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Rollup merge of #136895 - maurer:fix-enum-discr, r=nikic

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debuginfo: Set bitwidth appropriately in enum variant tags

Previously, we unconditionally set the bitwidth to 128-bits, the largest an enum would possibly be. Then, LLVM would cut down the constant by chopping off leading zeroes before emitting the DWARF. LLVM only supported 64-bit enumerators, so this would also have occasionally resulted in truncated data.

LLVM added support for 128-bit enumerators in llvm/llvm-project#125578

That patchset trusts the constant to describe how wide the variant tag is, so the high 64-bits of zeros are considered potentially load-bearing.

As a result, we went from emitting tags that looked like:
DW_AT_discr_value     (0xfe)

(because `dwarf::BestForm` selected `data1`)

to emitting tags that looked like:
DW_AT_discr_value	(<0x10> fe ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00 )

This makes the `DW_AT_discr_value` encode at the bitwidth of the tag, which:
1. Is probably closer to our intentions in terms of describing the data.
2. Doesn't invoke the 128-bit support which may not be supported by all debuggers / downstream tools.
3. Will result in smaller debug information.
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Jubilee 2025-02-13 17:46:08 -08:00 • committed by GitHub
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4
tests/codegen/enum/enum-debug-niche-2.rs
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@ -5,8 +5,8 @@
//@ ignore-msvc
//
// CHECK: {{.*}}DICompositeType{{.*}}tag: DW_TAG_variant_part,{{.*}}size: 32,{{.*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Placeholder",{{.*}}extraData: i128 4294967295{{[,)].*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Error",{{.*}}extraData: i128 0{{[,)].*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Placeholder",{{.*}}extraData: i32 -1{{[,)].*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Error",{{.*}}extraData: i32 0{{[,)].*}}
#![feature(never_type)]
#[derive(Copy, Clone)]