1 use clippy_utils::diagnostics::{span_lint_and_note, span_lint_and_sugg};
2 use clippy_utils::source::snippet_with_macro_callsite;
3 use clippy_utils::ty::{has_drop, is_copy};
5 any_parent_is_automatically_derived, contains_name, get_parent_expr, is_from_proc_macro, match_def_path, paths,
7 use if_chain::if_chain;
8 use rustc_data_structures::fx::FxHashSet;
9 use rustc_errors::Applicability;
10 use rustc_hir::def::Res;
11 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
12 use rustc_lint::{LateContext, LateLintPass};
14 use rustc_session::{declare_tool_lint, impl_lint_pass};
15 use rustc_span::symbol::{Ident, Symbol};
18 declare_clippy_lint! {
20 /// Checks for literal calls to `Default::default()`.
22 /// ### Why is this bad?
23 /// It's easier for the reader if the name of the type is used, rather than the
24 /// generic `Default`.
28 /// let s: String = Default::default();
33 /// let s = String::default();
35 #[clippy::version = "pre 1.29.0"]
36 pub DEFAULT_TRAIT_ACCESS,
38 "checks for literal calls to `Default::default()`"
41 declare_clippy_lint! {
43 /// Checks for immediate reassignment of fields initialized
44 /// with Default::default().
46 /// ### Why is this bad?
47 ///It's more idiomatic to use the [functional update syntax](https://doc.rust-lang.org/reference/expressions/struct-expr.html#functional-update-syntax).
49 /// ### Known problems
50 /// Assignments to patterns that are of tuple type are not linted.
54 /// # #[derive(Default)]
55 /// # struct A { i: i32 }
56 /// let mut a: A = Default::default();
62 /// # #[derive(Default)]
63 /// # struct A { i: i32 }
66 /// .. Default::default()
69 #[clippy::version = "1.49.0"]
70 pub FIELD_REASSIGN_WITH_DEFAULT,
72 "binding initialized with Default should have its fields set in the initializer"
77 // Spans linted by `field_reassign_with_default`.
78 reassigned_linted: FxHashSet<Span>,
81 impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);
83 impl<'tcx> LateLintPass<'tcx> for Default {
84 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
86 if !expr.span.from_expansion();
87 // Avoid cases already linted by `field_reassign_with_default`
88 if !self.reassigned_linted.contains(&expr.span);
89 if let ExprKind::Call(path, ..) = expr.kind;
90 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
91 if let ExprKind::Path(ref qpath) = path.kind;
92 if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
93 if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
94 if !is_update_syntax_base(cx, expr);
95 // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
96 if let QPath::Resolved(None, _path) = qpath;
97 let expr_ty = cx.typeck_results().expr_ty(expr);
98 if let ty::Adt(def, ..) = expr_ty.kind();
99 if !is_from_proc_macro(cx, expr);
101 // TODO: Work out a way to put "whatever the imported way of referencing
102 // this type in this file" rather than a fully-qualified type.
103 let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did()));
106 DEFAULT_TRAIT_ACCESS,
108 &format!("calling `{replacement}` is more clear than this expression"),
111 Applicability::Unspecified, // First resolve the TODO above
117 #[expect(clippy::too_many_lines)]
118 fn check_block(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
119 // start from the `let mut _ = _::default();` and look at all the following
120 // statements, see if they re-assign the fields of the binding
121 let stmts_head = match block.stmts {
122 // Skip the last statement since there cannot possibly be any following statements that re-assign fields.
123 [head @ .., _] if !head.is_empty() => head,
126 for (stmt_idx, stmt) in stmts_head.iter().enumerate() {
127 // find all binding statements like `let mut _ = T::default()` where `T::default()` is the
128 // `default` method of the `Default` trait, and store statement index in current block being
129 // checked and the name of the bound variable
130 let (local, variant, binding_name, binding_type, span) = if_chain! {
131 // only take `let ...` statements
132 if let StmtKind::Local(local) = stmt.kind;
133 if let Some(expr) = local.init;
134 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
135 if !expr.span.from_expansion();
136 // only take bindings to identifiers
137 if let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind;
138 // only when assigning `... = Default::default()`
139 if is_expr_default(expr, cx);
140 let binding_type = cx.typeck_results().node_type(binding_id);
141 if let Some(adt) = binding_type.ty_adt_def();
143 let variant = adt.non_enum_variant();
144 if adt.did().is_local() || !variant.is_field_list_non_exhaustive();
145 let module_did = cx.tcx.parent_module(stmt.hir_id);
149 .all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
150 let all_fields_are_copy = variant
154 is_copy(cx, cx.tcx.type_of(field.did))
156 if !has_drop(cx, binding_type) || all_fields_are_copy;
158 (local, variant, ident.name, binding_type, expr.span)
164 // find all "later statement"'s where the fields of the binding set as
165 // Default::default() get reassigned, unless the reassignment refers to the original binding
166 let mut first_assign = None;
167 let mut assigned_fields = Vec::new();
168 let mut cancel_lint = false;
169 for consecutive_statement in &block.stmts[stmt_idx + 1..] {
170 // find out if and which field was set by this `consecutive_statement`
171 if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consecutive_statement, binding_name) {
172 // interrupt and cancel lint if assign_rhs references the original binding
173 if contains_name(binding_name, assign_rhs) {
178 // if the field was previously assigned, replace the assignment, otherwise insert the assignment
179 if let Some(prev) = assigned_fields
181 .find(|(field_name, _)| field_name == &field_ident.name)
183 *prev = (field_ident.name, assign_rhs);
185 assigned_fields.push((field_ident.name, assign_rhs));
188 // also set first instance of error for help message
189 if first_assign.is_none() {
190 first_assign = Some(consecutive_statement);
193 // interrupt if no field was assigned, since we only want to look at consecutive statements
199 // if there are incorrectly assigned fields, do a span_lint_and_note to suggest
200 // construction using `Ty { fields, ..Default::default() }`
201 if !assigned_fields.is_empty() && !cancel_lint {
202 // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
203 let ext_with_default = !variant
206 .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.name));
208 let field_list = assigned_fields
210 .map(|(field, rhs)| {
211 // extract and store the assigned value for help message
212 let value_snippet = snippet_with_macro_callsite(cx, rhs.span, "..");
213 format!("{field}: {value_snippet}")
215 .collect::<Vec<String>>()
218 // give correct suggestion if generics are involved (see #6944)
219 let binding_type = if_chain! {
220 if let ty::Adt(adt_def, substs) = binding_type.kind();
221 if !substs.is_empty();
223 let adt_def_ty_name = cx.tcx.item_name(adt_def.did());
224 let generic_args = substs.iter().collect::<Vec<_>>();
225 let tys_str = generic_args
227 .map(ToString::to_string)
230 format!("{adt_def_ty_name}::<{}>", &tys_str)
232 binding_type.to_string()
236 let sugg = if ext_with_default {
237 if field_list.is_empty() {
238 format!("{binding_type}::default()")
240 format!("{binding_type} {{ {field_list}, ..Default::default() }}")
243 format!("{binding_type} {{ {field_list} }}")
246 // span lint once per statement that binds default
249 FIELD_REASSIGN_WITH_DEFAULT,
250 first_assign.unwrap().span,
251 "field assignment outside of initializer for an instance created with Default::default()",
253 &format!("consider initializing the variable with `{sugg}` and removing relevant reassignments"),
255 self.reassigned_linted.insert(span);
261 /// Checks if the given expression is the `default` method belonging to the `Default` trait.
262 fn is_expr_default<'tcx>(expr: &'tcx Expr<'tcx>, cx: &LateContext<'tcx>) -> bool {
264 if let ExprKind::Call(fn_expr, _) = &expr.kind;
265 if let ExprKind::Path(qpath) = &fn_expr.kind;
266 if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
268 // right hand side of assignment is `Default::default`
269 match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)
276 /// Returns the reassigned field and the assigning expression (right-hand side of assign).
277 fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Option<(Ident, &'tcx Expr<'tcx>)> {
279 // only take assignments
280 if let StmtKind::Semi(later_expr) = this.kind;
281 if let ExprKind::Assign(assign_lhs, assign_rhs, _) = later_expr.kind;
282 // only take assignments to fields where the left-hand side field is a field of
283 // the same binding as the previous statement
284 if let ExprKind::Field(binding, field_ident) = assign_lhs.kind;
285 if let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind;
286 if let Some(second_binding_name) = path.segments.last();
287 if second_binding_name.ident.name == binding_name;
289 Some((field_ident, assign_rhs))
296 /// Returns whether `expr` is the update syntax base: `Foo { a: 1, .. base }`
297 fn is_update_syntax_base<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
299 if let Some(parent) = get_parent_expr(cx, expr);
300 if let ExprKind::Struct(_, _, Some(base)) = parent.kind;
302 base.hir_id == expr.hir_id