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Fix regression in promotion of rvalues referencing a static #44442

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Sep 11, 2017
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23 changes: 22 additions & 1 deletion src/librustc_passes/consts.rs
Original file line number Diff line number Diff line change
Expand Up @@ -327,7 +327,28 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>, e: &hir::Expr, node
let def = v.tables.qpath_def(qpath, e.hir_id);
match def {
Def::VariantCtor(..) | Def::StructCtor(..) |
Def::Fn(..) | Def::Method(..) => {}
Def::Fn(..) | Def::Method(..) => {}

// References to a static are inherently promotable,
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Comment doesn't match code.

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References to statics outside of a static aren't promotable.

// with the exception of "#[thread_loca]" statics.
// The latter may not outlive the current function
Def::Static(did, _) => {
let mut thread_local = false;

for attr in &v.tcx.get_attrs(did)[..] {
if attr.check_name("thread_local") {
debug!("Static(id={:?}) is unpromotable \
due to a #[thread_local] attribute", did);
v.promotable = false;
thread_local = true;
break;
}
}

if !thread_local {
debug!("Allowing promotion of reference to Static(id={:?})", did);
}
}
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Hmm, this will allow promoting reads from statics, which is bad. Can you make sure to only promote if the mode is Static? i.e. only a static can refer to another static.

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@eddyb eddyb Sep 9, 2017

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Actually, wait, this is old borrowck, how is this even a problem at all? I need to investigate further. Realized [&TEST] is what doesn't get promoted in your original testcase, so this makes sense, but, again, it should be limited to statics referring to other statics.
Hopefully the MIR constant checking will prevent edge cases from compiling.


Def::Const(did) |
Def::AssociatedConst(did) => {
Expand Down
18 changes: 18 additions & 0 deletions src/test/run-pass/issue-44373.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

struct Foo(bool);

struct Container(&'static [&'static Foo]);

static FOO: Foo = Foo(true);
static CONTAINER: Container = Container(&[&FOO]);

fn main() {}