Make typeck aware of uninhabited types

compiler/rustc_hir_typeck/src/_match.rs

@@ -11,7 +11,7 @@ use rustc_trait_selection::traits::{
 use tracing::{debug, instrument};
 
 use crate::coercion::{AsCoercionSite, CoerceMany};
-use crate::{Diverges, Expectation, FnCtxt, Needs};
+use crate::{DivergeReason, Diverges, Expectation, FnCtxt, Needs};
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(skip(self), level = "debug", ret)]
@@ -31,7 +31,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // If there are no arms, that is a diverging match; a special case.
         if arms.is_empty() {
-            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            self.diverges
+                .set(self.diverges.get() | Diverges::Always(DivergeReason::Other, expr.span));
             return tcx.types.never;
         }
 
@@ -150,13 +151,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // we can emit a better note. Rather than pointing
         // at a diverging expression in an arbitrary arm,
         // we can point at the entire `match` expression
-        if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
-            all_arms_diverge = Diverges::Always {
-                span: expr.span,
-                custom_note: Some(
-                    "any code following this `match` expression is unreachable, as all arms diverge",
-                ),
-            };
+        if let (Diverges::Always(..), hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
+            all_arms_diverge = Diverges::Always(DivergeReason::AllArmsDiverge, expr.span);
         }
 
         // We won't diverge unless the scrutinee or all arms diverge.
@@ -253,7 +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/check.rs

@@ -16,7 +16,7 @@ use tracing::{debug, instrument};
 
 use crate::coercion::CoerceMany;
 use crate::gather_locals::GatherLocalsVisitor;
-use crate::{CoroutineTypes, Diverges, FnCtxt};
+use crate::{CoroutineTypes, DivergeReason, Diverges, FnCtxt};
 
 /// Helper used for fns and closures. Does the grungy work of checking a function
 /// body and returns the function context used for that purpose, since in the case of a fn item
@@ -89,10 +89,8 @@ pub(super) fn check_fn<'a, 'tcx>(
         let ty_span = ty.map(|ty| ty.span);
         fcx.check_pat_top(param.pat, param_ty, ty_span, None, None);
         if param.pat.is_never_pattern() {
-            fcx.function_diverges_because_of_empty_arguments.set(Diverges::Always {
-                span: param.pat.span,
-                custom_note: Some("any code following a never pattern is unreachable"),
-            });
+            fcx.function_diverges_because_of_empty_arguments
+                .set(Diverges::Always(DivergeReason::NeverPattern, param.pat.span));
         }
 
         // Check that argument is Sized.

compiler/rustc_hir_typeck/src/diverges.rs

@@ -1,34 +1,30 @@
 use std::{cmp, ops};
 
-use rustc_span::{DUMMY_SP, Span};
+use rustc_hir::HirId;
+use rustc_span::Span;
 
 /// Tracks whether executing a node may exit normally (versus
 /// return/break/panic, which "diverge", leaving dead code in their
 /// wake). Tracked semi-automatically (through type variables marked
 /// as diverging), with some manual adjustments for control-flow
 /// primitives (approximating a CFG).
-#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Copy, Clone, Debug)]
 pub(crate) enum Diverges {
     /// Potentially unknown, some cases converge,
     /// 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 {
-        /// The `Span` points to the expression
-        /// that caused us to diverge
-        /// (e.g. `return`, `break`, etc).
-        span: Span,
-        /// In some cases (e.g. a `match` expression
-        /// where all arms diverge), we may be
-        /// able to provide a more informative
-        /// message to the user.
-        /// If this is `None`, a default message
-        /// will be generated, which is suitable
-        /// for most cases.
-        custom_note: Option<&'static str>,
-    },
+    Always(DivergeReason, Span),
 
     /// Same as `Always` but with a reachability
     /// warning already emitted.
@@ -40,14 +36,15 @@ pub(crate) enum Diverges {
 impl ops::BitAnd for Diverges {
     type Output = Self;
     fn bitand(self, other: Self) -> Self {
-        cmp::min(self, other)
+        cmp::min_by_key(self, other, Self::ordinal)
     }
 }
 
 impl ops::BitOr for Diverges {
     type Output = Self;
     fn bitor(self, other: Self) -> Self {
-        cmp::max(self, other)
+        // argument order is to prefer `self` if ordinal is equal
+        cmp::max_by_key(other, self, Self::ordinal)
     }
 }
 
@@ -64,15 +61,28 @@ impl ops::BitOrAssign for Diverges {
 }
 
 impl Diverges {
-    /// Creates a `Diverges::Always` with the provided `span` and the default note message.
-    pub(super) fn always(span: Span) -> Diverges {
-        Diverges::Always { span, custom_note: None }
+    pub(super) fn is_always(self) -> bool {
+        match self {
+            Self::Maybe | Diverges::UninhabitedExpr(..) | Diverges::Warned => false,
+            Self::Always(..) | Self::WarnedAlways => true,
+        }
     }
 
-    pub(super) fn is_always(self) -> bool {
-        // Enum comparison ignores the
-        // contents of fields, so we just
-        // fill them in with garbage here.
-        self >= Diverges::Always { span: DUMMY_SP, custom_note: None }
+    fn ordinal(&self) -> u8 {
+        match self {
+            Self::Maybe => 0,
+            Self::UninhabitedExpr(..) => 1,
+            Self::Warned => 2,
+            Self::Always { .. } => 3,
+            Self::WarnedAlways => 4,
+        }
     }
 }
+
+#[derive(Clone, Copy, Debug)]
+pub(crate) enum DivergeReason {
+    AllArmsDiverge,
+    NeverPattern,
+    UninhabitedExpr(HirId),
+    Other,
+}

compiler/rustc_hir_typeck/src/expr.rs

@@ -50,8 +50,8 @@ use crate::errors::{
     YieldExprOutsideOfCoroutine,
 };
 use crate::{
-    BreakableCtxt, CoroutineTypes, Diverges, FnCtxt, Needs, cast, fatally_break_rust,
-    report_unexpected_variant_res, type_error_struct,
+    BreakableCtxt, CoroutineTypes, DivergeReason, Diverges, FnCtxt, Needs, cast,
+    fatally_break_rust, report_unexpected_variant_res, type_error_struct,
 };
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -234,19 +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(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.
@@ -352,6 +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(_)
@@ -381,7 +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
@@ -442,6 +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,
@@ -1507,7 +1556,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // of a `break` or an outer `break` or `return`.
             self.diverges.set(Diverges::Maybe);
         } else {
-            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            self.diverges
+                .set(self.diverges.get() | Diverges::Always(DivergeReason::Other, expr.span));
         }
 
         // If we permit break with a value, then result type is
@@ -1610,7 +1660,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 })
                 .unwrap_or_else(|| self.next_ty_var(expr.span));
             let mut coerce = CoerceMany::with_coercion_sites(coerce_to, args);
-            assert_eq!(self.diverges.get(), Diverges::Maybe);
+            assert!(matches!(self.diverges.get(), Diverges::Maybe));
             for e in args {
                 let e_ty = self.check_expr_with_hint(e, coerce_to);
                 let cause = self.misc(e.span);

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::{
@@ -42,14 +43,20 @@ use tracing::{debug, instrument};
 use crate::callee::{self, DeferredCallResolution};
 use crate::errors::{self, CtorIsPrivate};
 use crate::method::{self, MethodCallee};
-use crate::{BreakableCtxt, Diverges, Expectation, FnCtxt, LoweredTy, rvalue_scopes};
+use crate::{
+    BreakableCtxt, DivergeReason, Diverges, Expectation, FnCtxt, LoweredTy, rvalue_scopes,
+};
 
 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 { span: orig_span, custom_note } = 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() {
@@ -71,18 +78,44 @@ 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());
-            lint.span_label(span, msg).span_label(
-                orig_span,
-                custom_note.unwrap_or("any code following this expression is unreachable"),
-            );
+            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::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);
         })
     }