Auto merge of #142390 - cjgillot:mir-liveness, r=<try>

Perform unused assignment and unused variables lints on MIR.

Rebase of #101500

Fixes #51003.

The first commit moves detection of uninhabited types from the current liveness pass to MIR building.

In order to keep the same level of diagnostics, I had to instrument MIR a little more:
- keep for which original local a guard local is created;
- store in the `VarBindingForm` the list of introducer places and whether this was a shorthand pattern.

I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas.

Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.

<!-- homu-ignore:start -->
<!--
If this PR is related to an unstable feature or an otherwise tracked effort,
please link to the relevant tracking issue here. If you don't know of a related
tracking issue or there are none, feel free to ignore this.

This PR will get automatically assigned to a reviewer. In case you would like
a specific user to review your work, you can assign it to them by using

    r? <reviewer name>
-->
<!-- homu-ignore:end -->

Author: rust-bors[bot]

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -140,9 +140,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 // whether or not the right-hand side is a place expression
                 if let LocalInfo::User(BindingForm::Var(VarBindingForm {
                     opt_match_place: Some((opt_match_place, match_span)),
-                    binding_mode: _,
-                    opt_ty_info: _,
-                    pat_span: _,
+                    ..
                 })) = *local_decl.local_info()
                 {
                     let stmt_source_info = self.body.source_info(location);

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -307,8 +307,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         LocalInfo::User(BindingForm::Var(mir::VarBindingForm {
                             binding_mode: BindingMode(ByRef::No, Mutability::Not),
                             opt_ty_info: Some(sp),
-                            opt_match_place: _,
-                            pat_span: _,
+                            ..
                         })) => {
                             if suggest {
                                 err.span_note(sp, "the binding is already a mutable borrow");
@@ -738,6 +737,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 opt_ty_info: _,
                 opt_match_place: _,
                 pat_span,
+                introductions: _,
             })) => pat_span,
             _ => local_decl.source_info.span,
         };

compiler/rustc_interface/src/passes.rs

@@ -990,6 +990,7 @@ fn run_required_analyses(tcx: TyCtxt<'_>) {
                 tcx.ensure_ok().mir_borrowck(def_id)
             }
             tcx.ensure_ok().has_ffi_unwind_calls(def_id);
+            tcx.ensure_ok().check_liveness(def_id);
 
             // If we need to codegen, ensure that we emit all errors from
             // `mir_drops_elaborated_and_const_checked` now, to avoid discovering

compiler/rustc_middle/src/mir/mod.rs

@@ -926,6 +926,8 @@ pub struct VarBindingForm<'tcx> {
     pub opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
     /// The span of the pattern in which this variable was bound.
     pub pat_span: Span,
+    /// For each introduction place, record here the span and whether this was a shorthand pattern.
+    pub introductions: Vec<(Span, /* is_shorthand */ bool)>,
 }
 
 #[derive(Clone, Debug, TyEncodable, TyDecodable)]
@@ -935,7 +937,7 @@ pub enum BindingForm<'tcx> {
     /// Binding for a `self`/`&self`/`&mut self` binding where the type is implicit.
     ImplicitSelf(ImplicitSelfKind),
     /// Reference used in a guard expression to ensure immutability.
-    RefForGuard,
+    RefForGuard(Local),
 }
 
 mod binding_form_impl {
@@ -950,7 +952,7 @@ mod binding_form_impl {
             match self {
                 Var(binding) => binding.hash_stable(hcx, hasher),
                 ImplicitSelf(kind) => kind.hash_stable(hcx, hasher),
-                RefForGuard => (),
+                RefForGuard(local) => local.hash_stable(hcx, hasher),
             }
         }
     }
@@ -1131,12 +1133,8 @@ impl<'tcx> LocalDecl<'tcx> {
         matches!(
             self.local_info(),
             LocalInfo::User(
-                BindingForm::Var(VarBindingForm {
-                    binding_mode: BindingMode(ByRef::No, _),
-                    opt_ty_info: _,
-                    opt_match_place: _,
-                    pat_span: _,
-                }) | BindingForm::ImplicitSelf(ImplicitSelfKind::Imm),
+                BindingForm::Var(VarBindingForm { binding_mode: BindingMode(ByRef::No, _), .. })
+                    | BindingForm::ImplicitSelf(ImplicitSelfKind::Imm),
             )
         )
     }
@@ -1148,12 +1146,8 @@ impl<'tcx> LocalDecl<'tcx> {
         matches!(
             self.local_info(),
             LocalInfo::User(
-                BindingForm::Var(VarBindingForm {
-                    binding_mode: BindingMode(ByRef::No, _),
-                    opt_ty_info: _,
-                    opt_match_place: _,
-                    pat_span: _,
-                }) | BindingForm::ImplicitSelf(_),
+                BindingForm::Var(VarBindingForm { binding_mode: BindingMode(ByRef::No, _), .. })
+                    | BindingForm::ImplicitSelf(_),
             )
         )
     }
@@ -1169,7 +1163,7 @@ impl<'tcx> LocalDecl<'tcx> {
     /// expression that is used to access said variable for the guard of the
     /// match arm.
     pub fn is_ref_for_guard(&self) -> bool {
-        matches!(self.local_info(), LocalInfo::User(BindingForm::RefForGuard))
+        matches!(self.local_info(), LocalInfo::User(BindingForm::RefForGuard(_)))
     }
 
     /// Returns `Some` if this is a reference to a static item that is used to

compiler/rustc_middle/src/mir/statement.rs

@@ -228,7 +228,7 @@ impl<'tcx> PlaceTy<'tcx> {
 impl<V, T> ProjectionElem<V, T> {
     /// Returns `true` if the target of this projection may refer to a different region of memory
     /// than the base.
-    fn is_indirect(&self) -> bool {
+    pub fn is_indirect(&self) -> bool {
         match self {
             Self::Deref => true,
 

compiler/rustc_middle/src/query/mod.rs

@@ -1127,8 +1127,10 @@ rustc_queries! {
         desc { |tcx| "checking privacy in {}", describe_as_module(key.to_local_def_id(), tcx) }
     }
 
-    query check_liveness(key: LocalDefId) {
-        desc { |tcx| "checking liveness of variables in `{}`", tcx.def_path_str(key) }
+    query check_liveness(key: LocalDefId) -> &'tcx rustc_index::bit_set::DenseBitSet<abi::FieldIdx> {
+        arena_cache
+        desc { |tcx| "checking liveness of variables in `{}`", tcx.def_path_str(key.to_def_id()) }
+        cache_on_disk_if(tcx) { tcx.is_typeck_child(key.to_def_id()) }
     }
 
     /// Return the live symbols in the crate for dead code check.

compiler/rustc_middle/src/thir.rs

@@ -777,6 +777,7 @@ pub enum PatKind<'tcx> {
         /// (The same binding can occur multiple times in different branches of
         /// an or-pattern, but only one of them will be primary.)
         is_primary: bool,
+        is_shorthand: bool,
     },
 
     /// `Foo(...)` or `Foo{...}` or `Foo`, where `Foo` is a variant name from an ADT with

compiler/rustc_middle/src/ty/closure.rs

@@ -327,6 +327,11 @@ pub fn place_to_string_for_capture<'tcx>(tcx: TyCtxt<'tcx>, place: &HirPlace<'tc
                     )
                 }
             },
+            HirProjectionKind::UnwrapUnsafeBinder => {
+                curr_string = format!("unwrap_binder!({curr_string})");
+            }
+            // Just change the type to the hidden type, so we can actually project.
+            HirProjectionKind::OpaqueCast => {}
             proj => bug!("{:?} unexpected because it isn't captured", proj),
         }
     }

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/block.rs

@@ -204,6 +204,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                 block,
                                 node,
                                 span,
+                                false,
                                 OutsideGuard,
                                 ScheduleDrops::Yes,
                             );
@@ -296,6 +297,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                 block,
                                 node,
                                 span,
+                                false,
                                 OutsideGuard,
                                 ScheduleDrops::Yes,
                             );

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/matches/match_pair.rs

@@ -160,7 +160,7 @@ impl<'tcx> MatchPairTree<'tcx> {
                 None
             }
 
-            PatKind::Binding { mode, var, ref subpattern, .. } => {
+            PatKind::Binding { mode, var, is_shorthand, ref subpattern, .. } => {
                 // In order to please the borrow checker, when lowering a pattern
                 // like `x @ subpat` we must establish any bindings in `subpat`
                 // before establishing the binding for `x`.
@@ -199,6 +199,7 @@ impl<'tcx> MatchPairTree<'tcx> {
                         source,
                         var_id: var,
                         binding_mode: mode,
+                        is_shorthand,
                     });
                 }
 

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -600,6 +600,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     block,
                     var,
                     irrefutable_pat.span,
+                    false,
                     OutsideGuard,
                     ScheduleDrops::Yes,
                 );
@@ -629,6 +630,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     block,
                     var,
                     irrefutable_pat.span,
+                    false,
                     OutsideGuard,
                     ScheduleDrops::Yes,
                 );
@@ -761,6 +763,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             pattern,
             &ProjectedUserTypesNode::None,
             &mut |this, name, mode, var, span, ty, user_tys| {
+                let saved_scope = this.source_scope;
+                this.set_correct_source_scope_for_arg(var.0, saved_scope, span);
                 let vis_scope = *visibility_scope
                     .get_or_insert_with(|| this.new_source_scope(scope_span, LintLevel::Inherited));
                 let source_info = SourceInfo { span, scope: this.source_scope };
@@ -778,6 +782,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     opt_match_place.map(|(x, y)| (x.cloned(), y)),
                     pattern.span,
                 );
+                this.source_scope = saved_scope;
             },
         );
         if let Some(guard_expr) = guard {
@@ -821,6 +826,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         block: BasicBlock,
         var: LocalVarId,
         span: Span,
+        is_shorthand: bool,
         for_guard: ForGuard,
         schedule_drop: ScheduleDrops,
     ) -> Place<'tcx> {
@@ -834,6 +840,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         {
             self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
         }
+        let local_info = self.local_decls[local_id].local_info.as_mut().unwrap_crate_local();
+        if let LocalInfo::User(BindingForm::Var(var_info)) = &mut **local_info {
+            var_info.introductions.push((span, is_shorthand));
+        }
         Place::from(local_id)
     }
 
@@ -1230,6 +1240,7 @@ struct Binding<'tcx> {
     source: Place<'tcx>,
     var_id: LocalVarId,
     binding_mode: BindingMode,
+    is_shorthand: bool,
 }
 
 /// Indicates that the type of `source` must be a subtype of the
@@ -2693,6 +2704,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block,
                 binding.var_id,
                 binding.span,
+                binding.is_shorthand,
                 RefWithinGuard,
                 schedule_drops,
             );
@@ -2709,6 +2721,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         block,
                         binding.var_id,
                         binding.span,
+                        binding.is_shorthand,
                         OutsideGuard,
                         schedule_drops,
                     );
@@ -2748,6 +2761,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     block,
                     binding.var_id,
                     binding.span,
+                    binding.is_shorthand,
                     OutsideGuard,
                     schedule_drops,
                 ),
@@ -2801,6 +2815,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     opt_ty_info: None,
                     opt_match_place,
                     pat_span,
+                    introductions: Vec::new(),
                 },
             )))),
         };
@@ -2823,7 +2838,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 user_ty: None,
                 source_info,
                 local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(
-                    BindingForm::RefForGuard,
+                    BindingForm::RefForGuard(for_arm_body),
                 ))),
             });
             if self.should_emit_debug_info_for_binding(name, var_id) {