x.py build --stage 1 library/std --target hexagon-unknown-qurt fails: libunwind requires link.h; std then fails due to missing libc/platform items

Summary

Building library/std for hexagon-unknown-qurt from the current rust-lang/rust master (using a custom bootstrap.toml for Hexagon/QURT toolchain) fails. I hit a sequence of failures (libunwind headers, then libc target support), and even after switching to a local libc fork + allowing warnings, std still fails with many missing libc symbols/types and missing platform modules.

Steps to reproduce (Command used)

1. Clone Rust:

   git clone https://github.com/rust-lang/rust.git
   cd rust
   

   • rust git rev-parse HEAD: 7704328ba5ae8d6ce0ac303c9d5a1a1605906766

2. Create bootstrap.toml (example below) and point it at the Hexagon toolchain.

3. Clean and build std for Hexagon/QURT:

   ./x.py clean
   ./x.py build --stage 1 library/std --target hexagon-unknown-qurt
   

Expected behaviour

./x.py build --stage 1 library/std --target hexagon-unknown-qurt succeeds (or fails with a clear “target unsupported” message if this target is not expected to build std).

Actual behaviour

1. Baseline: build from current Rust git + recommended-ish bootstrap.toml
   Command:

   ./x.py build --stage 1 library/std --target hexagon-unknown-qurt
   

   Failure (libunwind build step) (terminal_output_ver1.txt):

   • While building libunwind.a for hexagon-unknown-qurt, compilation fails because link.h is missing:

   .../libunwind/src/AddressSpace.hpp: fatal error: 'link.h' file not found
   

2. Tried disabling libunwind
   Change:

   • Set llvm-libunwind = "no" and backtrace = false.

   Result terminal_output_ver2.txt:

   • This allowed progress past the libunwind failure, but the build later failed in libc/std compilation:
     • error[E0432]: unresolved import 'unistd' (coming from libc internals for the target).

3. Switched to local libc (fork/path override)
   Attempted:

   • cargo update -p libc (updated libc version in the lockfile, but did not resolve the target issue).
   • Switched to a local libc checkout/fork (path override) based on rust-lang/libc with QURT/hexagon adjustments.
   • clib git rev-parse HEAD: 4b38c69e31afadbead63af49794850b5a41f7dc4

   Result terminal_output_ver3.txt:

   • Build aborted due to “warnings are denied …” (warnings treated as errors).

4. Tried allowing warnings
   Attempt:

   • Added deny-warnings = false in bootstrap.toml.

   Result terminal_output_ver4.txt:

   • The build moved forward using the local libc, but then generated many libc-related issues:

     • lots of missing libc functions/constants/types (open, pread, pwrite, fcntl, O_*, socket constants, signals, etc.).
     • and additional errors where libc items are present but not publicly exported as expected (private type aliases/modules).

   • In addition, std fails with missing platform modules for unix paths (e.g. cannot find platform::fs / platform::raw).

Bootstrap configuration (bootstrap.toml)

[build]
build-stage = 1
host = ["x86_64-unknown-linux-gnu"]
target = ["x86_64-unknown-linux-gnu", "hexagon-unknown-qurt"]

[target.hexagon-unknown-qurt]
cc = "/home/user/Projects/qualcomm/5.5.2.0/tools/HEXAGON_Tools/8.7.06/Tools/bin/hexagon-clang"
cxx = "/home/user/Projects/qualcomm/5.5.2.0/tools/HEXAGON_Tools/8.7.06/Tools/bin/hexagon-clang++"
ar = "llvm-ar"
ranlib = "llvm-ranlib"
llvm-libunwind = 'in-tree'

Also tried:

[rust]
backtrace = false

[target.hexagon-unknown-qurt]
llvm-libunwind = "no"

And:

[rust]
deny-warnings = false

Operating system

• Host: x86_64-unknown-linux-gnu (Linux VM)
• Target: hexagon-unknown-qurt
• Hexagon toolchain: hexagon-clang/clang++ from Qualcomm HEXAGON Tools (8.7.06) (paths in bootstrap.toml)
• Build command:

./x.py build --stage 1 library/std --target hexagon-unknown-qurt

HEAD

rust git rev-parse HEAD: 7704328ba5ae8d6ce0ac303c9d5a1a1605906766
clib git rev-parse HEAD: 4b38c69e31afadbead63af49794850b5a41f7dc4

Additional context

• Is hexagon-unknown-qurt expected to support building std in-tree?
  • It seems like it suggests/targets std support based on:
    • rust-lang/rust target support PR: https://github.com/rust-lang/rust/pull/147173.
    • rust-lang/libc QuRT support commit: https://github.com/rust-lang/libc/commit/8ecae6cae62df6bf8123b87f60b964a554af1863.
    • target documentation (mentions std): https://doc.rust-lang.org/nightly/rustc/platform-support/hexagon-unknown-qurt.html.
• If yes: what is the intended libc + unwind setup for this target (in-tree libunwind vs SDK/unwindless, libc source/version/branch)?
• If no: is there an official stance / tracking issue for core/alloc-only vs std support for this target?

Start with the hexagon-unknown-qurt platform-support documentation and reproduce ./x.py build --stage 1 library/std --target hexagon-unknown-qurt using the reported bootstrap.toml. Trace the libunwind and libc failures, including missing platform modules in std, and compare them with the referenced Rust target-support and libc changes. Done means documenting the supported std setup or producing a clear unsupported-target result.