Windows `-Zbuild-std` with `windows_raw_dylib` broken after `nightly-2026-08-27`

[Librazy]

Problem

Windows -Zbuild-std -Zbuild-std-features="windows_raw_dylib" broken after nightly-2026-08-27 while working on nightly-2026-08-26, even on an empty bin crate.

x86_64-pc-windows-msvc

x86_64-pc-windows-msvc target fails with

error[E0152]: duplicate lang item in crate `core`: `sized`
  |
  = note: the lang item is first defined in crate `core` (which `std` depends on)
  = note: first definition in `core` loaded from \\?\D:\testbuildstd\target\debug\build\core\addd3c1cd86f6f7c\out\libcore-addd3c1cd86f6f7c.rmeta
  = note: second definition in `core` loaded from C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\x86_64-pc-windows-msvc\lib\libcore-a4b1d4e620d2fe7b.rmeta

For more information about this error, try `rustc --explain E0152`.
error: could not compile `std` (lib) due to 1 previous error

x86_64-win7-windows-msvc

x86_64-win7-windows-msvc target fails with

error[E0463]: can't find crate for `panic_unwind`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `std` (lib) due to 1 previous error

Full verification

 D:\  Remove-Item -Recurse -Force .\testbuildstd\
 D:\  cargo init testbuildstd
    Creating binary (application) package
note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 D:\  cd .\testbuildstd\
 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling hashbrown v0.17.1
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 24.24s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling hashbrown v0.17.1
error[E0152]: duplicate lang item in crate `core`: `sized`
  |
  = note: the lang item is first defined in crate `core` (which `std` depends on)
  = note: first definition in `core` loaded from \\?\D:\testbuildstd\target\debug\build\core\addd3c1cd86f6f7c\out\libcore-addd3c1cd86f6f7c.rmeta
  = note: second definition in `core` loaded from C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\x86_64-pc-windows-msvc\lib\libcore-a4b1d4e620d2fe7b.rmeta

For more information about this error, try `rustc --explain E0152`.
error: could not compile `std` (lib) due to 1 previous error
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib" --target="x86_64-win7-windows-msvc"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling hashbrown v0.17.1
error[E0463]: can't find crate for `panic_unwind`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `std` (lib) due to 1 previous error
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std --target="x86_64-win7-windows-msvc"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling hashbrown v0.17.1
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.39s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-26 build -Zbuild-std
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling hashbrown v0.17.1
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 22.34s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-26 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling hashbrown v0.17.1
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.67s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-26 build -Zbuild-std --target="x86_64-win7-windows-msvc"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling hashbrown v0.17.1
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 20.76s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-26 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib" --target="x86_64-win7-windows-msvc"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling rustc-demangle v0.1.28
   Compiling cfg-if v1.0.4
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling hashbrown v0.17.1
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-26-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.51s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib,panic-unwind" --target="x86_64-win7-windows-msvc"
   Compiling compiler_builtins v0.1.160 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
   Compiling core v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
   Compiling std v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
   Compiling windows-link v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
   Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
   Compiling alloc v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
   Compiling unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
   Compiling cfg-if v1.0.4
   Compiling rustc-demangle v0.1.28
   Compiling rustc-literal-escaper v0.0.8
   Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
   Compiling panic_unwind v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
   Compiling hashbrown v0.17.1
   Compiling std_detect v0.1.5 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
   Compiling proc_macro v0.0.0 (C:\Users\Librazy\.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
   Compiling testbuildstd v0.1.0 (D:\testbuildstd)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.33s

Steps

0. Install nightly-2026-08-27 with rust-src on Windows
1. Create a bin crate on windows
2. Run cargo +nightly-2026-08-26 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib"

Possible Solution(s)

No response

Notes

No response

Version

 testbuildstd  cargo +nightly-2026-08-27 version --verbose
cargo 1.100.0-nightly (e8cb624d5 2026-08-22)
release: 1.100.0-nightly
commit-hash: e8cb624d5701824f46a2ec5873cfd59ee3d2f66c
commit-date: 2026-08-22
host: x86_64-pc-windows-msvc
libgit2: 1.9.6 (sys:0.21.0 vendored)
libcurl: 8.21.0-DEV (sys:0.4.90+curl-8.21.0 vendored ssl:Schannel)
os: Windows 10.0.26100 (Windows 11 IoTEnterpriseS) [64-bit]
 testbuildstd  cargo +nightly-2026-08-26 version --verbose
cargo 1.100.0-nightly (e8cb624d5 2026-08-22)
release: 1.100.0-nightly
commit-hash: e8cb624d5701824f46a2ec5873cfd59ee3d2f66c
commit-date: 2026-08-22
host: x86_64-pc-windows-msvc
libgit2: 1.9.6 (sys:0.21.0 vendored)
libcurl: 8.21.0-DEV (sys:0.4.90+curl-8.21.0 vendored ssl:Schannel)
os: Windows 10.0.26100 (Windows 11 IoTEnterpriseS) [64-bit]

[Librazy]

Ah, I figured it out! We need to add panic-unwind to build-std-features too.

Both
cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib,panic-unwind"
and
cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib,panic-unwind" --target="x86_64-win7-windows-msvc"
works.

Details

 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib,panic-unwind"
Compiling compiler_builtins v0.1.160 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
Compiling core v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
Compiling std v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
Compiling windows-link v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
Compiling alloc v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
Compiling unwind v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
Compiling rustc-demangle v0.1.28
Compiling cfg-if v1.0.4
Compiling rustc-literal-escaper v0.0.8
Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
Compiling panic_unwind v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
Compiling panic_abort v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
Compiling hashbrown v0.17.1
Compiling std_detect v0.1.5 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
Compiling proc_macro v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
Compiling testbuildstd v0.1.0 (D:\testbuildstd)
Finished dev profile [unoptimized + debuginfo] target(s) in 18.88s
 testbuildstd  Remove-Item -Recurse -Force ./target
 testbuildstd  cargo +nightly-2026-08-27 build -Zbuild-std -Zbuild-std-features="windows_raw_dylib,panic-unwind" --target="x86_64-win7-windows-msvc"
Compiling compiler_builtins v0.1.160 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\compiler-builtins\compiler-builtins)
Compiling core v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core)
Compiling std v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std)
Compiling windows-link v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\windows_link)
Compiling rustc-std-workspace-core v1.99.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-core)
Compiling alloc v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc)
Compiling unwind v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\unwind)
Compiling rustc-demangle v0.1.28
Compiling cfg-if v1.0.4
Compiling rustc-literal-escaper v0.0.8
Compiling rustc-std-workspace-alloc v1.99.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\rustc-std-workspace-alloc)
Compiling panic_abort v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_abort)
Compiling panic_unwind v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\panic_unwind)
Compiling hashbrown v0.17.1
Compiling std_detect v0.1.5 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\std_detect)
Compiling proc_macro v0.0.0 (C:\Users\Librazy.rustup\toolchains\nightly-2026-08-27-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\proc_macro)
Compiling testbuildstd v0.1.0 (D:\testbuildstd)
Finished dev profile [unoptimized + debuginfo] target(s) in 20.01s

Since I'm not sure if this is an intended behavioral change, I'll keep this issue open for now.

[ChrisDenton]

Hm, sounds related to panic runtimes. cc @Bryanskiy @bjorn3 I figure rust-lang/rust#160007 is related. Was this intentional?

[Bryanskiy]

Was this intentional?

No, I'll take a look

[bjorn3]

I think specifying -Zbuild-std-features at all disables the default panic-unwind feature. I don't know why it worked before, but I think cargo should enable the panic-unwind feature for -Zbuild-std=std + panic=unwind.

[Bryanskiy]

I think specifying -Zbuild-std-features at all disables the default panic-unwind feature.

That's right:

cargo/src/compiler/standard_lib.rs

Lines 82 to 89 in 19e7b54

	let features = match &gctx.cli_unstable().build_std_features {
	Some(list) => list.clone(),
	None => vec![
	"panic-unwind".to_string(),
	"backtrace".to_string(),
	"default".to_string(),
	],
	};

I don't know why it worked before

panic_unwind is built regardless of the panic-unwind feature

cargo/src/compiler/standard_lib.rs

Line 30 in 19e7b54

crates.insert("panic_unwind");

and is passed with --extern to the final artifact.

Previously, the panic runtime was injected by rustc as a direct dependency, which is why it worked. With rust-lang/rust#160007 the panic runtime is now injected when building std, but Cargo does not pass --extern because the panic-unwind feature is missing. As a result, the compiler loads panic_unwind from the sysroot, which also brings in core as a dependency. This leads to two separate core crates: one from the target directory and one from the sysroot.

 but I think cargo should enable the panic-unwind feature for -Zbuild-std=std + panic=unwind.

This is implemented in #17185 and should fix the problem. But I need to update it.