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Rollup merge of #148878 - folkertdev:tail-call-unsupported-abi, r=WaffleLapkin

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error when ABI does not support guaranteed tail calls

Some ABIs cannot support guaranteed tail calls. There isn't really an exhaustive list, so this is a best effort. Conveniently, we already disallow calling most of these directly anyway. The only exception that I was able to trigger an LLVM assertion with so far was `cmse-nonsecure-entry`.

For that calling convention, LLVM specifically notes that  (guaranteed) tail calls cannot be supported:

28dbbba6c3/llvm/lib/Target/ARM/ARMISelLowering.cpp (L2331-L2335)

---

I have some doubts about the implementation here though. I think it would be nicer to use `CanonAbi`, and move the `become` ABI check into `rustc_hir_typeck`, similar to `check_call_abi`:

d6deffe2de/compiler/rustc_hir_typeck/src/callee.rs (L157-L194)

Both the check for whether an ABI is callable and whether it supports guaranteed tail calls can then be methods (containing exhaustive matches) on `CanonAbi`. I'm however not sure

- if the ABI checks are deliberately only performed when constructing MIR
- what assumptions can be made about the `call` expression in [`check_expr_become`](d6deffe2de/compiler/rustc_hir_typeck/src/expr.rs (L1126-L1150)), it looks like currently the check that the "argument" to `become` is a function call also only occurs later during MIR construction

Are there issues with validating the ABI earlier in `rustc_hir_typeck` that I'm overlooking? I believe that we should already know the call's ABI and whether it is c-variadic at that point.

cc ````@workingjubilee```` for `CanonAbi`, ````@davidtwco```` for cmse
r? ````@WaffleLapkin````
This commit is contained in:
Stuart Cook 2025-11-14 13:14:04 +11:00 • committed by GitHub
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45
compiler/rustc_abi/src/extern_abi.rs
View file

@ -276,6 +276,51 @@ impl ExternAbi {
_ => CVariadicStatus::NotSupported,
}
}
/// Returns whether the ABI supports guaranteed tail calls.
#[cfg(feature = "nightly")]
pub fn supports_guaranteed_tail_call(self) -> bool {
match self {
Self::CmseNonSecureCall | Self::CmseNonSecureEntry => {
// See https://godbolt.org/z/9jhdeqErv. The CMSE calling conventions clear registers
// before returning, and hence cannot guarantee a tail call.
false
}
Self::AvrInterrupt
| Self::AvrNonBlockingInterrupt
| Self::Msp430Interrupt
| Self::RiscvInterruptM
| Self::RiscvInterruptS
| Self::X86Interrupt => {
// See https://godbolt.org/z/Edfjnxxcq. Interrupts cannot be called directly.
false
}
Self::GpuKernel | Self::PtxKernel => {
// See https://godbolt.org/z/jq5TE5jK1.
false
}
Self::Custom => {
// This ABI does not support calls at all (except via assembly).
false
}
Self::C { .. }
| Self::System { .. }
| Self::Rust
| Self::RustCall
| Self::RustCold
| Self::RustInvalid
| Self::Unadjusted
| Self::EfiApi
| Self::Aapcs { .. }
| Self::Cdecl { .. }
| Self::Stdcall { .. }
| Self::Fastcall { .. }
| Self::Thiscall { .. }
| Self::Vectorcall { .. }
| Self::SysV64 { .. }
| Self::Win64 { .. } => true,
}
}
}
pub fn all_names() -> Vec<&'static str> {

14
compiler/rustc_mir_build/src/check_tail_calls.rs
View file

@ -135,6 +135,10 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
self.report_abi_mismatch(expr.span, caller_sig.abi, callee_sig.abi);
}
if !callee_sig.abi.supports_guaranteed_tail_call() {
self.report_unsupported_abi(expr.span, callee_sig.abi);
}
// FIXME(explicit_tail_calls): this currently fails for cases where opaques are used.
// e.g.
// ```
@ -358,6 +362,16 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
self.found_errors = Err(err);
}
fn report_unsupported_abi(&mut self, sp: Span, callee_abi: ExternAbi) {
let err = self
.tcx
.dcx()
.struct_span_err(sp, "ABI does not support guaranteed tail calls")
.with_note(format!("`become` is not supported for `extern {callee_abi}` functions"))
.emit();
self.found_errors = Err(err);
}
fn report_signature_mismatch(
&mut self,
sp: Span,

38
tests/ui/explicit-tail-calls/unsupported-abi/cmse-nonsecure-call.rs Normal file
View file

@ -0,0 +1,38 @@
//@ add-minicore
//@ ignore-backends: gcc
//@ compile-flags: --target thumbv8m.main-none-eabi --crate-type lib
//@ needs-llvm-components: arm
#![expect(incomplete_features)]
#![feature(no_core, explicit_tail_calls, abi_cmse_nonsecure_call)]
#![no_core]
extern crate minicore;
use minicore::*;
unsafe extern "C" {
safe fn magic() -> extern "cmse-nonsecure-call" fn(u32, u32) -> u32;
}
// The `cmse-nonsecure-call` ABI can only occur on function pointers:
//
// - a `cmse-nonsecure-call` definition throws an error
// - a `cmse-nonsecure-call` become in a definition with any other ABI is an ABI mismatch
#[no_mangle]
extern "cmse-nonsecure-call" fn become_nonsecure_call_1(x: u32, y: u32) -> u32 {
//~^ ERROR the `"cmse-nonsecure-call"` ABI is only allowed on function pointers
unsafe {
let f = magic();
become f(1, 2)
//~^ ERROR ABI does not support guaranteed tail calls
}
}
#[no_mangle]
extern "C" fn become_nonsecure_call_2(x: u32, y: u32) -> u32 {
unsafe {
let f = magic();
become f(1, 2)
//~^ ERROR mismatched function ABIs
//~| ERROR ABI does not support guaranteed tail calls
}
}

34
tests/ui/explicit-tail-calls/unsupported-abi/cmse-nonsecure-call.stderr Normal file
View file

@ -0,0 +1,34 @@
error[E0781]: the `"cmse-nonsecure-call"` ABI is only allowed on function pointers
--> $DIR/cmse-nonsecure-call.rs:21:1
|
LL | extern "cmse-nonsecure-call" fn become_nonsecure_call_1(x: u32, y: u32) -> u32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: ABI does not support guaranteed tail calls
--> $DIR/cmse-nonsecure-call.rs:25:9
|
LL | become f(1, 2)
| ^^^^^^^^^^^^^^
|
= note: `become` is not supported for `extern "cmse-nonsecure-call"` functions
error: mismatched function ABIs
--> $DIR/cmse-nonsecure-call.rs:34:9
|
LL | become f(1, 2)
| ^^^^^^^^^^^^^^
|
= note: `become` requires caller and callee to have the same ABI
= note: caller ABI is `"C"`, while callee ABI is `"cmse-nonsecure-call"`
error: ABI does not support guaranteed tail calls
--> $DIR/cmse-nonsecure-call.rs:34:9
|
LL | become f(1, 2)
| ^^^^^^^^^^^^^^
|
= note: `become` is not supported for `extern "cmse-nonsecure-call"` functions
error: aborting due to 4 previous errors
For more information about this error, try `rustc --explain E0781`.

22
tests/ui/explicit-tail-calls/unsupported-abi/cmse-nonsecure-entry.rs Normal file
View file

@ -0,0 +1,22 @@
//@ add-minicore
//@ ignore-backends: gcc
//@ compile-flags: --target thumbv8m.main-none-eabi --crate-type lib
//@ needs-llvm-components: arm
#![expect(incomplete_features)]
#![feature(no_core, explicit_tail_calls, cmse_nonsecure_entry)]
#![no_core]
extern crate minicore;
use minicore::*;
#[inline(never)]
extern "cmse-nonsecure-entry" fn entry(c: bool, x: u32, y: u32) -> u32 {
if c { x } else { y }
}
// A `cmse-nonsecure-entry` clears registers before returning, so a tail call cannot be guaranteed.
#[unsafe(no_mangle)]
extern "cmse-nonsecure-entry" fn become_nonsecure_entry(c: bool, x: u32, y: u32) -> u32 {
become entry(c, x, y)
//~^ ERROR ABI does not support guaranteed tail calls
}

10
tests/ui/explicit-tail-calls/unsupported-abi/cmse-nonsecure-entry.stderr Normal file
View file

@ -0,0 +1,10 @@
error: ABI does not support guaranteed tail calls
--> $DIR/cmse-nonsecure-entry.rs:20:5
|
LL | become entry(c, x, y)
| ^^^^^^^^^^^^^^^^^^^^^
|
= note: `become` is not supported for `extern "cmse-nonsecure-entry"` functions
error: aborting due to 1 previous error