src/tools/clippy/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::{
   5     any_parent_is_automatically_derived, contains_name, get_parent_expr, is_from_proc_macro, match_def_path, paths,
   6 };
   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};
  13 use rustc_middle::ty;
  14 use rustc_session::{declare_tool_lint, impl_lint_pass};
  15 use rustc_span::symbol::{Ident, Symbol};
  16 use rustc_span::Span;
  17
  18 declare_clippy_lint! {
  19     /// ### What it does
  20     /// Checks for literal calls to `Default::default()`.
  21     ///
  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`.
  25     ///
  26     /// ### Example
  27     /// ```rust
  28     /// let s: String = Default::default();
  29     /// ```
  30     ///
  31     /// Use instead:
  32     /// ```rust
  33     /// let s = String::default();
  34     /// ```
  35     #[clippy::version = "pre 1.29.0"]
  36     pub DEFAULT_TRAIT_ACCESS,
  37     pedantic,
  38     "checks for literal calls to `Default::default()`"
  39 }
  40
  41 declare_clippy_lint! {
  42     /// ### What it does
  43     /// Checks for immediate reassignment of fields initialized
  44     /// with Default::default().
  45     ///
  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).
  48     ///
  49     /// ### Known problems
  50     /// Assignments to patterns that are of tuple type are not linted.
  51     ///
  52     /// ### Example
  53     /// ```
  54     /// # #[derive(Default)]
  55     /// # struct A { i: i32 }
  56     /// let mut a: A = Default::default();
  57     /// a.i = 42;
  58     /// ```
  59     ///
  60     /// Use instead:
  61     /// ```
  62     /// # #[derive(Default)]
  63     /// # struct A { i: i32 }
  64     /// let a = A {
  65     ///     i: 42,
  66     ///     .. Default::default()
  67     /// };
  68     /// ```
  69     #[clippy::version = "1.49.0"]
  70     pub FIELD_REASSIGN_WITH_DEFAULT,
  71     style,
  72     "binding initialized with Default should have its fields set in the initializer"
  73 }
  74
  75 #[derive(Default)]
  76 pub struct Default {
  77     // Spans linted by `field_reassign_with_default`.
  78     reassigned_linted: FxHashSet<Span>,
  79 }
  80
  81 impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);
  82
  83 impl<'tcx> LateLintPass<'tcx> for Default {
  84     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  85         if_chain! {
  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);
 100             then {
 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()));
 104                 span_lint_and_sugg(
 105                     cx,
 106                     DEFAULT_TRAIT_ACCESS,
 107                     expr.span,
 108                     &format!("calling `{replacement}` is more clear than this expression"),
 109                     "try",
 110                     replacement,
 111                     Applicability::Unspecified, // First resolve the TODO above
 112                 );
 113             }
 114         }
 115     }
 116
 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,
 124             _ => return,
 125         };
 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();
 142                 if adt.is_struct();
 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);
 146                 if variant
 147                     .fields
 148                     .iter()
 149                     .all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
 150                 let all_fields_are_copy = variant
 151                     .fields
 152                     .iter()
 153                     .all(|field| {
 154                         is_copy(cx, cx.tcx.type_of(field.did))
 155                     });
 156                 if !has_drop(cx, binding_type) || all_fields_are_copy;
 157                 then {
 158                     (local, variant, ident.name, binding_type, expr.span)
 159                 } else {
 160                     continue;
 161                 }
 162             };
 163
 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) {
 174                         cancel_lint = true;
 175                         break;
 176                     }
 177
 178                     // if the field was previously assigned, replace the assignment, otherwise insert the assignment
 179                     if let Some(prev) = assigned_fields
 180                         .iter_mut()
 181                         .find(|(field_name, _)| field_name == &field_ident.name)
 182                     {
 183                         *prev = (field_ident.name, assign_rhs);
 184                     } else {
 185                         assigned_fields.push((field_ident.name, assign_rhs));
 186                     }
 187
 188                     // also set first instance of error for help message
 189                     if first_assign.is_none() {
 190                         first_assign = Some(consecutive_statement);
 191                     }
 192                 }
 193                 // interrupt if no field was assigned, since we only want to look at consecutive statements
 194                 else {
 195                     break;
 196                 }
 197             }
 198
 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
 204                     .fields
 205                     .iter()
 206                     .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.name));
 207
 208                 let field_list = assigned_fields
 209                     .into_iter()
 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}")
 214                     })
 215                     .collect::<Vec<String>>()
 216                     .join(", ");
 217
 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();
 222                     then {
 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
 226                             .iter()
 227                             .map(ToString::to_string)
 228                             .collect::<Vec<_>>()
 229                             .join(", ");
 230                         format!("{adt_def_ty_name}::<{}>", &tys_str)
 231                     } else {
 232                         binding_type.to_string()
 233                     }
 234                 };
 235
 236                 let sugg = if ext_with_default {
 237                     if field_list.is_empty() {
 238                         format!("{binding_type}::default()")
 239                     } else {
 240                         format!("{binding_type} {{ {field_list}, ..Default::default() }}")
 241                     }
 242                 } else {
 243                     format!("{binding_type} {{ {field_list} }}")
 244                 };
 245
 246                 // span lint once per statement that binds default
 247                 span_lint_and_note(
 248                     cx,
 249                     FIELD_REASSIGN_WITH_DEFAULT,
 250                     first_assign.unwrap().span,
 251                     "field assignment outside of initializer for an instance created with Default::default()",
 252                     Some(local.span),
 253                     &format!("consider initializing the variable with `{sugg}` and removing relevant reassignments"),
 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 }