Fix ICE on const eval of union field
MIR's `Const::get_field()` attempts to retrieve the value for a given field in a constant. In the case of a union constant it was falling through to a generic `const_get_elt` based on the field index. As union fields don't have an index this caused an ICE in `llvm_field_index`. Fix by simply returning the current value when accessing any field in a union. This works because all union fields start at byte offset 0. The added test uses `const_fn` it ensure the field is extracted using MIR's const evaluation. The crash is reproducible without it, however. Fixes #47788
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@ -140,7 +140,10 @@ impl<'a, 'tcx> Const<'tcx> {
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}
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}
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_ => {
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const_get_elt(self.llval, layout.llvm_field_index(i))
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match layout.fields {
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layout::FieldPlacement::Union(_) => self.llval,
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_ => const_get_elt(self.llval, layout.llvm_field_index(i)),
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}
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}
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}
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}
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@ -0,0 +1,26 @@
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// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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#![feature(const_fn)]
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union DummyUnion {
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field1: i32,
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field2: i32,
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}
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const fn read_field() -> i32 {
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const UNION: DummyUnion = DummyUnion { field1: 5 };
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const FIELD: i32 = unsafe { UNION.field2 };
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FIELD
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}
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fn main() {
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assert_eq!(read_field(), 5);
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}
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