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

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

@@ -710,6 +710,14 @@ impl<'a> AstValidator<'a> {
         match fn_ctxt {
             FnCtxt::Foreign => return,
             FnCtxt::Free | FnCtxt::Assoc(_) => {
+                if !self.sess.target.arch.supports_c_variadic_definitions() {
+                    self.dcx().emit_err(errors::CVariadicNotSupported {
+                        variadic_span: variadic_param.span,
+                        target: &*self.sess.target.llvm_target,
+                    });
+                    return;
+                }
+
                 match sig.header.ext {
                     Extern::Implicit(_) => {
                         if !matches!(sig.header.safety, Safety::Unsafe(_)) {

compiler/rustc_ast_passes/src/errors.rs

@@ -733,6 +733,14 @@ 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`)

compiler/rustc_target/src/spec/mod.rs

@@ -1925,6 +1925,24 @@ impl Arch {
             Self::Other(name) => rustc_span::Symbol::intern(name),
         }
     }
+
+    pub fn supports_c_variadic_definitions(&self) -> bool {
+        use Arch::*;
+
+        match self {
+            // These targets just do not support c-variadic definitions.
+            Bpf | SpirV => false,
+
+            // We don't know if the target supports c-variadic definitions, but we don't want
+            // to needlessly restrict custom target.json configurations.
+            Other(_) => true,
+
+            AArch64 | AmdGpu | Arm | Arm64EC | Avr | CSky | Hexagon | LoongArch32 | LoongArch64
+            | M68k | Mips | Mips32r6 | Mips64 | Mips64r6 | Msp430 | Nvptx64 | PowerPC
+            | PowerPC64 | PowerPC64LE | RiscV32 | RiscV64 | S390x | Sparc | Sparc64 | Wasm32
+            | Wasm64 | X86 | X86_64 | Xtensa => true,
+        }
+    }
 }
 
 crate::target_spec_enum! {