Consider fields to be inhabited if they are unstable

compiler/rustc_middle/src/ty/inhabitedness/mod.rs

@@ -43,6 +43,7 @@
 //! This code should only compile in modules where the uninhabitedness of `Foo`
 //! is visible.
 
+use rustc_span::sym;
 use rustc_type_ir::TyKind::*;
 use tracing::instrument;
 
@@ -84,6 +85,21 @@ impl<'tcx> VariantDef {
         InhabitedPredicate::all(
             tcx,
             self.fields.iter().map(|field| {
+                // Unstable fields are always considered to be inhabited. In the future,
+                // this could be extended to be conditional on the field being unstable
+                // only within the module that's querying the inhabitedness, like:
+                //     `let pred = pred.or(InhabitedPredicate::IsUnstable(field.did));`
+                // but this is unnecessary for now, since it would only affect nightly-only
+                // code or code within the standard library itself.
+                // HACK: We filter out `rustc_private` fields since with the flag
+                // `-Zforce-unstable-if-unmarked` we consider all unmarked fields to be
+                // unstable when building the compiler.
+                if tcx
+                    .lookup_stability(field.did)
+                    .is_some_and(|stab| stab.is_unstable() && stab.feature != sym::rustc_private)
+                {
+                    return InhabitedPredicate::True;
+                }
                 let pred = tcx.type_of(field.did).instantiate_identity().inhabited_predicate(tcx);
                 if adt.is_enum() {
                     return pred;

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -15,7 +15,7 @@ use rustc_middle::ty::{
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint;
-use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
+use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, sym};
 
 use crate::constructor::Constructor::*;
 use crate::constructor::{
@@ -232,7 +232,11 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                             let is_visible =
                                 adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
                             let is_uninhabited = cx.is_uninhabited(*ty);
-                            let skip = is_uninhabited && !is_visible;
+                            let is_unstable =
+                                cx.tcx.lookup_stability(field.did).is_some_and(|stab| {
+                                    stab.is_unstable() && stab.feature != sym::rustc_private
+                                });
+                            let skip = is_uninhabited && (!is_visible || is_unstable);
                             (ty, PrivateUninhabitedField(skip))
                         });
                         cx.dropless_arena.alloc_from_iter(tys)

tests/ui/uninhabited/auxiliary/staged-api.rs

@@ -0,0 +1,8 @@
+#![feature(staged_api)]
+#![stable(feature = "stable", since = "1.0.0")]
+
+#[stable(feature = "stable", since = "1.0.0")]
+pub struct Foo<T> {
+    #[unstable(feature = "unstable", issue = "none")]
+    pub field: T,
+}

tests/ui/uninhabited/uninhabited-pin-field.rs

@@ -0,0 +1,10 @@
+use std::pin::Pin;
+
+enum Void {}
+
+fn demo(x: Pin<Void>) {
+    match x {}
+    //~^ ERROR non-exhaustive patterns
+}
+
+fn main() {}

tests/ui/uninhabited/uninhabited-pin-field.stderr

@@ -0,0 +1,19 @@
+error[E0004]: non-exhaustive patterns: type `Pin<Void>` is non-empty
+  --> $DIR/uninhabited-pin-field.rs:6:11
+   |
+LL |     match x {}
+   |           ^
+   |
+note: `Pin<Void>` defined here
+  --> $SRC_DIR/core/src/pin.rs:LL:COL
+   = note: the matched value is of type `Pin<Void>`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
+   |
+LL ~     match x {
+LL +         _ => todo!(),
+LL ~     }
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0004`.

tests/ui/uninhabited/uninhabited-unstable-field.current.stderr

@@ -0,0 +1,22 @@
+error[E0004]: non-exhaustive patterns: type `Foo<Void>` is non-empty
+  --> $DIR/uninhabited-unstable-field.rs:13:11
+   |
+LL |     match x {}
+   |           ^
+   |
+note: `Foo<Void>` defined here
+  --> $DIR/auxiliary/staged-api.rs:5:1
+   |
+LL | pub struct Foo<T> {
+   | ^^^^^^^^^^^^^^^^^
+   = note: the matched value is of type `Foo<Void>`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
+   |
+LL ~     match x {
+LL +         _ => todo!(),
+LL ~     }
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0004`.

tests/ui/uninhabited/uninhabited-unstable-field.exhaustive.stderr

@@ -0,0 +1,22 @@
+error[E0004]: non-exhaustive patterns: type `Foo<Void>` is non-empty
+  --> $DIR/uninhabited-unstable-field.rs:13:11
+   |
+LL |     match x {}
+   |           ^
+   |
+note: `Foo<Void>` defined here
+  --> $DIR/auxiliary/staged-api.rs:5:1
+   |
+LL | pub struct Foo<T> {
+   | ^^^^^^^^^^^^^^^^^
+   = note: the matched value is of type `Foo<Void>`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
+   |
+LL ~     match x {
+LL +         _ => todo!(),
+LL ~     }
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0004`.

tests/ui/uninhabited/uninhabited-unstable-field.rs

@@ -0,0 +1,29 @@
+//@ aux-build: staged-api.rs
+//@ revisions: current exhaustive
+
+#![feature(exhaustive_patterns)]
+
+extern crate staged_api;
+
+use staged_api::Foo;
+
+enum Void {}
+
+fn demo(x: Foo<Void>) {
+    match x {}
+    //~^ ERROR non-exhaustive patterns
+}
+
+// Ensure that the pattern is not considered unreachable.
+fn demo2(x: Foo<Void>) {
+    match x {
+        Foo { .. } => {}
+    }
+}
+
+// Same as above, but for wildcard.
+fn demo3(x: Foo<Void>) {
+    match x { _ => {} }
+}
+
+fn main() {}