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