Auto merge of #142390 - cjgillot:mir-liveness, r=davidtwco

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.

Author: bors

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -139,9 +139,7 @@ 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

@@ -336,8 +336,7 @@ 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");
@@ -751,6 +750,7 @@ 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

@@ -1096,6 +1096,7 @@ fn run_required_analyses(tcx: TyCtxt<'_>) {
                 tcx.ensure_ok().check_transmutes(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

@@ -882,6 +882,9 @@ 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,
+    /// A binding can be introduced multiple times, with or patterns:
+    /// `Foo::A { x } | Foo::B { z: x }`. This stores information for each of those introductions.
+    pub introductions: Vec<VarBindingIntroduction>,
 }
 
 #[derive(Clone, Debug, TyEncodable, TyDecodable)]
@@ -891,7 +894,15 @@ 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),
+}
+
+#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct VarBindingIntroduction {
+    /// Where this additional introduction happened.
+    pub span: Span,
+    /// Is that introduction a shorthand struct pattern, i.e. `Foo { x }`.
+    pub is_shorthand: bool,
 }
 
 mod binding_form_impl {
@@ -906,7 +917,7 @@ 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),
             }
         }
     }
@@ -1088,12 +1099,8 @@ 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),
             )
         )
     }
@@ -1105,12 +1112,8 @@ 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(_),
             )
         )
     }
@@ -1126,7 +1129,7 @@ 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

@@ -237,7 +237,7 @@ 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

@@ -1195,8 +1195,10 @@ 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

@@ -798,6 +798,8 @@ 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 this binding a shorthand struct pattern, i.e. `Foo { a }`?
+        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

@@ -383,6 +383,11 @@ 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

@@ -286,6 +286,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                 block,
                                 node,
                                 span,
+                                false,
                                 OutsideGuard,
                                 ScheduleDrops::Yes,
                             );