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, 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 // Avoid cases already linted by `field_reassign_with_default`
79 if !self.reassigned_linted.contains(&expr.span);
80 if let ExprKind::Call(ref path, ..) = expr.kind;
81 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
82 if let ExprKind::Path(ref qpath) = path.kind;
83 if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
84 if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
85 // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
86 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() {
90 // TODO: Work out a way to put "whatever the imported way of referencing
91 // this type in this file" rather than a fully-qualified type.
92 let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did));
97 &format!("calling `{}` is more clear than this expression", replacement),
100 Applicability::Unspecified, // First resolve the TODO above
107 fn check_block<'tcx>(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
108 // start from the `let mut _ = _::default();` and look at all the following
109 // statements, see if they re-assign the fields of the binding
110 let stmts_head = match block.stmts {
111 // Skip the last statement since there cannot possibly be any following statements that re-assign fields.
112 [head @ .., _] if !head.is_empty() => head,
115 for (stmt_idx, stmt) in stmts_head.iter().enumerate() {
116 // find all binding statements like `let mut _ = T::default()` where `T::default()` is the
117 // `default` method of the `Default` trait, and store statement index in current block being
118 // checked and the name of the bound variable
119 let (local, variant, binding_name, binding_type, span) = if_chain! {
120 // only take `let ...` statements
121 if let StmtKind::Local(local) = stmt.kind;
122 if let Some(expr) = local.init;
123 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
124 if !in_external_macro(cx.tcx.sess, expr.span);
125 // only take bindings to identifiers
126 if let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind;
127 // only when assigning `... = Default::default()`
128 if is_expr_default(expr, cx);
129 let binding_type = cx.typeck_results().node_type(binding_id);
130 if let Some(adt) = binding_type.ty_adt_def();
132 let variant = adt.non_enum_variant();
133 if adt.did.is_local() || !variant.is_field_list_non_exhaustive();
134 let module_did = cx.tcx.parent_module(stmt.hir_id).to_def_id();
138 .all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
140 (local, variant, ident.name, binding_type, expr.span)
146 // find all "later statement"'s where the fields of the binding set as
147 // Default::default() get reassigned, unless the reassignment refers to the original binding
148 let mut first_assign = None;
149 let mut assigned_fields = Vec::new();
150 let mut cancel_lint = false;
151 for consecutive_statement in &block.stmts[stmt_idx + 1..] {
152 // find out if and which field was set by this `consecutive_statement`
153 if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consecutive_statement, binding_name) {
154 // interrupt and cancel lint if assign_rhs references the original binding
155 if contains_name(binding_name, assign_rhs) {
160 // if the field was previously assigned, replace the assignment, otherwise insert the assignment
161 if let Some(prev) = assigned_fields
163 .find(|(field_name, _)| field_name == &field_ident.name)
165 *prev = (field_ident.name, assign_rhs);
167 assigned_fields.push((field_ident.name, assign_rhs));
170 // also set first instance of error for help message
171 if first_assign.is_none() {
172 first_assign = Some(consecutive_statement);
175 // interrupt if no field was assigned, since we only want to look at consecutive statements
181 // if there are incorrectly assigned fields, do a span_lint_and_note to suggest
182 // construction using `Ty { fields, ..Default::default() }`
183 if !assigned_fields.is_empty() && !cancel_lint {
184 // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
185 let ext_with_default = !variant
188 .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.ident.name));
190 let field_list = assigned_fields
192 .map(|(field, rhs)| {
193 // extract and store the assigned value for help message
194 let value_snippet = snippet_with_macro_callsite(cx, rhs.span, "..");
195 format!("{}: {}", field, value_snippet)
197 .collect::<Vec<String>>()
200 let sugg = if ext_with_default {
201 if field_list.is_empty() {
202 format!("{}::default()", binding_type)
204 format!("{} {{ {}, ..Default::default() }}", binding_type, field_list)
207 format!("{} {{ {} }}", binding_type, field_list)
210 // span lint once per statement that binds default
213 FIELD_REASSIGN_WITH_DEFAULT,
214 first_assign.unwrap().span,
215 "field assignment outside of initializer for an instance created with Default::default()",
218 "consider initializing the variable with `{}` and removing relevant reassignments",
222 self.reassigned_linted.insert(span);
228 /// Checks if the given expression is the `default` method belonging to the `Default` trait.
229 fn is_expr_default<'tcx>(expr: &'tcx Expr<'tcx>, cx: &LateContext<'tcx>) -> bool {
231 if let ExprKind::Call(ref fn_expr, _) = &expr.kind;
232 if let ExprKind::Path(qpath) = &fn_expr.kind;
233 if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
235 // right hand side of assignment is `Default::default`
236 match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)
243 /// Returns the reassigned field and the assigning expression (right-hand side of assign).
244 fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Option<(Ident, &'tcx Expr<'tcx>)> {
246 // only take assignments
247 if let StmtKind::Semi(ref later_expr) = this.kind;
248 if let ExprKind::Assign(ref assign_lhs, ref assign_rhs, _) = later_expr.kind;
249 // only take assignments to fields where the left-hand side field is a field of
250 // the same binding as the previous statement
251 if let ExprKind::Field(ref binding, field_ident) = assign_lhs.kind;
252 if let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind;
253 if let Some(second_binding_name) = path.segments.last();
254 if second_binding_name.ident.name == binding_name;
256 Some((field_ident, assign_rhs))