try to improve control flow

Author: workingjubilee

compiler/rustc_hir_analysis/src/lib.rs

@@ -116,49 +116,42 @@ fn require_c_abi_if_c_variadic(
     abi: ExternAbi,
     span: 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";
 
+    // ABIs which can stably use varargs
     if !decl.c_variadic || matches!(abi, ExternAbi::C { .. } | ExternAbi::Cdecl { .. }) {
         return;
     }
 
+    // ABIs with feature-gated stability
     let extended_abi_support = tcx.features().extended_varargs_abi_support();
     let extern_system_varargs = tcx.features().extern_system_varargs();
-    let conventions = if extended_abi_support { CONVENTIONS_UNSTABLE } else { CONVENTIONS_STABLE };
+
+    // If the feature gate has been enabled, we can stop here
+    if extern_system_varargs && let ExternAbi::System { .. } = abi {
+        return;
+    };
+    if extended_abi_support && abi.supports_varargs() {
+        return;
+    };
+
+    // Looks like we need to pick an error to emit.
+    // Is there any feature which we could have enabled to make this work?
     match abi {
         ExternAbi::System { .. } => {
-            // User enabled additional ABI support for varargs and function ABI matches those ones.
-            if extern_system_varargs {
-                return;
-            } else {
-                // Using this ABI would be ok, if the feature for additional ABI support was enabled.
-                feature_err(&tcx.sess, sym::extern_system_varargs, span, UNSTABLE_EXPLAIN).emit();
-            }
+            feature_err(&tcx.sess, sym::extern_system_varargs, span, UNSTABLE_EXPLAIN)
         }
-        abi => {
-            if abi.supports_varargs() {
-                // User enabled additional ABI support for varargs and function ABI matches those ones.
-                if extended_abi_support {
-                    return;
-                } else {
-                    // Using this ABI would be ok, if the feature for additional ABI support was enabled.
-                    feature_err(
-                        &tcx.sess,
-                        sym::extended_varargs_abi_support,
-                        span,
-                        UNSTABLE_EXPLAIN,
-                    )
-                    .emit();
-                };
-            }
+        abi if abi.supports_varargs() => {
+            feature_err(&tcx.sess, sym::extended_varargs_abi_support, span, UNSTABLE_EXPLAIN)
         }
-    };
-
-    tcx.dcx().emit_err(errors::VariadicFunctionCompatibleConvention { span, conventions });
+        _ => tcx.dcx().create_err(errors::VariadicFunctionCompatibleConvention {
+            span,
+            conventions: CONVENTIONS_STABLE,
+        }),
+    }
+    .emit();
 }
 
 pub fn provide(providers: &mut Providers) {