Auto merge of #57937 - denzp:nvptx, r=nagisa

NVPTX target specification

This change adds a built-in `nvptx64-nvidia-cuda` GPGPU no-std target specification and a basic PTX assembly smoke tests.

The approach is taken here and the target spec is based on `ptx-linker`, a project started about 1.5 years ago. Key feature: bitcode object files being linked with LTO into the final module on the linker's side.

Prior to this change, the linker used a `ld` linker-flavor, but I think, having the special CLI convention is a more reliable way.

Questions about further progress on reliable CUDA workflow with Rust:
1. Is it possible to create a test suite `codegen-asm` to verify end-to-end integration with LLVM backend?
1. How would it be better to organise no-std `compile-fail` tests: add `#![no_std]` where possible and mark others as `ignore-nvptx` directive, or alternatively, introduce `compile-fail-no-std` test suite?
1. Can we have the `ptx-linker` eventually be integrated as `rls` or `clippy`? Hopefully, this should allow to statically link against LLVM used in Rust and get rid of the [current hacky solution](https://github.com/denzp/rustc-llvm-proxy).
1. Am I missing some methods from `rustc_codegen_ssa::back::linker::Linker` that can be useful for bitcode-only linking?

Currently, there are no major public CUDA projects written in Rust I'm aware of, but I'm expecting to have a built-in target will create a solid foundation for further experiments and awesome crates.

Related to #38789
Fixes #38787
Fixes #38786

2 files changed, 3 insertions, 2 deletions

diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 32b03c5fb1b..1aa2e116a5a 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -831,6 +831,7 @@ impl Build {
                   !target.contains("msvc") &&
                   !target.contains("emscripten") &&
                   !target.contains("wasm32") &&
+                  !target.contains("nvptx") &&
                   !target.contains("fuchsia") {
             Some(self.cc(target))
         } else {
diff --git a/src/bootstrap/sanity.rs b/src/bootstrap/sanity.rs
index fe547a6b151..ff4fb85bbfa 100644
--- a/src/bootstrap/sanity.rs
+++ b/src/bootstrap/sanity.rs
@@ -156,7 +156,7 @@ pub fn check(build: &mut Build) {
             panic!("the iOS target is only supported on macOS");
         }
 
-        if target.contains("-none-") {
+        if target.contains("-none-") || target.contains("nvptx") {
             if build.no_std(*target).is_none() {
                 let target = build.config.target_config.entry(target.clone())
                     .or_default();
@@ -165,7 +165,7 @@ pub fn check(build: &mut Build) {
             }
 
             if build.no_std(*target) == Some(false) {
-                panic!("All the *-none-* targets are no-std targets")
+                panic!("All the *-none-* and nvptx* targets are no-std targets")
             }
         }