clippy_lints/src/default.rs

   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};
   4 use clippy_utils::{any_parent_is_automatically_derived, contains_name, get_parent_expr, match_def_path, paths};
   5 use if_chain::if_chain;
   6 use rustc_data_structures::fx::FxHashSet;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::def::Res;
   9 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
  10 use rustc_lint::{LateContext, LateLintPass};
  11 use rustc_middle::ty;
  12 use rustc_session::{declare_tool_lint, impl_lint_pass};
  13 use rustc_span::symbol::{Ident, Symbol};
  14 use rustc_span::Span;
  15
  16 declare_clippy_lint! {
  17     /// ### What it does
  18     /// Checks for literal calls to `Default::default()`.
  19     ///
  20     /// ### Why is this bad?
  21     /// It's easier for the reader if the name of the type is used, rather than the
  22     /// generic `Default`.
  23     ///
  24     /// ### Example
  25     /// ```rust
  26     /// let s: String = Default::default();
  27     /// ```
  28     ///
  29     /// Use instead:
  30     /// ```rust
  31     /// let s = String::default();
  32     /// ```
  33     #[clippy::version = "pre 1.29.0"]
  34     pub DEFAULT_TRAIT_ACCESS,
  35     pedantic,
  36     "checks for literal calls to `Default::default()`"
  37 }
  38
  39 declare_clippy_lint! {
  40     /// ### What it does
  41     /// Checks for immediate reassignment of fields initialized
  42     /// with Default::default().
  43     ///
  44     /// ### Why is this bad?
  45     ///It's more idiomatic to use the [functional update syntax](https://doc.rust-lang.org/reference/expressions/struct-expr.html#functional-update-syntax).
  46     ///
  47     /// ### Known problems
  48     /// Assignments to patterns that are of tuple type are not linted.
  49     ///
  50     /// ### Example
  51     /// ```
  52     /// # #[derive(Default)]
  53     /// # struct A { i: i32 }
  54     /// let mut a: A = Default::default();
  55     /// a.i = 42;
  56     /// ```
  57     ///
  58     /// Use instead:
  59     /// ```
  60     /// # #[derive(Default)]
  61     /// # struct A { i: i32 }
  62     /// let a = A {
  63     ///     i: 42,
  64     ///     .. Default::default()
  65     /// };
  66     /// ```
  67     #[clippy::version = "1.49.0"]
  68     pub FIELD_REASSIGN_WITH_DEFAULT,
  69     style,
  70     "binding initialized with Default should have its fields set in the initializer"
  71 }
  72
  73 #[derive(Default)]
  74 pub struct Default {
  75     // Spans linted by `field_reassign_with_default`.
  76     reassigned_linted: FxHashSet<Span>,
  77 }
  78
  79 impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);
  80
  81 impl<'tcx> LateLintPass<'tcx> for Default {
  82     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  83         if_chain! {
  84             if !expr.span.from_expansion();
  85             // Avoid cases already linted by `field_reassign_with_default`
  86             if !self.reassigned_linted.contains(&expr.span);
  87             if let ExprKind::Call(path, ..) = expr.kind;
  88             if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
  89             if let ExprKind::Path(ref qpath) = path.kind;
  90             if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
  91             if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
  92             if !is_update_syntax_base(cx, expr);
  93             // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
  94             if let QPath::Resolved(None, _path) = qpath;
  95             let expr_ty = cx.typeck_results().expr_ty(expr);
  96             if let ty::Adt(def, ..) = expr_ty.kind();
  97             then {
  98                 // TODO: Work out a way to put "whatever the imported way of referencing
  99                 // this type in this file" rather than a fully-qualified type.
 100                 let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did()));
 101                 span_lint_and_sugg(
 102                     cx,
 103                     DEFAULT_TRAIT_ACCESS,
 104                     expr.span,
 105                     &format!("calling `{}` is more clear than this expression", replacement),
 106                     "try",
 107                     replacement,
 108                     Applicability::Unspecified, // First resolve the TODO above
 109                 );
 110             }
 111         }
 112     }
 113
 114     #[expect(clippy::too_many_lines)]
 115     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
 116         // start from the `let mut _ = _::default();` and look at all the following
 117         // statements, see if they re-assign the fields of the binding
 118         let stmts_head = match block.stmts {
 119             // Skip the last statement since there cannot possibly be any following statements that re-assign fields.
 120             [head @ .., _] if !head.is_empty() => head,
 121             _ => return,
 122         };
 123         for (stmt_idx, stmt) in stmts_head.iter().enumerate() {
 124             // find all binding statements like `let mut _ = T::default()` where `T::default()` is the
 125             // `default` method of the `Default` trait, and store statement index in current block being
 126             // checked and the name of the bound variable
 127             let (local, variant, binding_name, binding_type, span) = if_chain! {
 128                 // only take `let ...` statements
 129                 if let StmtKind::Local(local) = stmt.kind;
 130                 if let Some(expr) = local.init;
 131                 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
 132                 if !expr.span.from_expansion();
 133                 // only take bindings to identifiers
 134                 if let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind;
 135                 // only when assigning `... = Default::default()`
 136                 if is_expr_default(expr, cx);
 137                 let binding_type = cx.typeck_results().node_type(binding_id);
 138                 if let Some(adt) = binding_type.ty_adt_def();
 139                 if adt.is_struct();
 140                 let variant = adt.non_enum_variant();
 141                 if adt.did().is_local() || !variant.is_field_list_non_exhaustive();
 142                 let module_did = cx.tcx.parent_module(stmt.hir_id).to_def_id();
 143                 if variant
 144                     .fields
 145                     .iter()
 146                     .all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
 147                 let all_fields_are_copy = variant
 148                     .fields
 149                     .iter()
 150                     .all(|field| {
 151                         is_copy(cx, cx.tcx.type_of(field.did))
 152                     });
 153                 if !has_drop(cx, binding_type) || all_fields_are_copy;
 154                 then {
 155                     (local, variant, ident.name, binding_type, expr.span)
 156                 } else {
 157                     continue;
 158                 }
 159             };
 160
 161             // find all "later statement"'s where the fields of the binding set as
 162             // Default::default() get reassigned, unless the reassignment refers to the original binding
 163             let mut first_assign = None;
 164             let mut assigned_fields = Vec::new();
 165             let mut cancel_lint = false;
 166             for consecutive_statement in &block.stmts[stmt_idx + 1..] {
 167                 // find out if and which field was set by this `consecutive_statement`
 168                 if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consecutive_statement, binding_name) {
 169                     // interrupt and cancel lint if assign_rhs references the original binding
 170                     if contains_name(binding_name, assign_rhs) {
 171                         cancel_lint = true;
 172                         break;
 173                     }
 174
 175                     // if the field was previously assigned, replace the assignment, otherwise insert the assignment
 176                     if let Some(prev) = assigned_fields
 177                         .iter_mut()
 178                         .find(|(field_name, _)| field_name == &field_ident.name)
 179                     {
 180                         *prev = (field_ident.name, assign_rhs);
 181                     } else {
 182                         assigned_fields.push((field_ident.name, assign_rhs));
 183                     }
 184
 185                     // also set first instance of error for help message
 186                     if first_assign.is_none() {
 187                         first_assign = Some(consecutive_statement);
 188                     }
 189                 }
 190                 // interrupt if no field was assigned, since we only want to look at consecutive statements
 191                 else {
 192                     break;
 193                 }
 194             }
 195
 196             // if there are incorrectly assigned fields, do a span_lint_and_note to suggest
 197             // construction using `Ty { fields, ..Default::default() }`
 198             if !assigned_fields.is_empty() && !cancel_lint {
 199                 // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
 200                 let ext_with_default = !variant
 201                     .fields
 202                     .iter()
 203                     .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.name));
 204
 205                 let field_list = assigned_fields
 206                     .into_iter()
 207                     .map(|(field, rhs)| {
 208                         // extract and store the assigned value for help message
 209                         let value_snippet = snippet_with_macro_callsite(cx, rhs.span, "..");
 210                         format!("{}: {}", field, value_snippet)
 211                     })
 212                     .collect::<Vec<String>>()
 213                     .join(", ");
 214
 215                 // give correct suggestion if generics are involved (see #6944)
 216                 let binding_type = if_chain! {
 217                     if let ty::Adt(adt_def, substs) = binding_type.kind();
 218                     if !substs.is_empty();
 219                     then {
 220                         let adt_def_ty_name = cx.tcx.item_name(adt_def.did());
 221                         let generic_args = substs.iter().collect::<Vec<_>>();
 222                         let tys_str = generic_args
 223                             .iter()
 224                             .map(ToString::to_string)
 225                             .collect::<Vec<_>>()
 226                             .join(", ");
 227                         format!("{}::<{}>", adt_def_ty_name, &tys_str)
 228                     } else {
 229                         binding_type.to_string()
 230                     }
 231                 };
 232
 233                 let sugg = if ext_with_default {
 234                     if field_list.is_empty() {
 235                         format!("{}::default()", binding_type)
 236                     } else {
 237                         format!("{} {{ {}, ..Default::default() }}", binding_type, field_list)
 238                     }
 239                 } else {
 240                     format!("{} {{ {} }}", binding_type, field_list)
 241                 };
 242
 243                 // span lint once per statement that binds default
 244                 span_lint_and_note(
 245                     cx,
 246                     FIELD_REASSIGN_WITH_DEFAULT,
 247                     first_assign.unwrap().span,
 248                     "field assignment outside of initializer for an instance created with Default::default()",
 249                     Some(local.span),
 250                     &format!(
 251                         "consider initializing the variable with `{}` and removing relevant reassignments",
 252                         sugg
 253                     ),
 254                 );
 255                 self.reassigned_linted.insert(span);
 256             }
 257         }
 258     }
 259 }
 260
 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 {
 263     if_chain! {
 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);
 267         then {
 268             // right hand side of assignment is `Default::default`
 269             match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)
 270         } else {
 271             false
 272         }
 273     }
 274 }
 275
 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>)> {
 278     if_chain! {
 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;
 288         then {
 289             Some((field_ident, assign_rhs))
 290         } else {
 291             None
 292         }
 293     }
 294 }
 295
 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 {
 298     if_chain! {
 299         if let Some(parent) = get_parent_expr(cx, expr);
 300         if let ExprKind::Struct(_, _, Some(base)) = parent.kind;
 301         then {
 302             base.hir_id == expr.hir_id
 303         } else {
 304             false
 305         }
 306     }
 307 }