Diagnose liveness on MIR.

Author: cjgillot

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

@@ -894,7 +894,7 @@ 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),
 }
 
 #[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
@@ -917,7 +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),
             }
         }
     }
@@ -1129,7 +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/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/src/builder/matches/mod.rs

@@ -2864,7 +2864,7 @@ 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) {

compiler/rustc_mir_build/src/builder/mod.rs

@@ -69,11 +69,6 @@ pub fn build_mir<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> Body<'tcx> {
                 }
             };
 
-            // Checking liveness after building the THIR ensures there were no typeck errors.
-            //
-            // maybe move the check to a MIR pass?
-            tcx.ensure_ok().check_liveness(def);
-
             // Don't steal here, instead steal in unsafeck. This is so that
             // pattern inline constants can be evaluated as part of building the
             // THIR of the parent function without a cycle.

compiler/rustc_mir_transform/messages.ftl

@@ -34,12 +34,16 @@ mir_transform_force_inline_attr =
 mir_transform_force_inline_justification =
     `{$callee}` is required to be inlined to: {$sym}
 
+mir_transform_maybe_string_interpolation = you might have meant to use string interpolation in this string literal
+
 mir_transform_must_not_suspend = {$pre}`{$def_path}`{$post} held across a suspend point, but should not be
     .label = the value is held across this suspend point
     .note = {$reason}
     .help = consider using a block (`{"{ ... }"}`) to shrink the value's scope, ending before the suspend point
 mir_transform_operation_will_panic = this operation will panic at runtime
 
+mir_transform_string_interpolation_only_works = string interpolation only works in `format!` invocations
+
 mir_transform_tail_expr_drop_order = relative drop order changing in Rust 2024
     .temporaries = in Rust 2024, this temporary value will be dropped first
     .observers = in Rust 2024, this local variable or temporary value will be dropped second
@@ -77,3 +81,26 @@ mir_transform_unconditional_recursion = function cannot return without recursing
 mir_transform_unconditional_recursion_call_site_label = recursive call site
 
 mir_transform_unknown_pass_name = MIR pass `{$name}` is unknown and will be ignored
+
+mir_transform_unused_assign = value assigned to `{$name}` is never read
+    .help = maybe it is overwritten before being read?
+
+mir_transform_unused_assign_passed = value passed to `{$name}` is never read
+    .help = maybe it is overwritten before being read?
+
+mir_transform_unused_assign_suggestion =
+    you might have meant to mutate the pointed at value being passed in, instead of changing the reference in the local binding
+
+mir_transform_unused_capture_maybe_capture_ref = value captured by `{$name}` is never read
+    .help = did you mean to capture by reference instead?
+
+mir_transform_unused_var_assigned_only = variable `{$name}` is assigned to, but never used
+    .note = consider using `_{$name}` instead
+
+mir_transform_unused_var_underscore = if this is intentional, prefix it with an underscore
+
+mir_transform_unused_variable = unused variable: `{$name}`
+
+mir_transform_unused_variable_args_in_macro = `{$name}` is captured in macro and introduced a unused variable
+
+mir_transform_unused_variable_try_ignore = try ignoring the field

compiler/rustc_mir_transform/src/errors.rs

@@ -1,5 +1,5 @@
 use rustc_errors::codes::*;
-use rustc_errors::{Diag, LintDiagnostic};
+use rustc_errors::{Applicability, Diag, EmissionGuarantee, LintDiagnostic, Subdiagnostic};
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::mir::AssertKind;
 use rustc_middle::query::Key;
@@ -158,6 +158,114 @@ pub(crate) struct FnItemRef {
     pub ident: Ident,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_unused_capture_maybe_capture_ref)]
+#[help]
+pub(crate) struct UnusedCaptureMaybeCaptureRef {
+    pub name: Symbol,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_unused_var_assigned_only)]
+#[note]
+pub(crate) struct UnusedVarAssignedOnly {
+    pub name: Symbol,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_unused_assign)]
+pub(crate) struct UnusedAssign {
+    pub name: Symbol,
+    #[subdiagnostic]
+    pub suggestion: Option<UnusedAssignSuggestion>,
+    #[help]
+    pub help: bool,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(mir_transform_unused_assign_suggestion, applicability = "maybe-incorrect")]
+pub(crate) struct UnusedAssignSuggestion {
+    pub pre: &'static str,
+    #[suggestion_part(code = "{pre}mut ")]
+    pub ty_span: Option<Span>,
+    #[suggestion_part(code = "")]
+    pub ty_ref_span: Span,
+    #[suggestion_part(code = "*")]
+    pub pre_lhs_span: Span,
+    #[suggestion_part(code = "")]
+    pub rhs_borrow_span: Span,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_unused_assign_passed)]
+#[help]
+pub(crate) struct UnusedAssignPassed {
+    pub name: Symbol,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_unused_variable)]
+pub(crate) struct UnusedVariable {
+    pub name: Symbol,
+    #[subdiagnostic]
+    pub string_interp: Vec<UnusedVariableStringInterp>,
+    #[subdiagnostic]
+    pub sugg: UnusedVariableSugg,
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum UnusedVariableSugg {
+    #[multipart_suggestion(
+        mir_transform_unused_variable_try_ignore,
+        applicability = "machine-applicable"
+    )]
+    TryIgnore {
+        #[suggestion_part(code = "{name}: _")]
+        shorthands: Vec<Span>,
+        #[suggestion_part(code = "_")]
+        non_shorthands: Vec<Span>,
+        name: Symbol,
+    },
+
+    #[multipart_suggestion(
+        mir_transform_unused_var_underscore,
+        applicability = "machine-applicable"
+    )]
+    TryPrefix {
+        #[suggestion_part(code = "_{name}")]
+        spans: Vec<Span>,
+        name: Symbol,
+    },
+
+    #[help(mir_transform_unused_variable_args_in_macro)]
+    NoSugg {
+        #[primary_span]
+        span: Span,
+        name: Symbol,
+    },
+}
+
+pub(crate) struct UnusedVariableStringInterp {
+    pub lit: Span,
+}
+
+impl Subdiagnostic for UnusedVariableStringInterp {
+    fn add_to_diag<G: EmissionGuarantee>(self, diag: &mut Diag<'_, G>) {
+        diag.span_label(
+            self.lit,
+            crate::fluent_generated::mir_transform_maybe_string_interpolation,
+        );
+        diag.multipart_suggestion(
+            crate::fluent_generated::mir_transform_string_interpolation_only_works,
+            vec![
+                (self.lit.shrink_to_lo(), String::from("format!(")),
+                (self.lit.shrink_to_hi(), String::from(")")),
+            ],
+            Applicability::MachineApplicable,
+        );
+    }
+}
+
 pub(crate) struct MustNotSupend<'a, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
     pub yield_sp: Span,

compiler/rustc_mir_transform/src/lib.rs

@@ -51,6 +51,7 @@ mod errors;
 mod ffi_unwind_calls;
 mod lint;
 mod lint_tail_expr_drop_order;
+mod liveness;
 mod patch;
 mod shim;
 mod ssa;
@@ -216,6 +217,7 @@ pub fn provide(providers: &mut Providers) {
         mir_for_ctfe,
         mir_coroutine_witnesses: coroutine::mir_coroutine_witnesses,
         optimized_mir,
+        check_liveness: liveness::check_liveness,
         is_mir_available,
         is_ctfe_mir_available: is_mir_available,
         mir_callgraph_cyclic: inline::cycle::mir_callgraph_cyclic,
@@ -513,6 +515,8 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
         }
     }
 
+    tcx.ensure_done().check_liveness(def);
+
     let (body, _) = tcx.mir_promoted(def);
     let mut body = body.steal();