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::{any_parent_is_automatically_derived, contains_name, in_macro, match_def_path, paths};
4 use if_chain::if_chain;
5 use rustc_data_structures::fx::FxHashSet;
6 use rustc_errors::Applicability;
7 use rustc_hir::def::Res;
8 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
9 use rustc_lint::{LateContext, LateLintPass};
10 use rustc_middle::lint::in_external_macro;
12 use rustc_session::{declare_tool_lint, impl_lint_pass};
13 use rustc_span::symbol::{Ident, Symbol};
16 declare_clippy_lint! {
17 /// **What it does:** Checks for literal calls to `Default::default()`.
19 /// **Why is this bad?** It's more clear to the reader to use the name of the type whose default is
20 /// being gotten than the generic `Default`.
22 /// **Known problems:** None.
27 /// let s: String = Default::default();
30 /// let s = String::default();
32 pub DEFAULT_TRAIT_ACCESS,
34 "checks for literal calls to `Default::default()`"
37 declare_clippy_lint! {
38 /// **What it does:** Checks for immediate reassignment of fields initialized
39 /// with Default::default().
41 /// **Why is this bad?**It's more idiomatic to use the [functional update syntax](https://doc.rust-lang.org/reference/expressions/struct-expr.html#functional-update-syntax).
43 /// **Known problems:** Assignments to patterns that are of tuple type are not linted.
48 /// # #[derive(Default)]
49 /// # struct A { i: i32 }
50 /// let mut a: A = Default::default();
55 /// # #[derive(Default)]
56 /// # struct A { i: i32 }
59 /// .. Default::default()
62 pub FIELD_REASSIGN_WITH_DEFAULT,
64 "binding initialized with Default should have its fields set in the initializer"
69 // Spans linted by `field_reassign_with_default`.
70 reassigned_linted: FxHashSet<Span>,
73 impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);
75 impl LateLintPass<'_> for Default {
76 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
78 if !in_macro(expr.span);
79 // Avoid cases already linted by `field_reassign_with_default`
80 if !self.reassigned_linted.contains(&expr.span);
81 if let ExprKind::Call(path, ..) = expr.kind;
82 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
83 if let ExprKind::Path(ref qpath) = path.kind;
84 if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
85 if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
86 // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
87 if let QPath::Resolved(None, _path) = qpath;
88 let expr_ty = cx.typeck_results().expr_ty(expr);
89 if let ty::Adt(def, ..) = expr_ty.kind();
91 // TODO: Work out a way to put "whatever the imported way of referencing
92 // this type in this file" rather than a fully-qualified type.
93 let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did));
98 &format!("calling `{}` is more clear than this expression", replacement),
101 Applicability::Unspecified, // First resolve the TODO above
107 #[allow(clippy::too_many_lines)]
108 fn check_block<'tcx>(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
109 // start from the `let mut _ = _::default();` and look at all the following
110 // statements, see if they re-assign the fields of the binding
111 let stmts_head = match block.stmts {
112 // Skip the last statement since there cannot possibly be any following statements that re-assign fields.
113 [head @ .., _] if !head.is_empty() => head,
116 for (stmt_idx, stmt) in stmts_head.iter().enumerate() {
117 // find all binding statements like `let mut _ = T::default()` where `T::default()` is the
118 // `default` method of the `Default` trait, and store statement index in current block being
119 // checked and the name of the bound variable
120 let (local, variant, binding_name, binding_type, span) = if_chain! {
121 // only take `let ...` statements
122 if let StmtKind::Local(local) = stmt.kind;
123 if let Some(expr) = local.init;
124 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
125 if !in_external_macro(cx.tcx.sess, expr.span);
126 // only take bindings to identifiers
127 if let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind;
128 // only when assigning `... = Default::default()`
129 if is_expr_default(expr, cx);
130 let binding_type = cx.typeck_results().node_type(binding_id);
131 if let Some(adt) = binding_type.ty_adt_def();
133 let variant = adt.non_enum_variant();
134 if adt.did.is_local() || !variant.is_field_list_non_exhaustive();
135 let module_did = cx.tcx.parent_module(stmt.hir_id).to_def_id();
139 .all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
141 (local, variant, ident.name, binding_type, expr.span)
147 // find all "later statement"'s where the fields of the binding set as
148 // Default::default() get reassigned, unless the reassignment refers to the original binding
149 let mut first_assign = None;
150 let mut assigned_fields = Vec::new();
151 let mut cancel_lint = false;
152 for consecutive_statement in &block.stmts[stmt_idx + 1..] {
153 // find out if and which field was set by this `consecutive_statement`
154 if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consecutive_statement, binding_name) {
155 // interrupt and cancel lint if assign_rhs references the original binding
156 if contains_name(binding_name, assign_rhs) {
161 // if the field was previously assigned, replace the assignment, otherwise insert the assignment
162 if let Some(prev) = assigned_fields
164 .find(|(field_name, _)| field_name == &field_ident.name)
166 *prev = (field_ident.name, assign_rhs);
168 assigned_fields.push((field_ident.name, assign_rhs));
171 // also set first instance of error for help message
172 if first_assign.is_none() {
173 first_assign = Some(consecutive_statement);
176 // interrupt if no field was assigned, since we only want to look at consecutive statements
182 // if there are incorrectly assigned fields, do a span_lint_and_note to suggest
183 // construction using `Ty { fields, ..Default::default() }`
184 if !assigned_fields.is_empty() && !cancel_lint {
185 // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
186 let ext_with_default = !variant
189 .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.ident.name));
191 let field_list = assigned_fields
193 .map(|(field, rhs)| {
194 // extract and store the assigned value for help message
195 let value_snippet = snippet_with_macro_callsite(cx, rhs.span, "..");
196 format!("{}: {}", field, value_snippet)
198 .collect::<Vec<String>>()
201 // give correct suggestion if generics are involved (see #6944)
202 let binding_type = if_chain! {
203 if let ty::Adt(adt_def, substs) = binding_type.kind();
204 if !substs.is_empty();
206 let adt_def_ty_name = cx.tcx.item_name(adt_def.did);
207 let generic_args = substs.iter().collect::<Vec<_>>();
208 let tys_str = generic_args
210 .map(ToString::to_string)
213 format!("{}::<{}>", adt_def_ty_name, &tys_str)
215 binding_type.to_string()
219 let sugg = if ext_with_default {
220 if field_list.is_empty() {
221 format!("{}::default()", binding_type)
223 format!("{} {{ {}, ..Default::default() }}", binding_type, field_list)
226 format!("{} {{ {} }}", binding_type, field_list)
229 // span lint once per statement that binds default
232 FIELD_REASSIGN_WITH_DEFAULT,
233 first_assign.unwrap().span,
234 "field assignment outside of initializer for an instance created with Default::default()",
237 "consider initializing the variable with `{}` and removing relevant reassignments",
241 self.reassigned_linted.insert(span);
247 /// Checks if the given expression is the `default` method belonging to the `Default` trait.
248 fn is_expr_default<'tcx>(expr: &'tcx Expr<'tcx>, cx: &LateContext<'tcx>) -> bool {
250 if let ExprKind::Call(fn_expr, _) = &expr.kind;
251 if let ExprKind::Path(qpath) = &fn_expr.kind;
252 if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
254 // right hand side of assignment is `Default::default`
255 match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)
262 /// Returns the reassigned field and the assigning expression (right-hand side of assign).
263 fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Option<(Ident, &'tcx Expr<'tcx>)> {
265 // only take assignments
266 if let StmtKind::Semi(later_expr) = this.kind;
267 if let ExprKind::Assign(assign_lhs, assign_rhs, _) = later_expr.kind;
268 // only take assignments to fields where the left-hand side field is a field of
269 // the same binding as the previous statement
270 if let ExprKind::Field(binding, field_ident) = assign_lhs.kind;
271 if let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind;
272 if let Some(second_binding_name) = path.segments.last();
273 if second_binding_name.ident.name == binding_name;
275 Some((field_ident, assign_rhs))