Auto merge of #75573 - Aaron1011:feature/const-mutation-lint, r=oli-obk

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.

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),
             }
         }
 

compiler/rustc_mir_build/src/thir/mod.rs

@@ -273,6 +273,10 @@ crate enum ExprKind<'tcx> {
     Literal {
         literal: &'tcx Const<'tcx>,
         user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+        /// The `DefId` of the `const` item this literal
+        /// was produced from, if this is not a user-written
+        /// literal value.
+        const_id: Option<DefId>,
     },
     /// A literal containing the address of a `static`.
     ///