Add CONST_ITEM_MUTATION lint

Fixes #74053
Fixes #55721

This PR adds a new lint `CONST_ITEM_MUTATION`.
Given an item `const FOO: SomeType = ..`, this lint fires on:

* Attempting to write directly to a field (`FOO.field = some_val`) or
  array entry (`FOO.array_field[0] = val`)
* Taking a mutable reference to the `const` item (`&mut FOO`), including
  through an autoderef `FOO.some_mut_self_method()`

The lint message explains that since each use of a constant creates a
new temporary, the original `const` item will not be modified.

Author: Aaron1011

compiler/rustc_middle/src/mir/mod.rs

@@ -899,6 +899,8 @@ pub enum LocalInfo<'tcx> {
     User(ClearCrossCrate<BindingForm<'tcx>>),
     /// A temporary created that references the static with the given `DefId`.
     StaticRef { def_id: DefId, is_thread_local: bool },
+    /// A temporary created that references the const with the given `DefId`
+    ConstRef { def_id: DefId },
 }
 
 impl<'tcx> LocalDecl<'tcx> {

compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs

@@ -804,68 +804,51 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         debug!("move_spans: target_temp = {:?}", target_temp);
 
         if let Some(Terminator {
-            kind: TerminatorKind::Call { func, args, fn_span, from_hir_call, .. },
-            ..
+            kind: TerminatorKind::Call { fn_span, from_hir_call, .. }, ..
         }) = &self.body[location.block].terminator
         {
-            let mut method_did = None;
-            if let Operand::Constant(box Constant { literal: ty::Const { ty, .. }, .. }) = func {
-                if let ty::FnDef(def_id, _) = *ty.kind() {
-                    debug!("move_spans: fn = {:?}", def_id);
-                    if let Some(ty::AssocItem { fn_has_self_parameter, .. }) =
-                        self.infcx.tcx.opt_associated_item(def_id)
-                    {
-                        if *fn_has_self_parameter {
-                            method_did = Some(def_id);
-                        }
-                    }
-                }
-            }
+            let method_did = if let Some(method_did) =
+                crate::util::find_self_call(self.infcx.tcx, &self.body, target_temp, location.block)
+            {
+                method_did
+            } else {
+                return normal_ret;
+            };
 
             let tcx = self.infcx.tcx;
-            let method_did = if let Some(did) = method_did { did } else { return normal_ret };
-
-            if let [Operand::Move(self_place), ..] = **args {
-                if self_place.as_local() == Some(target_temp) {
-                    let parent = tcx.parent(method_did);
-                    let is_fn_once = parent == tcx.lang_items().fn_once_trait();
-                    let is_operator = !from_hir_call
-                        && parent.map_or(false, |p| {
-                            tcx.lang_items().group(LangItemGroup::Op).contains(&p)
-                        });
-                    let fn_call_span = *fn_span;
-
-                    let self_arg = tcx.fn_arg_names(method_did)[0];
-
-                    let kind = if is_fn_once {
-                        FnSelfUseKind::FnOnceCall
-                    } else if is_operator {
-                        FnSelfUseKind::Operator { self_arg }
-                    } else {
-                        debug!(
-                            "move_spans: method_did={:?}, fn_call_span={:?}",
-                            method_did, fn_call_span
-                        );
-                        let implicit_into_iter = matches!(
-                            fn_call_span.desugaring_kind(),
-                            Some(DesugaringKind::ForLoop(ForLoopLoc::IntoIter))
-                        );
-                        FnSelfUseKind::Normal { self_arg, implicit_into_iter }
-                    };
 
-                    return FnSelfUse {
-                        var_span: stmt.source_info.span,
-                        fn_call_span,
-                        fn_span: self
-                            .infcx
-                            .tcx
-                            .sess
-                            .source_map()
-                            .guess_head_span(self.infcx.tcx.def_span(method_did)),
-                        kind,
-                    };
-                }
-            }
+            let parent = tcx.parent(method_did);
+            let is_fn_once = parent == tcx.lang_items().fn_once_trait();
+            let is_operator = !from_hir_call
+                && parent.map_or(false, |p| tcx.lang_items().group(LangItemGroup::Op).contains(&p));
+            let fn_call_span = *fn_span;
+
+            let self_arg = tcx.fn_arg_names(method_did)[0];
+
+            let kind = if is_fn_once {
+                FnSelfUseKind::FnOnceCall
+            } else if is_operator {
+                FnSelfUseKind::Operator { self_arg }
+            } else {
+                debug!("move_spans: method_did={:?}, fn_call_span={:?}", method_did, fn_call_span);
+                let implicit_into_iter = matches!(
+                    fn_call_span.desugaring_kind(),
+                    Some(DesugaringKind::ForLoop(ForLoopLoc::IntoIter))
+                );
+                FnSelfUseKind::Normal { self_arg, implicit_into_iter }
+            };
+
+            return FnSelfUse {
+                var_span: stmt.source_info.span,
+                fn_call_span,
+                fn_span: self
+                    .infcx
+                    .tcx
+                    .sess
+                    .source_map()
+                    .guess_head_span(self.infcx.tcx.def_span(method_did)),
+                kind,
+            };
         }
         normal_ret
     }

compiler/rustc_mir/src/transform/check_const_item_mutation.rs

@@ -0,0 +1,114 @@
+use rustc_errors::DiagnosticBuilder;
+use rustc_middle::lint::LintDiagnosticBuilder;
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::lint::builtin::CONST_ITEM_MUTATION;
+use rustc_span::def_id::DefId;
+
+use crate::transform::{MirPass, MirSource};
+
+pub struct CheckConstItemMutation;
+
+impl<'tcx> MirPass<'tcx> for CheckConstItemMutation {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, _src: MirSource<'tcx>, body: &mut Body<'tcx>) {
+        let mut checker = ConstMutationChecker { body, tcx, target_local: None };
+        checker.visit_body(&body);
+    }
+}
+
+struct ConstMutationChecker<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    target_local: Option<Local>,
+}
+
+impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
+    fn is_const_item(&self, local: Local) -> Option<DefId> {
+        if let Some(box LocalInfo::ConstRef { def_id }) = self.body.local_decls[local].local_info {
+            Some(def_id)
+        } else {
+            None
+        }
+    }
+    fn lint_const_item_usage(
+        &self,
+        const_item: DefId,
+        location: Location,
+        decorate: impl for<'b> FnOnce(LintDiagnosticBuilder<'b>) -> DiagnosticBuilder<'b>,
+    ) {
+        let source_info = self.body.source_info(location);
+        let lint_root = self.body.source_scopes[source_info.scope]
+            .local_data
+            .as_ref()
+            .assert_crate_local()
+            .lint_root;
+
+        self.tcx.struct_span_lint_hir(CONST_ITEM_MUTATION, lint_root, source_info.span, |lint| {
+            decorate(lint)
+                .span_note(self.tcx.def_span(const_item), "`const` item defined here")
+                .emit()
+        });
+    }
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
+    fn visit_statement(&mut self, stmt: &Statement<'tcx>, loc: Location) {
+        if let StatementKind::Assign(box (lhs, _)) = &stmt.kind {
+            // Check for assignment to fields of a constant
+            // Assigning directly to a constant (e.g. `FOO = true;`) is a hard error,
+            // so emitting a lint would be redundant.
+            if !lhs.projection.is_empty() {
+                if let Some(def_id) = self.is_const_item(lhs.local) {
+                    self.lint_const_item_usage(def_id, loc, |lint| {
+                        let mut lint = lint.build("attempting to modify a `const` item");
+                        lint.note("each usage of a `const` item creates a new temporary - the original `const` item will not be modified");
+                        lint
+                    })
+                }
+            }
+            // We are looking for MIR of the form:
+            //
+            // ```
+            // _1 = const FOO;
+            // _2 = &mut _1;
+            // method_call(_2, ..)
+            // ```
+            //
+            // Record our current LHS, so that we can detect this
+            // pattern in `visit_rvalue`
+            self.target_local = lhs.as_local();
+        }
+        self.super_statement(stmt, loc);
+        self.target_local = None;
+    }
+    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, loc: Location) {
+        if let Rvalue::Ref(_, BorrowKind::Mut { .. }, place) = rvalue {
+            let local = place.local;
+            if let Some(def_id) = self.is_const_item(local) {
+                // If this Rvalue is being used as the right-hand side of a
+                // `StatementKind::Assign`, see if it ends up getting used as
+                // the `self` parameter of a method call (as the terminator of our current
+                // BasicBlock). If so, we emit a more specific lint.
+                let method_did = self.target_local.and_then(|target_local| {
+                    crate::util::find_self_call(self.tcx, &self.body, target_local, loc.block)
+                });
+                let lint_loc =
+                    if method_did.is_some() { self.body.terminator_loc(loc.block) } else { loc };
+                self.lint_const_item_usage(def_id, lint_loc, |lint| {
+                    let mut lint = lint.build("taking a mutable reference to a `const` item");
+                    lint
+                        .note("each usage of a `const` item creates a new temporary")
+                        .note("the mutable reference will refer to this temporary, not the original `const` item");
+
+                    if let Some(method_did) = method_did {
+                        lint.span_note(self.tcx.def_span(method_did), "mutable reference created due to call to this method");
+                    }
+
+                    lint
+                });
+            }
+        }
+        self.super_rvalue(rvalue, loc);
+    }
+}

compiler/rustc_mir/src/transform/mod.rs

@@ -16,6 +16,7 @@ use std::borrow::Cow;
 pub mod add_call_guards;
 pub mod add_moves_for_packed_drops;
 pub mod add_retag;
+pub mod check_const_item_mutation;
 pub mod check_consts;
 pub mod check_packed_ref;
 pub mod check_unsafety;
@@ -307,6 +308,7 @@ fn mir_const<'tcx>(
         &[&[
             // MIR-level lints.
             &check_packed_ref::CheckPackedRef,
+            &check_const_item_mutation::CheckConstItemMutation,
             // What we need to do constant evaluation.
             &simplify::SimplifyCfg::new("initial"),
             &rustc_peek::SanityCheck,

compiler/rustc_mir/src/util/find_self_call.rs

@@ -0,0 +1,35 @@
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_span::def_id::DefId;
+
+/// Checks if the specified `local` is used as the `self` prameter of a method call
+/// in the provided `BasicBlock`. If it is, then the `DefId` of the called method is
+/// returned.
+pub fn find_self_call(
+    tcx: TyCtxt<'_>,
+    body: &Body<'_>,
+    local: Local,
+    block: BasicBlock,
+) -> Option<DefId> {
+    debug!("find_self_call(local={:?}): terminator={:?}", local, &body[block].terminator);
+    if let Some(Terminator { kind: TerminatorKind::Call { func, args, .. }, .. }) =
+        &body[block].terminator
+    {
+        debug!("find_self_call: func={:?}", func);
+        if let Operand::Constant(box Constant { literal: ty::Const { ty, .. }, .. }) = func {
+            if let ty::FnDef(def_id, _) = *ty.kind() {
+                if let Some(ty::AssocItem { fn_has_self_parameter: true, .. }) =
+                    tcx.opt_associated_item(def_id)
+                {
+                    debug!("find_self_call: args={:?}", args);
+                    if let [Operand::Move(self_place) | Operand::Copy(self_place), ..] = **args {
+                        if self_place.as_local() == Some(local) {
+                            return Some(def_id);
+                        }
+                    }
+                }
+            }
+        }
+    }
+    None
+}

compiler/rustc_mir/src/util/mod.rs

@@ -7,12 +7,14 @@ pub mod storage;
 
 mod alignment;
 pub mod collect_writes;
+mod find_self_call;
 mod graphviz;
 pub(crate) mod pretty;
 pub(crate) mod spanview;
 
 pub use self::aggregate::expand_aggregate;
 pub use self::alignment::is_disaligned;
+pub use self::find_self_call::find_self_call;
 pub use self::graphviz::write_node_label as write_graphviz_node_label;
 pub use self::graphviz::{graphviz_safe_def_name, write_mir_graphviz};
 pub use self::pretty::{dump_enabled, dump_mir, write_mir_pretty, PassWhere};

compiler/rustc_mir_build/src/build/expr/as_constant.rs

@@ -21,7 +21,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let Expr { ty, temp_lifetime: _, span, kind } = expr;
         match kind {
             ExprKind::Scope { region_scope: _, lint_level: _, value } => this.as_constant(value),
-            ExprKind::Literal { literal, user_ty } => {
+            ExprKind::Literal { literal, user_ty, const_id: _ } => {
                 let user_ty = user_ty.map(|user_ty| {
                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span,

compiler/rustc_mir_build/src/build/expr/as_temp.rs

@@ -76,6 +76,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     local_decl.local_info =
                         Some(box LocalInfo::StaticRef { def_id, is_thread_local: true });
                 }
+                ExprKind::Literal { const_id: Some(def_id), .. } => {
+                    local_decl.local_info = Some(box LocalInfo::ConstRef { def_id });
+                }
                 _ => {}
             }
             this.local_decls.push(local_decl)

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -247,6 +247,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
         hir::ExprKind::Lit(ref lit) => ExprKind::Literal {
             literal: cx.const_eval_literal(&lit.node, expr_ty, lit.span, false),
             user_ty: None,
+            const_id: None,
         },
 
         hir::ExprKind::Binary(op, ref lhs, ref rhs) => {
@@ -306,6 +307,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
                     ExprKind::Literal {
                         literal: cx.const_eval_literal(&lit.node, expr_ty, lit.span, true),
                         user_ty: None,
+                        const_id: None,
                     }
                 } else {
                     ExprKind::Unary { op: UnOp::Neg, arg: arg.to_ref() }
@@ -447,6 +449,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
                                             kind: ExprKind::Literal {
                                                 literal: ty::Const::zero_sized(cx.tcx, ty),
                                                 user_ty,
+                                                const_id: None,
                                             },
                                         }
                                         .to_ref(),
@@ -473,6 +476,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
                                             kind: ExprKind::Literal {
                                                 literal: ty::Const::zero_sized(cx.tcx, ty),
                                                 user_ty: None,
+                                                const_id: None,
                                             },
                                         }
                                         .to_ref(),
@@ -585,7 +589,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
                             temp_lifetime,
                             ty: var_ty,
                             span: expr.span,
-                            kind: ExprKind::Literal { literal, user_ty: None },
+                            kind: ExprKind::Literal { literal, user_ty: None, const_id: None },
                         }
                         .to_ref()
                     };
@@ -714,7 +718,11 @@ fn method_callee<'a, 'tcx>(
         temp_lifetime,
         ty,
         span,
-        kind: ExprKind::Literal { literal: ty::Const::zero_sized(cx.tcx(), ty), user_ty },
+        kind: ExprKind::Literal {
+            literal: ty::Const::zero_sized(cx.tcx(), ty),
+            user_ty,
+            const_id: None,
+        },
     }
 }
 
@@ -777,6 +785,7 @@ fn convert_path_expr<'a, 'tcx>(
             ExprKind::Literal {
                 literal: ty::Const::zero_sized(cx.tcx, cx.typeck_results().node_type(expr.hir_id)),
                 user_ty,
+                const_id: None,
             }
         }
 
@@ -794,6 +803,7 @@ fn convert_path_expr<'a, 'tcx>(
                     .tcx
                     .mk_const(ty::Const { val, ty: cx.typeck_results().node_type(expr.hir_id) }),
                 user_ty: None,
+                const_id: Some(def_id),
             }
         }
 
@@ -810,6 +820,7 @@ fn convert_path_expr<'a, 'tcx>(
                     ty: cx.typeck_results().node_type(expr.hir_id),
                 }),
                 user_ty,
+                const_id: Some(def_id),
             }
         }