c-variadic: bpf and spirv do not support c-variadic definitions

Author: folkertdev

compiler/rustc_ast_passes/messages.ftl

@@ -80,6 +80,8 @@ ast_passes_c_variadic_must_be_unsafe =
 ast_passes_c_variadic_no_extern = `...` is not supported for non-extern functions
     .help = only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
 
+ast_passes_c_variadic_not_supported = the `{$target}` target does not support c-variadic functions
+
 ast_passes_const_and_c_variadic = functions cannot be both `const` and C-variadic
     .const = `const` because of this
     .variadic = C-variadic because of this

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -37,7 +37,7 @@ use rustc_session::lint::builtin::{
 };
 use rustc_session::parse::feature_err;
 use rustc_span::{Ident, Span, kw, sym};
-use rustc_target::spec::{AbiMap, AbiMapping};
+use rustc_target::spec::{AbiMap, AbiMapping, Arch};
 use thin_vec::thin_vec;
 
 use crate::errors::{self, TildeConstReason};
@@ -710,6 +710,15 @@ impl<'a> AstValidator<'a> {
         match fn_ctxt {
             FnCtxt::Foreign => return,
             FnCtxt::Free | FnCtxt::Assoc(_) => {
+                // Some targets just do not allow c-variadic definitions.
+                let target = &self.sess.target;
+                if matches!(target.arch, Arch::SpirV | Arch::Bpf) {
+                    self.dcx().emit_err(errors::CVariadicNotSupported {
+                        variadic_span: variadic_param.span,
+                        target: &*target.llvm_target,
+                    });
+                }
+
                 match sig.header.ext {
                     Extern::Implicit(_) => {
                         if !matches!(sig.header.safety, Safety::Unsafe(_)) {

compiler/rustc_ast_passes/src/errors.rs

@@ -725,6 +725,14 @@ pub(crate) struct CoroutineAndCVariadic {
     pub variadic_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_c_variadic_not_supported)]
+pub(crate) struct CVariadicNotSupported<'a> {
+    #[primary_span]
+    pub variadic_span: Span,
+    pub target: &'a str,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_pattern_in_foreign, code = E0130)]
 // FIXME: deduplicate with rustc_lint (`BuiltinLintDiag::PatternsInFnsWithoutBody`)

tests/ui/c-variadic/not-supported.bpf.stderr

@@ -0,0 +1,8 @@
+error: the `bpfel` target does not support c-variadic functions
+  --> $DIR/not-supported.rs:23:31
+   |
+LL | unsafe extern "C" fn variadic(_: ...) {}
+   |                               ^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/c-variadic/not-supported.rs

@@ -0,0 +1,24 @@
+//@ add-minicore
+//@ revisions: bpf
+//@ [bpf] compile-flags: --target=bpfel-unknown-none
+//@ [bpf] needs-llvm-components: bpf
+//@ ignore-backends: gcc
+#![no_core]
+#![feature(no_core, lang_items, c_variadic)]
+#![crate_type = "lib"]
+
+// Some targets do not support c-variadic definitions. Examples are bpf and spirv.
+
+extern crate minicore;
+use minicore::*;
+
+#[lang = "va_list"]
+struct VaList(*mut i32);
+
+extern "C" {
+    fn foreign_variadic(...);
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn variadic(_: ...) {}
+//[bpf]~^ ERROR the `bpfel` target does not support c-variadic functions