Add arm64ec-pc-windows-msvc target

Introduces the `arm64ec-pc-windows-msvc` target for building Arm64EC ("Emulation Compatible") binaries for Windows.

For more information about Arm64EC see <https://learn.microsoft.com/en-us/windows/arm/arm64ec>.

Tier 3 policy:

> A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

I will be the maintainer for this target.

> Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.

Target uses the `arm64ec` architecture to match LLVM and MSVC, and the `-pc-windows-msvc` suffix to indicate that it targets Windows via the MSVC environment.

> Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.

Target name exactly specifies the type of code that will be produced.

> If possible, use only letters, numbers, dashes and underscores for the name. Periods (.) are known to cause issues in Cargo.

Done.

> Tier 3 targets may have unusual requirements to build or use, but must not create legal issues or impose onerous legal terms for the Rust project or for Rust developers or users.

> The target must not introduce license incompatibilities.

Uses the same dependencies, requirements and licensing as the other `*-pc-windows-msvc` targets.

> Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).

Understood.

> The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.

> Compiling, linking, and emitting functional binaries, libraries, or other code for the target (whether hosted on the target itself or cross-compiling from another target) must not depend on proprietary (non-FOSS) libraries. Host tools built for the target itself may depend on the ordinary runtime libraries supplied by the platform and commonly used by other applications built for the target, but those libraries must not be required for code generation for the target; cross-compilation to the target must not require such libraries at all. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.

> "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

Uses the same dependencies, requirements and licensing as the other `*-pc-windows-msvc` targets.

> Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.

> This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.

Understood, I am not a member of the Rust team.

> Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.

Both `core` and `alloc` are supported.

Support for `std` dependends on making changes to the standard library, `stdarch` and `backtrace` which cannot be done yet as the bootstrapping compiler raises a warning ("unexpected `cfg` condition value") for `target_arch = "arm64ec"`.

> The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.

Documentation is provided in src/doc/rustc/src/platform-support/arm64ec-pc-windows-msvc.md

> Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.

> Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.

> Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target.

> In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

Understood.

Author: dpaoliello

Cargo.lock

@@ -469,9 +469,9 @@ version = "0.1.0"
 
 [[package]]
 name = "cc"
-version = "1.0.79"
+version = "1.0.90"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "50d30906286121d95be3d479533b458f87493b30a4b5f79a607db8f5d11aa91f"
+checksum = "8cd6604a82acf3039f1144f54b8eb34e91ffba622051189e71b781822d5ee1f5"
 
 [[package]]
 name = "cfg-if"

compiler/rustc_codegen_llvm/src/back/archive.rs

@@ -55,6 +55,7 @@ fn llvm_machine_type(cpu: &str) -> LLVMMachineType {
         "x86_64" => LLVMMachineType::AMD64,
         "x86" => LLVMMachineType::I386,
         "aarch64" => LLVMMachineType::ARM64,
+        "arm64ec" => LLVMMachineType::ARM64EC,
         "arm" => LLVMMachineType::ARM,
         _ => panic!("unsupported cpu type {cpu}"),
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -56,6 +56,7 @@ pub enum LLVMMachineType {
     AMD64 = 0x8664,
     I386 = 0x14c,
     ARM64 = 0xaa64,
+    ARM64EC = 0xa641,
     ARM = 0x01c0,
 }
 

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -201,7 +201,13 @@ impl<'a> IntoIterator for LLVMFeature<'a> {
 // which might lead to failures if the oldest tested / supported LLVM version
 // doesn't yet support the relevant intrinsics
 pub fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a> {
-    let arch = if sess.target.arch == "x86_64" { "x86" } else { &*sess.target.arch };
+    let arch = if sess.target.arch == "x86_64" {
+        "x86"
+    } else if sess.target.arch == "arm64ec" {
+        "aarch64"
+    } else {
+        &*sess.target.arch
+    };
     match (arch, s) {
         ("x86", "sse4.2") => {
             LLVMFeature::with_dependency("sse4.2", TargetFeatureFoldStrength::EnableOnly("crc32"))

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -288,7 +288,7 @@ pub(super) fn emit_va_arg<'ll, 'tcx>(
         // Generic x86
         "x86" => emit_ptr_va_arg(bx, addr, target_ty, false, Align::from_bytes(4).unwrap(), true),
         // Windows AArch64
-        "aarch64" if target.is_like_windows => {
+        "aarch64" | "arm64ec" if target.is_like_windows => {
             emit_ptr_va_arg(bx, addr, target_ty, false, Align::from_bytes(8).unwrap(), false)
         }
         // macOS / iOS AArch64

compiler/rustc_codegen_ssa/Cargo.toml

@@ -7,7 +7,7 @@ edition = "2021"
 # tidy-alphabetical-start
 ar_archive_writer = "0.1.5"
 bitflags = "2.4.1"
-cc = "1.0.69"
+cc = "1.0.90"
 itertools = "0.11"
 jobserver = "0.1.28"
 pathdiff = "0.2.0"

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -8,7 +8,7 @@ use std::path::Path;
 use object::write::{self, StandardSegment, Symbol, SymbolSection};
 use object::{
     elf, pe, xcoff, Architecture, BinaryFormat, Endianness, FileFlags, Object, ObjectSection,
-    ObjectSymbol, SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope,
+    ObjectSymbol, SectionFlags, SectionKind, SubArchitecture, SymbolFlags, SymbolKind, SymbolScope,
 };
 
 use rustc_data_structures::memmap::Mmap;
@@ -182,37 +182,40 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
         Endian::Little => Endianness::Little,
         Endian::Big => Endianness::Big,
     };
-    let architecture = match &sess.target.arch[..] {
-        "arm" => Architecture::Arm,
-        "aarch64" => {
+    let (architecture, sub_architecture) = match &sess.target.arch[..] {
+        "arm" => (Architecture::Arm, None),
+        "aarch64" => (
             if sess.target.pointer_width == 32 {
                 Architecture::Aarch64_Ilp32
             } else {
                 Architecture::Aarch64
-            }
-        }
-        "x86" => Architecture::I386,
-        "s390x" => Architecture::S390x,
-        "mips" | "mips32r6" => Architecture::Mips,
-        "mips64" | "mips64r6" => Architecture::Mips64,
-        "x86_64" => {
+            },
+            None,
+        ),
+        "x86" => (Architecture::I386, None),
+        "s390x" => (Architecture::S390x, None),
+        "mips" | "mips32r6" => (Architecture::Mips, None),
+        "mips64" | "mips64r6" => (Architecture::Mips64, None),
+        "x86_64" => (
             if sess.target.pointer_width == 32 {
                 Architecture::X86_64_X32
             } else {
                 Architecture::X86_64
-            }
-        }
-        "powerpc" => Architecture::PowerPc,
-        "powerpc64" => Architecture::PowerPc64,
-        "riscv32" => Architecture::Riscv32,
-        "riscv64" => Architecture::Riscv64,
-        "sparc64" => Architecture::Sparc64,
-        "avr" => Architecture::Avr,
-        "msp430" => Architecture::Msp430,
-        "hexagon" => Architecture::Hexagon,
-        "bpf" => Architecture::Bpf,
-        "loongarch64" => Architecture::LoongArch64,
-        "csky" => Architecture::Csky,
+            },
+            None,
+        ),
+        "powerpc" => (Architecture::PowerPc, None),
+        "powerpc64" => (Architecture::PowerPc64, None),
+        "riscv32" => (Architecture::Riscv32, None),
+        "riscv64" => (Architecture::Riscv64, None),
+        "sparc64" => (Architecture::Sparc64, None),
+        "avr" => (Architecture::Avr, None),
+        "msp430" => (Architecture::Msp430, None),
+        "hexagon" => (Architecture::Hexagon, None),
+        "bpf" => (Architecture::Bpf, None),
+        "loongarch64" => (Architecture::LoongArch64, None),
+        "csky" => (Architecture::Csky, None),
+        "arm64ec" => (Architecture::Aarch64, Some(SubArchitecture::Arm64EC)),
         // Unsupported architecture.
         _ => return None,
     };
@@ -227,6 +230,7 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
     };
 
     let mut file = write::Object::new(binary_format, architecture, endianness);
+    file.set_sub_architecture(sub_architecture);
     if sess.target.is_like_osx {
         if macho_is_arm64e(&sess.target) {
             file.set_macho_cpu_subtype(object::macho::CPU_SUBTYPE_ARM64E);

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -567,9 +567,10 @@ pub fn linking_symbol_name_for_instance_in_crate<'tcx>(
         return undecorated;
     }
 
-    let x86 = match &target.arch[..] {
-        "x86" => true,
-        "x86_64" => false,
+    let prefix = match &target.arch[..] {
+        "x86" => Some('_'),
+        "x86_64" => None,
+        "arm64ec" => Some('#'),
         // Only x86/64 use symbol decorations.
         _ => return undecorated,
     };
@@ -606,8 +607,8 @@ pub fn linking_symbol_name_for_instance_in_crate<'tcx>(
         Conv::X86Stdcall => ("_", "@"),
         Conv::X86VectorCall => ("", "@@"),
         _ => {
-            if x86 {
-                undecorated.insert(0, '_');
+            if let Some(prefix) = prefix {
+                undecorated.insert(0, prefix);
             }
             return undecorated;
         }

compiler/rustc_codegen_ssa/src/base.rs

@@ -907,7 +907,11 @@ impl CrateInfo {
                     lang_items::required(tcx, l).then_some(name)
                 })
                 .collect();
-            let prefix = if target.is_like_windows && target.arch == "x86" { "_" } else { "" };
+            let prefix = match (target.is_like_windows, target.arch.as_ref()) {
+                (true, "x86") => "_",
+                (true, "arm64ec") => "#",
+                _ => "",
+            };
 
             // This loop only adds new items to values of the hash map, so the order in which we
             // iterate over the values is not important.

compiler/rustc_llvm/Cargo.toml

@@ -8,12 +8,7 @@ edition = "2021"
 libc = "0.2.73"
 # tidy-alphabetical-end
 
-# FIXME: updating cc past 1.0.79 breaks libstd bootstrapping, pin
-# to the last working version here so `cargo update` doesn't cause the
-# a higher version to be selected
-# https://github.com/rust-lang/cc-rs/issues/913
-# 1.0.{84, 85} fix this but have been yanked
 [build-dependencies]
 # tidy-alphabetical-start
-cc = "=1.0.79"
+cc = "1.0.90"
 # tidy-alphabetical-end

compiler/rustc_target/src/abi/call/mod.rs

@@ -814,7 +814,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
                     }
                 }
             },
-            "aarch64" => {
+            "aarch64" | "arm64ec" => {
                 let kind = if cx.target_spec().is_like_osx {
                     aarch64::AbiKind::DarwinPCS
                 } else if cx.target_spec().is_like_windows {

compiler/rustc_target/src/spec/mod.rs

@@ -1564,6 +1564,7 @@ supported_targets! {
 
     ("aarch64-pc-windows-msvc", aarch64_pc_windows_msvc),
     ("aarch64-uwp-windows-msvc", aarch64_uwp_windows_msvc),
+    ("arm64ec-pc-windows-msvc", arm64ec_pc_windows_msvc),
     ("x86_64-pc-windows-msvc", x86_64_pc_windows_msvc),
     ("x86_64-uwp-windows-msvc", x86_64_uwp_windows_msvc),
     ("x86_64-win7-windows-msvc", x86_64_win7_windows_msvc),

compiler/rustc_target/src/spec/targets/arm64ec_pc_windows_msvc.rs

@@ -0,0 +1,21 @@
+use crate::spec::{add_link_args, base, LinkerFlavor, Lld, Target};
+
+pub fn target() -> Target {
+    let mut base = base::windows_msvc::opts();
+    base.max_atomic_width = Some(128);
+    base.features = "+v8a,+neon,+fp-armv8".into();
+    add_link_args(
+        &mut base.late_link_args,
+        LinkerFlavor::Msvc(Lld::No),
+        &["/machine:arm64ec", "softintrin.lib"],
+    );
+
+    Target {
+        llvm_target: "arm64ec-pc-windows-msvc".into(),
+        description: None,
+        pointer_width: 64,
+        data_layout: "e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128".into(),
+        arch: "arm64ec".into(),
+        options: base,
+    }
+}

compiler/rustc_target/src/target_features.rs

@@ -409,7 +409,7 @@ impl super::spec::Target {
     pub fn supported_target_features(&self) -> &'static [(&'static str, Stability)] {
         match &*self.arch {
             "arm" => ARM_ALLOWED_FEATURES,
-            "aarch64" => AARCH64_ALLOWED_FEATURES,
+            "aarch64" | "arm64ec" => AARCH64_ALLOWED_FEATURES,
             "x86" | "x86_64" => X86_ALLOWED_FEATURES,
             "hexagon" => HEXAGON_ALLOWED_FEATURES,
             "mips" | "mips32r6" | "mips64" | "mips64r6" => MIPS_ALLOWED_FEATURES,
@@ -425,7 +425,7 @@ impl super::spec::Target {
 
     pub fn tied_target_features(&self) -> &'static [&'static [&'static str]] {
         match &*self.arch {
-            "aarch64" => AARCH64_TIED_FEATURES,
+            "aarch64" | "arm64ec" => AARCH64_TIED_FEATURES,
             _ => &[],
         }
     }

src/bootstrap/src/lib.rs

@@ -91,7 +91,7 @@ const EXTRA_CHECK_CFGS: &[(Option<Mode>, &str, Option<&[&'static str]>)] = &[
     /* Extra values not defined in the built-in targets yet, but used in std */
     (Some(Mode::Std), "target_env", Some(&["libnx", "p2"])),
     // (Some(Mode::Std), "target_os", Some(&[])),
-    (Some(Mode::Std), "target_arch", Some(&["spirv", "nvptx", "xtensa"])),
+    (Some(Mode::Std), "target_arch", Some(&["arm64ec", "spirv", "nvptx", "xtensa"])),
     /* Extra names used by dependencies */
     // FIXME: Used by serde_json, but we should not be triggering on external dependencies.
     (Some(Mode::Rustc), "no_btreemap_remove_entry", None),

src/doc/rustc/src/SUMMARY.md

@@ -18,6 +18,7 @@
     - [arm64e-apple-ios.md](platform-support/arm64e-apple-ios.md)
     - [arm64e-apple-darwin.md](platform-support/arm64e-apple-darwin.md)
     - [aarch64-apple-ios-sim](platform-support/aarch64-apple-ios-sim.md)
+    - [arm64ec-pc-windows-msvc](platform-support/arm64ec-pc-windows-msvc.md)
     - [\*-apple-tvos](platform-support/apple-tvos.md)
     - [\*-apple-watchos\*](platform-support/apple-watchos.md)
     - [aarch64-nintendo-switch-freestanding](platform-support/aarch64-nintendo-switch-freestanding.md)

src/doc/rustc/src/platform-support.md

@@ -241,6 +241,7 @@ target | std | host | notes
 -------|:---:|:----:|-------
 [`arm64e-apple-ios`](platform-support/arm64e-apple-ios.md) | ✓ | | ARM64e Apple iOS
 [`arm64e-apple-darwin`](platform-support/arm64e-apple-darwin.md)  | ✓ | ✓ | ARM64e Apple Darwin
+[`arm64ec-pc-windows-msvc`](platform-support/arm64ec-pc-windows-msvc.md) | ? | | Arm64EC Windows MSVC
 `aarch64-apple-ios-macabi` | ? |  | Apple Catalyst on ARM64
 [`aarch64-apple-tvos`](platform-support/apple-tvos.md) | ? |  | ARM64 tvOS
 [`aarch64-apple-tvos-sim`](platform-support/apple-tvos.md) | ? |  | ARM64 tvOS Simulator

src/doc/rustc/src/platform-support/arm64ec-pc-windows-msvc.md

@@ -0,0 +1,69 @@
+# `arm64ec-pc-windows-msvc`
+
+**Tier: 3**
+
+Arm64EC ("Emulation Compatible") for mixed architecture (AArch64 and x86_64)
+applications on AArch64 Windows 11. See <https://learn.microsoft.com/en-us/windows/arm/arm64ec>.
+
+## Target maintainers
+
+- [@dpaoliello](https://github.com/dpaoliello)
+
+## Requirements
+
+Target only supports cross-compilation, `core` and `alloc` are supported but
+`std` is not.
+
+Builds Arm64EC static and dynamic libraries and executables which can be run on
+AArch64 Windows 11 devices. Arm64EC static libraries can also be linked into
+Arm64X dynamic libraries and executables.
+
+Uses `arm64ec` as its `target_arch` - code built for Arm64EC must be compatible
+with x86_64 code (e.g., same structure layouts, function signatures, etc.) but
+use AArch64 intrinsics.
+
+Only supported backend is LLVM 18 (or above).
+
+## Building the target
+
+You can build Rust with support for the targets by adding it to the `target`
+list in `config.toml` and disabling `std`:
+
+```toml
+[build]
+target = [ "arm64ec-pc-windows-msvc" ]
+
+[target.arm64ec-pc-windows-msvc]
+no-std = true
+```
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will either need to build Rust with the target enabled (see
+"Building the target" above), or build your own copy of `core` by using
+`build-std` or similar.
+
+## Testing
+
+Tests can be run on AArch64 Windows 11 devices.
+
+Since this is a `no_std` target, the Rust test suite is not supported.
+
+## Cross-compilation toolchains and C code
+
+C code can be built using the Arm64-targetting MSVC toolchain.
+
+To compile:
+
+```bash
+cl /arm64EC /c ...
+```
+
+To link:
+
+```bash
+link /MACHINE:ARM64EC ...
+```
+
+Further reading: <https://learn.microsoft.com/en-us/windows/arm/arm64ec-build>

tests/assembly/targets/targets-pe.rs

@@ -12,6 +12,9 @@
 //@ revisions: aarch64_uwp_windows_msvc
 //@ [aarch64_uwp_windows_msvc] compile-flags: --target aarch64-uwp-windows-msvc
 //@ [aarch64_uwp_windows_msvc] needs-llvm-components: aarch64
+//@ revisions: arm64ec_pc_windows_msvc
+//@ [arm64ec_pc_windows_msvc] compile-flags: --target arm64ec-pc-windows-msvc
+//@ [arm64ec_pc_windows_msvc] needs-llvm-components: aarch64
 //@ revisions: avr_unknown_gnu_atmega328
 //@ [avr_unknown_gnu_atmega328] compile-flags: --target avr-unknown-gnu-atmega328
 //@ [avr_unknown_gnu_atmega328] needs-llvm-components: avr

tests/ui/check-cfg/well-known-values.stderr

@@ -118,7 +118,7 @@ warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
 LL |     target_arch = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_arch` are: `aarch64`, `arm`, `avr`, `bpf`, `csky`, `hexagon`, `loongarch64`, `m68k`, `mips`, `mips32r6`, `mips64`, `mips64r6`, `msp430`, `nvptx64`, `powerpc`, `powerpc64`, `riscv32`, `riscv64`, `s390x`, `sparc`, `sparc64`, `wasm32`, `wasm64`, `x86`, `x86_64`
+   = note: expected values for `target_arch` are: `aarch64`, `arm`, `arm64ec`, `avr`, `bpf`, `csky`, `hexagon`, `loongarch64`, `m68k`, `mips`, `mips32r6`, `mips64`, `mips64r6`, `msp430`, `nvptx64`, `powerpc`, `powerpc64`, `riscv32`, `riscv64`, `s390x`, `sparc`, `sparc64`, `wasm32`, `wasm64`, `x86`, `x86_64`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`