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1b1a35ee XL |
1 | use rustc_errors::DiagnosticBuilder; |
2 | use rustc_middle::lint::LintDiagnosticBuilder; | |
3 | use rustc_middle::mir::visit::Visitor; | |
4 | use rustc_middle::mir::*; | |
5 | use rustc_middle::ty::TyCtxt; | |
6 | use rustc_session::lint::builtin::CONST_ITEM_MUTATION; | |
7 | use rustc_span::def_id::DefId; | |
8 | ||
a2a8927a | 9 | use crate::MirLint; |
1b1a35ee XL |
10 | |
11 | pub struct CheckConstItemMutation; | |
12 | ||
a2a8927a XL |
13 | impl<'tcx> MirLint<'tcx> for CheckConstItemMutation { |
14 | fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) { | |
1b1a35ee XL |
15 | let mut checker = ConstMutationChecker { body, tcx, target_local: None }; |
16 | checker.visit_body(&body); | |
17 | } | |
18 | } | |
19 | ||
20 | struct ConstMutationChecker<'a, 'tcx> { | |
21 | body: &'a Body<'tcx>, | |
22 | tcx: TyCtxt<'tcx>, | |
23 | target_local: Option<Local>, | |
24 | } | |
25 | ||
a2a8927a | 26 | impl<'tcx> ConstMutationChecker<'_, 'tcx> { |
1b1a35ee XL |
27 | fn is_const_item(&self, local: Local) -> Option<DefId> { |
28 | if let Some(box LocalInfo::ConstRef { def_id }) = self.body.local_decls[local].local_info { | |
29 | Some(def_id) | |
30 | } else { | |
31 | None | |
32 | } | |
33 | } | |
34 | ||
35 | fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> { | |
36 | let def_id = self.is_const_item(local)?; | |
1b1a35ee XL |
37 | |
38 | // We avoid linting mutation of a const item if the const's type has a | |
39 | // Drop impl. The Drop logic observes the mutation which was performed. | |
40 | // | |
41 | // pub struct Log { msg: &'static str } | |
42 | // pub const LOG: Log = Log { msg: "" }; | |
43 | // impl Drop for Log { | |
44 | // fn drop(&mut self) { println!("{}", self.msg); } | |
45 | // } | |
46 | // | |
47 | // LOG.msg = "wow"; // prints "wow" | |
48 | // | |
49 | // FIXME(https://github.com/rust-lang/rust/issues/77425): | |
50 | // Drop this exception once there is a stable attribute to suppress the | |
51 | // const item mutation lint for a single specific const only. Something | |
52 | // equivalent to: | |
53 | // | |
54 | // #[const_mutation_allowed] | |
55 | // pub const LOG: Log = Log { msg: "" }; | |
29967ef6 | 56 | match self.tcx.calculate_dtor(def_id, |_, _| Ok(())) { |
1b1a35ee XL |
57 | Some(_) => None, |
58 | None => Some(def_id), | |
59 | } | |
60 | } | |
61 | ||
62 | fn lint_const_item_usage( | |
63 | &self, | |
29967ef6 | 64 | place: &Place<'tcx>, |
1b1a35ee XL |
65 | const_item: DefId, |
66 | location: Location, | |
67 | decorate: impl for<'b> FnOnce(LintDiagnosticBuilder<'b>) -> DiagnosticBuilder<'b>, | |
68 | ) { | |
fc512014 XL |
69 | // Don't lint on borrowing/assigning when a dereference is involved. |
70 | // If we 'leave' the temporary via a dereference, we must | |
71 | // be modifying something else | |
29967ef6 XL |
72 | // |
73 | // `unsafe { *FOO = 0; *BAR.field = 1; }` | |
74 | // `unsafe { &mut *FOO }` | |
fc512014 XL |
75 | // `unsafe { (*ARRAY)[0] = val; } |
76 | if !place.projection.iter().any(|p| matches!(p, PlaceElem::Deref)) { | |
29967ef6 XL |
77 | let source_info = self.body.source_info(location); |
78 | let lint_root = self.body.source_scopes[source_info.scope] | |
79 | .local_data | |
80 | .as_ref() | |
81 | .assert_crate_local() | |
82 | .lint_root; | |
1b1a35ee | 83 | |
29967ef6 XL |
84 | self.tcx.struct_span_lint_hir( |
85 | CONST_ITEM_MUTATION, | |
86 | lint_root, | |
87 | source_info.span, | |
88 | |lint| { | |
89 | decorate(lint) | |
90 | .span_note(self.tcx.def_span(const_item), "`const` item defined here") | |
91 | .emit() | |
92 | }, | |
93 | ); | |
94 | } | |
1b1a35ee XL |
95 | } |
96 | } | |
97 | ||
a2a8927a | 98 | impl<'tcx> Visitor<'tcx> for ConstMutationChecker<'_, 'tcx> { |
1b1a35ee XL |
99 | fn visit_statement(&mut self, stmt: &Statement<'tcx>, loc: Location) { |
100 | if let StatementKind::Assign(box (lhs, _)) = &stmt.kind { | |
101 | // Check for assignment to fields of a constant | |
102 | // Assigning directly to a constant (e.g. `FOO = true;`) is a hard error, | |
103 | // so emitting a lint would be redundant. | |
104 | if !lhs.projection.is_empty() { | |
105 | if let Some(def_id) = self.is_const_item_without_destructor(lhs.local) { | |
29967ef6 XL |
106 | self.lint_const_item_usage(&lhs, def_id, loc, |lint| { |
107 | let mut lint = lint.build("attempting to modify a `const` item"); | |
108 | lint.note("each usage of a `const` item creates a new temporary; the original `const` item will not be modified"); | |
109 | lint | |
110 | }) | |
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111 | } |
112 | } | |
113 | // We are looking for MIR of the form: | |
114 | // | |
115 | // ``` | |
116 | // _1 = const FOO; | |
117 | // _2 = &mut _1; | |
118 | // method_call(_2, ..) | |
119 | // ``` | |
120 | // | |
121 | // Record our current LHS, so that we can detect this | |
122 | // pattern in `visit_rvalue` | |
123 | self.target_local = lhs.as_local(); | |
124 | } | |
125 | self.super_statement(stmt, loc); | |
126 | self.target_local = None; | |
127 | } | |
128 | fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, loc: Location) { | |
129 | if let Rvalue::Ref(_, BorrowKind::Mut { .. }, place) = rvalue { | |
130 | let local = place.local; | |
131 | if let Some(def_id) = self.is_const_item(local) { | |
132 | // If this Rvalue is being used as the right-hand side of a | |
133 | // `StatementKind::Assign`, see if it ends up getting used as | |
134 | // the `self` parameter of a method call (as the terminator of our current | |
135 | // BasicBlock). If so, we emit a more specific lint. | |
136 | let method_did = self.target_local.and_then(|target_local| { | |
137 | crate::util::find_self_call(self.tcx, &self.body, target_local, loc.block) | |
138 | }); | |
139 | let lint_loc = | |
140 | if method_did.is_some() { self.body.terminator_loc(loc.block) } else { loc }; | |
29967ef6 | 141 | self.lint_const_item_usage(place, def_id, lint_loc, |lint| { |
1b1a35ee XL |
142 | let mut lint = lint.build("taking a mutable reference to a `const` item"); |
143 | lint | |
144 | .note("each usage of a `const` item creates a new temporary") | |
145 | .note("the mutable reference will refer to this temporary, not the original `const` item"); | |
146 | ||
147 | if let Some((method_did, _substs)) = method_did { | |
148 | lint.span_note(self.tcx.def_span(method_did), "mutable reference created due to call to this method"); | |
149 | } | |
150 | ||
151 | lint | |
152 | }); | |
153 | } | |
154 | } | |
155 | self.super_rvalue(rvalue, loc); | |
156 | } | |
157 | } |