compiler: Embed consensus in lint::types::improper_ctypes

Extracting this logic into a module makes it easier to write down, and
more importantly, later find, the actual decisions we've made.

compiler/rustc_lint/src/types.rs

@@ -3,7 +3,6 @@ use std::ops::ControlFlow;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::DiagMessage;
-use rustc_hir::def::CtorKind;
 use rustc_hir::{is_range_literal, Expr, ExprKind, Node};
 use rustc_middle::bug;
 use rustc_middle::ty::layout::{IntegerExt, LayoutOf, SizeSkeleton};
@@ -19,6 +18,8 @@ use rustc_target::spec::abi::Abi as SpecAbi;
 use tracing::debug;
 use {rustc_ast as ast, rustc_attr as attr, rustc_hir as hir};
 
+mod improper_ctypes;
+
 use crate::lints::{
     AmbiguousWidePointerComparisons, AmbiguousWidePointerComparisonsAddrMetadataSuggestion,
     AmbiguousWidePointerComparisonsAddrSuggestion, AtomicOrderingFence, AtomicOrderingLoad,
@@ -1405,38 +1406,23 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                             };
                         }
 
-                        // non_exhaustive suggests it is possible that someone might break ABI
-                        // see: https://github.com/rust-lang/rust/issues/44109#issuecomment-537583344
-                        // so warn on complex enums being used outside their crate
-                        let nonexhaustive_nonlocal_ffi =
-                            def.is_variant_list_non_exhaustive() && !def.did().is_local();
+                        use improper_ctypes::{
+                            check_non_exhaustive_variant, non_local_and_non_exhaustive,
+                        };
 
+                        let non_local_def = non_local_and_non_exhaustive(def);
                         // Check the contained variants.
-                        for variant in def.variants() {
-                            // but only warn about really_tagged_union reprs,
-                            // exempt enums with unit ctors like C's (like rust-bindgen)
-                            if nonexhaustive_nonlocal_ffi
-                                && !matches!(variant.ctor_kind(), Some(CtorKind::Const))
-                            {
-                                return FfiUnsafe {
-                                    ty,
-                                    reason: fluent::lint_improper_ctypes_non_exhaustive,
-                                    help: None,
-                                };
-                            };
-                            let is_non_exhaustive = variant.is_field_list_non_exhaustive();
-                            if is_non_exhaustive && !variant.def_id.is_local() {
-                                return FfiUnsafe {
-                                    ty,
-                                    reason: fluent::lint_improper_ctypes_non_exhaustive_variant,
-                                    help: None,
-                                };
-                            }
+                        let ret = def.variants().iter().try_for_each(|variant| {
+                            check_non_exhaustive_variant(non_local_def, variant)
+                                .map_break(|reason| FfiUnsafe { ty, reason, help: None })?;
 
                             match self.check_variant_for_ffi(acc, ty, def, variant, args) {
-                                FfiSafe => (),
-                                r => return r,
+                                FfiSafe => ControlFlow::Continue(()),
+                                r => ControlFlow::Break(r),
                             }
+                        });
+                        if let ControlFlow::Break(result) = ret {
+                            return result;
                         }
 
                         FfiSafe

compiler/rustc_lint/src/types/improper_ctypes.rs

@@ -0,0 +1,51 @@
+use std::ops::ControlFlow;
+
+use rustc_errors::DiagMessage;
+use rustc_hir::def::CtorKind;
+use rustc_middle::ty;
+
+use crate::fluent_generated as fluent;
+
+/// Check a variant of a non-exhaustive enum for improper ctypes
+///
+/// We treat `#[non_exhaustive] enum` as "ensure that code will compile if new variants are added".
+/// This includes linting, on a best-effort basis. There are valid additions that are unlikely.
+///
+/// Adding a data-carrying variant to an existing C-like enum that is passed to C is "unlikely",
+/// so we don't need the lint to account for it.
+/// e.g. going from enum Foo { A, B, C } to enum Foo { A, B, C, D(u32) }.
+pub(crate) fn check_non_exhaustive_variant(
+    non_local_def: bool,
+    variant: &ty::VariantDef,
+) -> ControlFlow<DiagMessage, ()> {
+    // non_exhaustive suggests it is possible that someone might break ABI
+    // see: https://github.com/rust-lang/rust/issues/44109#issuecomment-537583344
+    // so warn on complex enums being used outside their crate
+    if non_local_def {
+        // which is why we only warn about really_tagged_union reprs from https://rust.tf/rfc2195
+        // with an enum like `#[repr(u8)] enum Enum { A(DataA), B(DataB), }`
+        // but exempt enums with unit ctors like C's (e.g. from rust-bindgen)
+        if variant_has_complex_ctor(variant) {
+            return ControlFlow::Break(fluent::lint_improper_ctypes_non_exhaustive);
+        }
+    }
+
+    let non_exhaustive_variant_fields = variant.is_field_list_non_exhaustive();
+    if non_exhaustive_variant_fields && !variant.def_id.is_local() {
+        return ControlFlow::Break(fluent::lint_improper_ctypes_non_exhaustive_variant);
+    }
+
+    ControlFlow::Continue(())
+}
+
+fn variant_has_complex_ctor(variant: &ty::VariantDef) -> bool {
+    // CtorKind::Const means a "unit" ctor
+    !matches!(variant.ctor_kind(), Some(CtorKind::Const))
+}
+
+// non_exhaustive suggests it is possible that someone might break ABI
+// see: https://github.com/rust-lang/rust/issues/44109#issuecomment-537583344
+// so warn on complex enums being used outside their crate
+pub(crate) fn non_local_and_non_exhaustive(def: ty::AdtDef<'_>) -> bool {
+    def.is_variant_list_non_exhaustive() && !def.did().is_local()
+}