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