add extern "rust-tail" calling convention

[folkertdev]

Maps to LLVM's tailcc and hence inherits all its problems.

This calling convention at least compiles on:

• x86_64
• aarch64
• s390x
• loongarch64

For most backends LLVM does not have support and fails loudly. At least for loongarch64 I noticed that LLVM does not actually have any tests for it.

[bjorn3]

Please emit an error as a fallback to extern "C" would not be ABI compatible with LLVM.

View changes since the review

[folkertdev]

Isn't the same true for RustPreserveNone? I can adjust that one too.

[bjorn3]

Yes

[bjorn3]

Suggested change

	sess.dcx().fatal("call conv {c:?} is LLVM-specific")
	sess.dcx().fatal(format!("call conv {c:?} is LLVM-specific"))

View changes since the review

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [codegen] tests/codegen-llvm/zip.rs ... ok

failures:

---- [codegen] tests/codegen-llvm/tailcc.rs#UNSUPPORTED stdout ----
------rustc stdout------------------------------

------rustc stderr------------------------------
warning: unused variable: `x`
##[warning]  --> /checkout/tests/codegen-llvm/tailcc.rs:21:33
   |
21 | pub extern "rust-tail" fn peach(x: u16) {
   |                                 ^ help: if this is intentional, prefix it with an underscore: `_x`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

error: extern "rust-tail" is only supported on x86_644 and aarch64
---
------------------------------------------

error in revision `UNSUPPORTED`: compilation failed!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/codegen-llvm/tailcc.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--cfg" "unsupported" "--check-cfg" "cfg(test,FALSE,x86,aarch64,unsupported)" "-O" "-Cdebug-assertions=no" "-Zcodegen-source-order" "--emit" "llvm-ir" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/codegen-llvm/tailcc.UNSUPPORTED/tailcc.ll" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Cpanic=abort" "-Cforce-unwind-tables=yes" "-C" "no-prepopulate-passes" "--target=powerpc64le-unknown-linux-gnu" "--extern" "minicore=/checkout/obj/build/aarch64-unknown-linux-gnu/test/codegen-llvm/tailcc.UNSUPPORTED/libminicore.rlib"
stdout: none
--- stderr -------------------------------
warning: unused variable: `x`
##[warning]  --> /checkout/tests/codegen-llvm/tailcc.rs:21:33
   |
21 | pub extern "rust-tail" fn peach(x: u16) {
   |                                 ^ help: if this is intentional, prefix it with an underscore: `_x`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

error: extern "rust-tail" is only supported on x86_644 and aarch64

error: aborting due to 1 previous error; 1 warning emitted
------------------------------------------

---- [codegen] tests/codegen-llvm/tailcc.rs#UNSUPPORTED stdout end ----

failures:
    [codegen] tests/codegen-llvm/tailcc.rs#UNSUPPORTED

test result: FAILED. 881 passed; 1 failed; 185 ignored; 0 measured; 3 filtered out; finished in 15.84s