Auto merge of #136897 - workingjubilee:revert-unfcped-stab, r=WaffleLapkin

Revert "Stabilize `extended_varargs_abi_support`"

I cannot find an FCP for this, despite it being a stabilization PR which normally means we do an FCP of some kind? It would seem reasonable for _either_ compiler or lang to have FCPed it? I am thus opening a revert PR, which mostly-cleanly applies, so that we can later actually land this properly with a stability report and FCP.

- https://github.com/rust-lang/rust/issues/136896
- https://github.com/rust-lang/rust/pull/116161
- https://github.com/rust-lang/rust/issues/100189

compiler/rustc_hir_analysis/messages.ftl

@@ -602,7 +602,7 @@ hir_analysis_value_of_associated_struct_already_specified =
     .label = re-bound here
     .previous_bound_label = `{$item_name}` bound here first
 
-hir_analysis_variadic_function_compatible_convention = C-variadic function must have a compatible calling convention, like `C`, `cdecl`, `system`, `aapcs`, `win64`, `sysv64` or `efiapi`
+hir_analysis_variadic_function_compatible_convention = C-variadic function must have a compatible calling convention, like {$conventions}
     .label = C-variadic function must have a compatible calling convention
 
 hir_analysis_variances_of = {$variances}

compiler/rustc_hir_analysis/src/errors.rs

@@ -646,10 +646,11 @@ pub(crate) struct MainFunctionGenericParameters {
 
 #[derive(Diagnostic)]
 #[diag(hir_analysis_variadic_function_compatible_convention, code = E0045)]
-pub(crate) struct VariadicFunctionCompatibleConvention {
+pub(crate) struct VariadicFunctionCompatibleConvention<'a> {
     #[primary_span]
     #[label]
     pub span: Span,
+    pub conventions: &'a str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_hir_analysis/src/lib.rs

@@ -100,6 +100,8 @@ use rustc_middle::middle;
 use rustc_middle::mir::interpret::GlobalId;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, Const, Ty, TyCtxt};
+use rustc_session::parse::feature_err;
+use rustc_span::symbol::sym;
 use rustc_span::{ErrorGuaranteed, Span};
 use rustc_trait_selection::traits;
 
@@ -114,9 +116,34 @@ fn require_c_abi_if_c_variadic(
     abi: ExternAbi,
     span: Span,
 ) {
-    if decl.c_variadic && !abi.supports_varargs() {
-        tcx.dcx().emit_err(errors::VariadicFunctionCompatibleConvention { span });
+    const CONVENTIONS_UNSTABLE: &str =
+        "`C`, `cdecl`, `system`, `aapcs`, `win64`, `sysv64` or `efiapi`";
+    const CONVENTIONS_STABLE: &str = "`C` or `cdecl`";
+    const UNSTABLE_EXPLAIN: &str =
+        "using calling conventions other than `C` or `cdecl` for varargs functions is unstable";
+
+    if !decl.c_variadic || matches!(abi, ExternAbi::C { .. } | ExternAbi::Cdecl { .. }) {
+        return;
     }
+
+    let extended_abi_support = tcx.features().extended_varargs_abi_support();
+    let conventions = match (extended_abi_support, abi.supports_varargs()) {
+        // User enabled additional ABI support for varargs and function ABI matches those ones.
+        (true, true) => return,
+
+        // Using this ABI would be ok, if the feature for additional ABI support was enabled.
+        // Return CONVENTIONS_STABLE, because we want the other error to look the same.
+        (false, true) => {
+            feature_err(&tcx.sess, sym::extended_varargs_abi_support, span, UNSTABLE_EXPLAIN)
+                .emit();
+            CONVENTIONS_STABLE
+        }
+
+        (false, false) => CONVENTIONS_STABLE,
+        (true, false) => CONVENTIONS_UNSTABLE,
+    };
+
+    tcx.dcx().emit_err(errors::VariadicFunctionCompatibleConvention { span, conventions });
 }
 
 pub fn provide(providers: &mut Providers) {