Check for uninhabited types in typeck

Author: camsteffen

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
@@ -72,5 +73,6 @@ impl Diverges {
 pub enum DivergeReason {
     AllArmsDiverge,
     NeverPattern,
+    UninhabitedExpr(HirId),
     Other,
 }

compiler/rustc_hir_typeck/src/expr.rs

@@ -230,17 +230,40 @@ 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, _) => {
+                let emit_warning = if let ExprKind::Path(ref qpath) = callee.kind {
+                    // Do not emit a warning for a call to a constructor.
+                    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 and the field are both uninhabited
+            ExprKind::Field(..)
+                if matches!(
+                    self.diverges.get(),
+                    Diverges::Always(DivergeReason::UninhabitedExpr(_), _)
+                ) && self.ty_is_uninhabited(ty) => {}
             _ => self.warn_if_unreachable(expr.hir_id, expr.span, "expression"),
         }
 
-        // Any expression that produces a value of type `!` must have diverged
-        if ty.is_never() {
-            self.diverges
-                .set(self.diverges.get() | Diverges::Always(DivergeReason::Other, expr.span));
+        if !self.diverges.get().is_always() {
+            if ty.is_never() {
+                // Any expression that produces a value of type `!` must have diverged.
+                self.diverges.set(Diverges::Always(DivergeReason::Other, expr.span));
+            } else if expr_may_be_uninhabited(expr) && self.ty_is_uninhabited(ty) {
+                // This expression produces a value of uninhabited type.
+                // This means it has diverged somehow.
+                self.diverges
+                    .set(Diverges::Always(DivergeReason::UninhabitedExpr(expr.hir_id), expr.span));
+            }
         }
 
         // Record the type, which applies it effects.
@@ -257,6 +280,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ty
     }
 
+    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,
@@ -3524,3 +3554,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.tcx.types.usize
     }
 }
+
+fn expr_may_be_uninhabited(expr: &hir::Expr<'_>) -> bool {
+    match expr.kind {
+        ExprKind::Call(..)
+        | ExprKind::MethodCall(..)
+        | ExprKind::Cast(..)
+        | ExprKind::Unary(hir::UnOp::Deref, _)
+        | ExprKind::Field(..)
+        | ExprKind::Path(..)
+        | ExprKind::Struct(..) => true,
+        ExprKind::ConstBlock(..)
+        | ExprKind::Array(..)
+        | ExprKind::Tup(..)
+        | ExprKind::Binary(..)
+        | ExprKind::Unary(hir::UnOp::Neg | hir::UnOp::Not, _)
+        | ExprKind::Lit(..)
+        | ExprKind::Type(..)
+        | ExprKind::DropTemps(..)
+        | ExprKind::OffsetOf(..)
+        | ExprKind::Let(..)
+        | ExprKind::If(..)
+        | ExprKind::Loop(..)
+        | ExprKind::Match(..)
+        | ExprKind::Closure(..)
+        | ExprKind::Block(..)
+        | ExprKind::Assign(..)
+        | ExprKind::AssignOp(..)
+        | ExprKind::Index(..)
+        | ExprKind::AddrOf(..)
+        | ExprKind::Break(..)
+        | ExprKind::Continue(..)
+        | ExprKind::Ret(..)
+        | ExprKind::Become(..)
+        | ExprKind::InlineAsm(..)
+        | ExprKind::Repeat(..)
+        | ExprKind::Yield(..)
+        | ExprKind::Err(_) => false,
+    }
+}

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;
 
@@ -76,17 +77,35 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Don't warn twice.
         self.diverges.set(Diverges::WarnedAlways);
 
+        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;
+                }
+            }
+        }
+
         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 +49,8 @@ pub(crate) struct FnCtxt<'a, 'tcx> {
     /// eventually).
     pub(super) param_env: ty::ParamEnv<'tcx>,
 
+    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 +129,7 @@ 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/src/liveness.rs

@@ -94,7 +94,7 @@ use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet};
 use rustc_index::IndexVec;
 use rustc_middle::query::Providers;
 use rustc_middle::span_bug;
-use rustc_middle::ty::{self, RootVariableMinCaptureList, Ty, TyCtxt};
+use rustc_middle::ty::{self, RootVariableMinCaptureList, TyCtxt};
 use rustc_session::lint;
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::{BytePos, Span};
@@ -119,8 +119,8 @@ rustc_index::newtype_index! {
 #[derive(Copy, Clone, PartialEq, Debug)]
 enum LiveNodeKind {
     UpvarNode(Span),
-    ExprNode(Span, HirId),
-    VarDefNode(Span, HirId),
+    ExprNode(Span),
+    VarDefNode(Span),
     ClosureNode,
     ExitNode,
     ErrNode,
@@ -130,8 +130,8 @@ fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
     let sm = tcx.sess.source_map();
     match lnk {
         UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_diagnostic_string(s)),
-        ExprNode(s, _) => format!("Expr node [{}]", sm.span_to_diagnostic_string(s)),
-        VarDefNode(s, _) => format!("Var def node [{}]", sm.span_to_diagnostic_string(s)),
+        ExprNode(s) => format!("Expr node [{}]", sm.span_to_diagnostic_string(s)),
+        VarDefNode(s) => format!("Var def node [{}]", sm.span_to_diagnostic_string(s)),
         ClosureNode => "Closure node".to_owned(),
         ExitNode => "Exit node".to_owned(),
         ErrNode => "Error node".to_owned(),
@@ -331,7 +331,7 @@ impl<'tcx> IrMaps<'tcx> {
         let shorthand_field_ids = self.collect_shorthand_field_ids(pat);
 
         pat.each_binding(|_, hir_id, _, ident| {
-            self.add_live_node_for_node(hir_id, VarDefNode(ident.span, hir_id));
+            self.add_live_node_for_node(hir_id, VarDefNode(ident.span));
             self.add_variable(Local(LocalInfo {
                 id: hir_id,
                 name: ident.name,
@@ -345,7 +345,7 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
     fn visit_local(&mut self, local: &'tcx hir::LetStmt<'tcx>) {
         self.add_from_pat(local.pat);
         if local.els.is_some() {
-            self.add_live_node_for_node(local.hir_id, ExprNode(local.span, local.hir_id));
+            self.add_live_node_for_node(local.hir_id, ExprNode(local.span));
         }
         intravisit::walk_local(self, local);
     }
@@ -377,13 +377,13 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             hir::ExprKind::Path(hir::QPath::Resolved(_, path)) => {
                 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
                 if let Res::Local(_var_hir_id) = path.res {
-                    self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
+                    self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
                 }
             }
             hir::ExprKind::Closure(closure) => {
                 // Interesting control flow (for loops can contain labeled
                 // breaks or continues)
-                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
 
                 // Make a live_node for each mentioned variable, with the span
                 // being the location that the variable is used. This results
@@ -409,15 +409,15 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             | hir::ExprKind::Match(..)
             | hir::ExprKind::Loop(..)
             | hir::ExprKind::Yield(..) => {
-                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
             }
             hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
-                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
             }
 
             // Inline assembly may contain labels.
             hir::ExprKind::InlineAsm(asm) if asm.contains_label() => {
-                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
+                self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
                 intravisit::walk_expr(self, expr);
             }
 
@@ -1297,52 +1297,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     fn check_is_ty_uninhabited(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
         let ty = self.typeck_results.expr_ty(expr);
         let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
-        if ty.is_inhabited_from(self.ir.tcx, m, self.param_env) {
-            return succ;
-        }
-        match self.ir.lnks[succ] {
-            LiveNodeKind::ExprNode(succ_span, succ_id) => {
-                self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "expression");
-            }
-            LiveNodeKind::VarDefNode(succ_span, succ_id) => {
-                self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "definition");
-            }
-            _ => {}
-        };
-        self.exit_ln
-    }
-
-    fn warn_about_unreachable<'desc>(
-        &mut self,
-        orig_span: Span,
-        orig_ty: Ty<'tcx>,
-        expr_span: Span,
-        expr_id: HirId,
-        descr: &'desc str,
-    ) {
-        if !orig_ty.is_never() {
-            // Unreachable code warnings are already emitted during type checking.
-            // However, during type checking, full type information is being
-            // calculated but not yet available, so the check for diverging
-            // expressions due to uninhabited result types is pretty crude and
-            // only checks whether ty.is_never(). Here, we have full type
-            // information available and can issue warnings for less obviously
-            // uninhabited types (e.g. empty enums). The check above is used so
-            // that we do not emit the same warning twice if the uninhabited type
-            // is indeed `!`.
-
-            self.ir.tcx.emit_node_span_lint(
-                lint::builtin::UNREACHABLE_CODE,
-                expr_id,
-                expr_span,
-                errors::UnreachableDueToUninhabited {
-                    expr: expr_span,
-                    orig: orig_span,
-                    descr,
-                    ty: orig_ty,
-                },
-            );
-        }
+        if ty.is_inhabited_from(self.ir.tcx, m, self.param_env) { succ } else { self.exit_ln }
     }
 }