.

@@ -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)

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@eddyb

eddyb Sep 26, 2018

Member

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

@@ -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>(

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@eddyb

eddyb Sep 26, 2018

Member

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.

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@qnighy

qnighy Sep 26, 2018

Contributor

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

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@arielb1

arielb1 Oct 1, 2018

Contributor

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.

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@qnighy

qnighy Oct 6, 2018

Contributor

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

@eddyb

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eddyb commented Sep 28, 2018

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

@michaelwoerister

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michaelwoerister commented Oct 1, 2018

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

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);

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@arielb1

arielb1 Oct 1, 2018

Contributor

Could is_vtable_shim become some sort of enum?

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@qnighy

qnighy Oct 6, 2018

Contributor

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

@@ -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>(

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@arielb1

arielb1 Oct 1, 2018

Contributor

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

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michaelwoerister commented Oct 4, 2018

@bors

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bors commented Oct 5, 2018

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

@bors

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bors commented Oct 11, 2018

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

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bors commented Oct 14, 2018

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

@TimNN

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TimNN commented Oct 23, 2018

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

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

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@oli-obk

oli-obk Oct 23, 2018

Contributor

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

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@qnighy

qnighy Oct 24, 2018

Contributor

Sure, done.

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

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@oli-obk

oli-obk Oct 23, 2018

Contributor

remove the copyright header here and in all other new tests

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@qnighy

qnighy Oct 24, 2018

Contributor

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

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

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@oli-obk

oli-obk Oct 23, 2018

Contributor

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.

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@qnighy

qnighy Oct 24, 2018

Contributor

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

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@oli-obk

oli-obk Oct 24, 2018

Contributor

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?)

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@qnighy

qnighy Oct 27, 2018

Contributor

Sure, added three tests.

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@oli-obk

oli-obk Oct 27, 2018

Contributor

Thanks!

@oli-obk

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oli-obk commented Oct 27, 2018

@bors r+

@bors

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bors commented Oct 27, 2018

📌 Commit 2f7ea4a has been approved by oli-obk

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bors commented Oct 27, 2018

⌛️ Testing commit 2f7ea4a with merge cae6efc...

bors added a commit that referenced this pull request Oct 27, 2018

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
@bors

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bors commented Oct 27, 2018

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

@bors bors merged commit 2f7ea4a into rust-lang:master Oct 27, 2018

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

@qnighy qnighy referenced this pull request Oct 27, 2018

Open

Tracking issue for RFC #1909: Unsized Rvalues #48055

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