Report uninhabited call return types on MIR.

Author: cjgillot

compiler/rustc_mir_build/messages.ftl

@@ -346,6 +346,11 @@ mir_build_union_field_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
 mir_build_union_pattern = cannot use unions in constant patterns
     .label = can't use a `union` here
 
+mir_build_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
+
 mir_build_unreachable_making_this_unreachable = collectively making this unreachable
 
 mir_build_unreachable_making_this_unreachable_n_more = ...and {$covered_by_many_n_more_count} other patterns collectively make this unreachable

compiler/rustc_mir_build/src/builder/expr/into.rs

@@ -269,18 +269,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         args,
                         unwind: UnwindAction::Continue,
                         destination,
-                        // The presence or absence of a return edge affects control-flow sensitive
-                        // MIR checks and ultimately whether code is accepted or not. We can only
-                        // omit the return edge if a return type is visibly uninhabited to a module
-                        // that makes the call.
-                        target: expr
-                            .ty
-                            .is_inhabited_from(
-                                this.tcx,
-                                this.parent_module,
-                                this.infcx.typing_env(this.param_env),
-                            )
-                            .then_some(success),
+                        target: Some(success),
                         call_source: if from_hir_call {
                             CallSource::Normal
                         } else {

compiler/rustc_mir_build/src/builder/mod.rs

@@ -23,10 +23,12 @@ use rustc_middle::mir::*;
 use rustc_middle::thir::{self, ExprId, LintLevel, LocalVarId, Param, ParamId, PatKind, Thir};
 use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt, TypeVisitableExt, TypingMode};
 use rustc_middle::{bug, span_bug};
+use rustc_session::lint;
 use rustc_span::{Span, Symbol, sym};
 
 use crate::builder::expr::as_place::PlaceBuilder;
 use crate::builder::scope::DropKind;
+use crate::errors;
 
 pub(crate) fn closure_saved_names_of_captured_variables<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -534,6 +536,7 @@ fn construct_fn<'tcx>(
         return_block.unit()
     });
 
+    builder.lint_and_remove_uninhabited();
     let mut body = builder.finish();
 
     body.spread_arg = if abi == ExternAbi::RustCall {
@@ -591,6 +594,7 @@ fn construct_const<'a, 'tcx>(
 
     builder.build_drop_trees();
 
+    builder.lint_and_remove_uninhabited();
     builder.finish()
 }
 
@@ -789,6 +793,75 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         builder
     }
 
+    fn lint_and_remove_uninhabited(&mut self) {
+        let mut lints = vec![];
+
+        for bbdata in self.cfg.basic_blocks.iter_mut() {
+            let term = bbdata.terminator_mut();
+            let TerminatorKind::Call { ref mut target, destination, .. } = term.kind else {
+                continue;
+            };
+            let Some(target_bb) = *target else { continue };
+
+            let ty = destination.ty(&self.local_decls, self.tcx).ty;
+            let ty_is_inhabited = ty.is_inhabited_from(
+                self.tcx,
+                self.parent_module,
+                self.infcx.typing_env(self.param_env),
+            );
+
+            if !ty_is_inhabited {
+                // 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 `!`.
+                if !ty.is_never() {
+                    lints.push((target_bb, ty, term.source_info.span));
+                }
+
+                // The presence or absence of a return edge affects control-flow sensitive
+                // MIR checks and ultimately whether code is accepted or not. We can only
+                // omit the return edge if a return type is visibly uninhabited to a module
+                // that makes the call.
+                *target = None;
+            }
+        }
+
+        for (target_bb, orig_ty, orig_span) in lints {
+            let target_bb = &self.cfg.basic_blocks[target_bb];
+            let (target_loc, descr) = target_bb
+                .statements
+                .iter()
+                .find_map(|stmt| match stmt.kind {
+                    StatementKind::StorageLive(_) | StatementKind::StorageDead(_) => None,
+                    StatementKind::FakeRead(..) => Some((stmt.source_info, "definition")),
+                    _ => Some((stmt.source_info, "expression")),
+                })
+                .unwrap_or_else(|| (target_bb.terminator().source_info, "expression"));
+            let lint_root = self.source_scopes[target_loc.scope]
+                .local_data
+                .as_ref()
+                .unwrap_crate_local()
+                .lint_root;
+            self.tcx.emit_node_span_lint(
+                lint::builtin::UNREACHABLE_CODE,
+                lint_root,
+                target_loc.span,
+                errors::UnreachableDueToUninhabited {
+                    expr: target_loc.span,
+                    orig: orig_span,
+                    descr,
+                    ty: orig_ty,
+                },
+            );
+        }
+    }
+
     fn finish(self) -> Body<'tcx> {
         let mut body = Body::new(
             MirSource::item(self.def_id.to_def_id()),

compiler/rustc_mir_build/src/errors.rs

@@ -721,6 +721,18 @@ pub(crate) struct WantedConstant {
     pub(crate) const_path: String,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(mir_build_unreachable_due_to_uninhabited)]
+pub(crate) struct UnreachableDueToUninhabited<'desc, 'tcx> {
+    pub descr: &'desc str,
+    #[label]
+    pub expr: Span,
+    #[label(mir_build_label_orig)]
+    #[note]
+    pub orig: Span,
+    pub ty: Ty<'tcx>,
+}
+
 #[derive(Diagnostic)]
 #[diag(mir_build_const_pattern_depends_on_generic_parameter, code = E0158)]
 pub(crate) struct ConstPatternDependsOnGenericParameter {

compiler/rustc_passes/messages.ftl

@@ -726,11 +726,6 @@ 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_argument =
     unrecognized argument
 

compiler/rustc_passes/src/errors.rs

@@ -1539,18 +1539,6 @@ pub(crate) struct ProcMacroBadSig {
     pub kind: ProcMacroKind,
 }
 
-#[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]

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::{BytePos, Span, Symbol, sym};
 use tracing::{debug, instrument};
@@ -1314,52 +1314,7 @@ 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.typing_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.typing_env) { succ } else { self.exit_ln }
     }
 }
 

tests/ui/enum-discriminant/issue-46519.rs

@@ -7,6 +7,7 @@
 #[should_panic(expected = "creating inhabited type")]
 fn test() {
     FontLanguageOverride::system_font(SystemFont::new());
+    //~^ WARNING unreachable expression
 }
 
 pub enum FontLanguageOverride {

tests/ui/enum-discriminant/issue-46519.stderr

@@ -0,0 +1,18 @@
+warning: unreachable expression
+  --> $DIR/issue-46519.rs:9:5
+   |
+LL |     FontLanguageOverride::system_font(SystemFont::new());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-----------------^
+   |     |                                 |
+   |     |                                 any code following this expression is unreachable
+   |     unreachable expression
+   |
+note: this expression has type `SystemFont`, which is uninhabited
+  --> $DIR/issue-46519.rs:9:39
+   |
+LL |     FontLanguageOverride::system_font(SystemFont::new());
+   |                                       ^^^^^^^^^^^^^^^^^
+   = note: `#[warn(unreachable_code)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/intrinsics/panic-uninitialized-zeroed.rs

@@ -5,7 +5,7 @@
 //@ [strict]compile-flags: -Zstrict-init-checks
 //@ needs-subprocess
 
-#![allow(deprecated, invalid_value)]
+#![allow(deprecated, invalid_value, unreachable_code)]
 #![feature(never_type)]
 
 use std::{

tests/ui/lint/dead-code/issue-85071-2.rs

@@ -17,6 +17,6 @@ fn main() {
     let s = S;
     let x = s.f();
     //~^ WARNING: unused variable: `x`
+    //~| WARNING: unreachable definition
     let _y = x;
-    //~^ WARNING: unreachable definition
 }

tests/ui/lint/dead-code/issue-85071-2.stderr

@@ -1,34 +1,33 @@
-warning: unreachable definition
-  --> $DIR/issue-85071-2.rs:20:9
+warning: unused variable: `x`
+  --> $DIR/issue-85071-2.rs:18:9
    |
 LL |     let x = s.f();
-   |             ----- any code following this expression is unreachable
-LL |
-LL |     let _y = x;
-   |         ^^ unreachable definition
-   |
-note: this expression has type `Foo`, which is uninhabited
-  --> $DIR/issue-85071-2.rs:18:13
+   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
    |
-LL |     let x = s.f();
-   |             ^^^^^
 note: the lint level is defined here
-  --> $DIR/issue-85071-2.rs:7:26
+  --> $DIR/issue-85071-2.rs:7:9
    |
 LL | #![warn(unused_variables,unreachable_code)]
-   |                          ^^^^^^^^^^^^^^^^
+   |         ^^^^^^^^^^^^^^^^
 
-warning: unused variable: `x`
+warning: unreachable definition
   --> $DIR/issue-85071-2.rs:18:9
    |
 LL |     let x = s.f();
-   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
+   |         ^   ----- any code following this expression is unreachable
+   |         |
+   |         unreachable definition
+   |
+note: this expression has type `Foo`, which is uninhabited
+  --> $DIR/issue-85071-2.rs:18:13
    |
+LL |     let x = s.f();
+   |             ^^^^^
 note: the lint level is defined here
-  --> $DIR/issue-85071-2.rs:7:9
+  --> $DIR/issue-85071-2.rs:7:26
    |
 LL | #![warn(unused_variables,unreachable_code)]
-   |         ^^^^^^^^^^^^^^^^
+   |                          ^^^^^^^^^^^^^^^^
 
 warning: 2 warnings emitted
 

tests/ui/lint/dead-code/issue-85071.rs

@@ -14,6 +14,6 @@ fn f() -> Foo {todo!()}
 fn main() {
     let x = f();
     //~^ WARNING: unused variable: `x`
+    //~| WARNING: unreachable definition
     let _ = x;
-    //~^ WARNING: unreachable expression
 }