Lint uninhabited types in typeck

Author: camsteffen

compiler/rustc_hir_typeck/src/_match.rs

@@ -249,7 +249,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// warn the user about the match arms being unreachable.
     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
         for arm in arms {
-            self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
+            if !arm.pat.is_never_pattern() {
+                self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
+            }
         }
     }
 

compiler/rustc_hir_typeck/src/diverges.rs

@@ -1,5 +1,6 @@
 use std::{cmp, ops};
 
+use rustc_hir::HirId;
 use rustc_span::Span;
 
 /// Tracks whether executing a node may exit normally (versus
@@ -13,6 +14,14 @@ pub(crate) enum Diverges {
     /// others require a CFG to determine them.
     Maybe,
 
+    /// This expression is uninhabited, we want to
+    /// emit a diagnostic but not pollute type checking.
+    UninhabitedExpr(HirId, Span),
+
+    /// Same as `UninhabitedExpr` but with reachability
+    /// warning already emitted.
+    Warned,
+
     /// Definitely known to diverge and therefore
     /// not reach the next sibling or its parent.
     Always(DivergeReason, Span),
@@ -54,16 +63,18 @@ impl ops::BitOrAssign for Diverges {
 impl Diverges {
     pub(super) fn is_always(self) -> bool {
         match self {
-            Self::Maybe => false,
+            Self::Maybe | Diverges::UninhabitedExpr(..) | Diverges::Warned => false,
             Self::Always(..) | Self::WarnedAlways => true,
         }
     }
 
     fn ordinal(&self) -> u8 {
         match self {
             Self::Maybe => 0,
-            Self::Always { .. } => 1,
-            Self::WarnedAlways => 2,
+            Self::UninhabitedExpr(..) => 1,
+            Self::Warned => 2,
+            Self::Always { .. } => 3,
+            Self::WarnedAlways => 4,
         }
     }
 }
@@ -72,5 +83,6 @@ impl Diverges {
 pub(crate) enum DivergeReason {
     AllArmsDiverge,
     NeverPattern,
+    UninhabitedExpr(HirId),
     Other,
 }

compiler/rustc_hir_typeck/src/expr.rs

@@ -234,20 +234,41 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // diverging expression (e.g. it arose from desugaring of `try { return }`),
             // we skip issuing a warning because it is autogenerated code.
             ExprKind::Call(..) if expr.span.is_desugaring(DesugaringKind::TryBlock) => {}
-            ExprKind::Call(callee, _) => self.warn_if_unreachable(expr.hir_id, callee.span, "call"),
+            ExprKind::Call(callee, _) => {
+                // Do not emit a warning for a call to a constructor.
+                let emit_warning = if let ExprKind::Path(ref qpath) = callee.kind {
+                    let res = self.typeck_results.borrow().qpath_res(qpath, callee.hir_id);
+                    !matches!(res, Res::Def(DefKind::Ctor(..), _))
+                } else {
+                    true
+                };
+                if emit_warning {
+                    self.warn_if_unreachable(expr.hir_id, callee.span, "call")
+                }
+            }
             ExprKind::MethodCall(segment, ..) => {
                 self.warn_if_unreachable(expr.hir_id, segment.ident.span, "call")
             }
+            // Allow field access when the struct is uninhabited.
+            ExprKind::Field(..)
+                if matches!(self.diverges.get(), Diverges::UninhabitedExpr(_, _)) => {}
             _ => self.warn_if_unreachable(expr.hir_id, expr.span, "expression"),
         }
 
         // Any expression that produces a value of type `!` must have diverged,
         // unless it's a place expression that isn't being read from, in which case
         // diverging would be unsound since we may never actually read the `!`.
         // e.g. `let _ = *never_ptr;` with `never_ptr: *const !`.
-        if ty.is_never() && self.expr_guaranteed_to_constitute_read_for_never(expr) {
-            self.diverges
-                .set(self.diverges.get() | Diverges::Always(DivergeReason::Other, expr.span));
+        let cur_diverges = self.diverges.get();
+        if !cur_diverges.is_always() && self.expr_guaranteed_to_constitute_read_for_never(expr) {
+            if ty.is_never() {
+                // Any expression that produces a value of type `!` must have diverged.
+                self.diverges.set(cur_diverges | Diverges::Always(DivergeReason::Other, expr.span));
+            } else if self.ty_is_uninhabited(ty) {
+                // This expression produces a value of uninhabited type.
+                // This means it has diverged somehow.
+                self.diverges.set(cur_diverges | Diverges::UninhabitedExpr(expr.hir_id, expr.span));
+            }
         }
 
         // Record the type, which applies it effects.
@@ -353,6 +374,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 self.pat_guaranteed_to_constitute_read_for_never(*pat)
             }
 
+            hir::Node::Pat(parent_pat) => match parent_pat.kind {
+                hir::PatKind::Lit(..) | hir::PatKind::Range(..) => false,
+
+                // These nodes do not have direct sub-exprs.
+                hir::PatKind::Wild
+                | hir::PatKind::Binding(..)
+                | hir::PatKind::Struct(..)
+                | hir::PatKind::TupleStruct(..)
+                | hir::PatKind::Or(..)
+                | hir::PatKind::Never
+                | hir::PatKind::Path(..)
+                | hir::PatKind::Tuple(..)
+                | hir::PatKind::Box(..)
+                | hir::PatKind::Deref(..)
+                | hir::PatKind::Ref(..)
+                | hir::PatKind::Slice(..)
+                | hir::PatKind::Err(..) => {
+                    unreachable!("no sub-expr expected for {parent_pat:?}")
+                }
+            },
+
             // These nodes (if they have a sub-expr) do constitute a read.
             hir::Node::Block(_)
             | hir::Node::Arm(_)
@@ -382,7 +424,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | hir::Node::Ty(_)
             | hir::Node::AssocItemConstraint(_)
             | hir::Node::TraitRef(_)
-            | hir::Node::Pat(_)
             | hir::Node::PatField(_)
             | hir::Node::LetStmt(_)
             | hir::Node::Synthetic
@@ -443,6 +484,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    fn ty_is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
+        let ty = self.resolve_vars_if_possible(ty);
+        // Freshen the type as `is_inhabited_from` may call a query on `ty`.
+        let ty = self.freshen(ty);
+        !ty.is_inhabited_from(self.tcx, self.parent_module, self.param_env)
+    }
+
     #[instrument(skip(self, expr), level = "debug")]
     fn check_expr_kind(
         &self,

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::collections::hash_map::Entry;
 use std::slice;
 
@@ -31,7 +32,7 @@ use rustc_session::lint;
 use rustc_span::Span;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::hygiene::DesugaringKind;
-use rustc_span::symbol::kw;
+use rustc_span::symbol::{kw, sym};
 use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::error_reporting::infer::need_type_info::TypeAnnotationNeeded;
 use rustc_trait_selection::traits::{
@@ -50,8 +51,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Produces warning on the given node, if the current point in the
     /// function is unreachable, and there hasn't been another warning.
     pub(crate) fn warn_if_unreachable(&self, id: HirId, span: Span, kind: &str) {
-        let Diverges::Always(reason, orig_span) = self.diverges.get() else {
-            return;
+        let (reason, orig_span) = match self.diverges.get() {
+            Diverges::UninhabitedExpr(hir_id, orig_span) => {
+                (DivergeReason::UninhabitedExpr(hir_id), orig_span)
+            }
+            Diverges::Always(reason, orig_span) => (reason, orig_span),
+            Diverges::Maybe | Diverges::Warned | Diverges::WarnedAlways => return,
         };
 
         match span.desugaring_kind() {
@@ -73,20 +78,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => {}
         }
 
+        if matches!(reason, DivergeReason::UninhabitedExpr(_)) {
+            if let Some(impl_of) = self.tcx.impl_of_method(self.body_id.to_def_id()) {
+                if self.tcx.has_attr(impl_of, sym::automatically_derived) {
+                    // Built-in derives are generated before typeck,
+                    // so they may contain unreachable code if there are uninhabited types
+                    return;
+                }
+            }
+        }
+
         // Don't warn twice.
-        self.diverges.set(Diverges::WarnedAlways);
+        self.diverges.set(match self.diverges.get() {
+            Diverges::UninhabitedExpr(..) => Diverges::Warned,
+            Diverges::Always(..) => Diverges::WarnedAlways,
+            Diverges::Maybe | Diverges::Warned | Diverges::WarnedAlways => bug!(),
+        });
 
         debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
 
         let msg = format!("unreachable {kind}");
         self.tcx().node_span_lint(lint::builtin::UNREACHABLE_CODE, id, span, |lint| {
             lint.primary_message(msg.clone());
-            let custom_note = match reason {
+            let custom_note: Cow<'_, _> = match reason {
                 DivergeReason::AllArmsDiverge => {
                     "any code following this `match` expression is unreachable, as all arms diverge"
+                        .into()
+                }
+                DivergeReason::NeverPattern => {
+                    "any code following a never pattern is unreachable".into()
                 }
-                DivergeReason::NeverPattern => "any code following a never pattern is unreachable",
-                DivergeReason::Other => "any code following this expression is unreachable",
+                DivergeReason::UninhabitedExpr(hir_id) => format!(
+                    "this expression has type `{}`, which is uninhabited",
+                    self.typeck_results.borrow().node_type(hir_id)
+                )
+                .into(),
+                DivergeReason::Other => "any code following this expression is unreachable".into(),
             };
             lint.span_label(span, msg).span_label(orig_span, custom_note);
         })

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -49,6 +49,11 @@ pub(crate) struct FnCtxt<'a, 'tcx> {
     /// eventually).
     pub(super) param_env: ty::ParamEnv<'tcx>,
 
+    /// The module in which the current function is defined. This
+    /// is used to compute type inhabitedness, which accounts for
+    /// visibility information.
+    pub(super) parent_module: DefId,
+
     /// If `Some`, this stores coercion information for returned
     /// expressions. If `None`, this is in a context where return is
     /// inappropriate, such as a const expression.
@@ -127,6 +132,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         FnCtxt {
             body_id,
             param_env,
+            parent_module: root_ctxt.tcx.parent_module_from_def_id(body_id).to_def_id(),
             ret_coercion: None,
             ret_coercion_span: Cell::new(None),
             coroutine_types: None,

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

@@ -92,27 +92,7 @@ impl<'tcx> InhabitedPredicate<'tcx> {
             Self::NotInModule(id) => in_module(id).map(|in_mod| !in_mod),
             // `t` may be a projection, for which `inhabited_predicate` returns a `GenericType`. As
             // we have a param_env available, we can do better.
-            Self::GenericType(t) => {
-                let normalized_pred = tcx
-                    .try_normalize_erasing_regions(param_env, t)
-                    .map_or(self, |t| t.inhabited_predicate(tcx));
-                match normalized_pred {
-                    // We don't have more information than we started with, so consider inhabited.
-                    Self::GenericType(_) => Ok(true),
-                    pred => {
-                        // A type which is cyclic when monomorphized can happen here since the
-                        // layout error would only trigger later. See e.g. `tests/ui/sized/recursive-type-2.rs`.
-                        if eval_stack.contains(&t) {
-                            return Ok(true); // Recover; this will error later.
-                        }
-                        eval_stack.push(t);
-                        let ret =
-                            pred.apply_inner(tcx, param_env, eval_stack, in_module, reveal_opaque);
-                        eval_stack.pop();
-                        ret
-                    }
-                }
-            }
+            Self::GenericType(_) => Ok(true),
             Self::OpaqueType(key) => match reveal_opaque(key) {
                 // Unknown opaque is assumed inhabited.
                 None => Ok(true),

compiler/rustc_passes/messages.ftl

@@ -752,11 +752,6 @@ passes_unnecessary_partial_stable_feature = the feature `{$feature}` has been pa
 
 passes_unnecessary_stable_feature = the feature `{$feature}` has been stable since {$since} and no longer requires an attribute to enable
 
-passes_unreachable_due_to_uninhabited = unreachable {$descr}
-    .label = unreachable {$descr}
-    .label_orig = any code following this expression is unreachable
-    .note = this expression has type `{$ty}`, which is uninhabited
-
 passes_unrecognized_field =
     unrecognized field name `{$name}`
 

compiler/rustc_passes/src/errors.rs

@@ -1679,18 +1679,6 @@ pub(crate) struct SkippingConstChecks {
     pub span: Span,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(passes_unreachable_due_to_uninhabited)]
-pub(crate) struct UnreachableDueToUninhabited<'desc, 'tcx> {
-    pub descr: &'desc str,
-    #[label]
-    pub expr: Span,
-    #[label(passes_label_orig)]
-    #[note]
-    pub orig: Span,
-    pub ty: Ty<'tcx>,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(passes_unused_var_maybe_capture_ref)]
 #[help]