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::{any_parent_is_automatically_derived, contains_name, in_macro, 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::ty;
  11 use rustc_session::{declare_tool_lint, impl_lint_pass};
  12 use rustc_span::symbol::{Ident, Symbol};
  13 use rustc_span::Span;
  14
  15 declare_clippy_lint! {
  16     /// ### What it does
  17     /// Checks for literal calls to `Default::default()`.
  18     ///
  19     /// ### Why is this bad?
  20     /// It's more clear to the reader to use the name of the type whose default is
  21     /// being gotten than the generic `Default`.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// // Bad
  26     /// let s: String = Default::default();
  27     ///
  28     /// // Good
  29     /// let s = String::default();
  30     /// ```
  31     pub DEFAULT_TRAIT_ACCESS,
  32     pedantic,
  33     "checks for literal calls to `Default::default()`"
  34 }
  35
  36 declare_clippy_lint! {
  37     /// ### What it does
  38     /// Checks for immediate reassignment of fields initialized
  39     /// with Default::default().
  40     ///
  41     /// ### Why is this bad?
  42     ///It's more idiomatic to use the [functional update syntax](https://doc.rust-lang.org/reference/expressions/struct-expr.html#functional-update-syntax).
  43     ///
  44     /// ### Known problems
  45     /// Assignments to patterns that are of tuple type are not linted.
  46     ///
  47     /// ### Example
  48     /// Bad:
  49     /// ```
  50     /// # #[derive(Default)]
  51     /// # struct A { i: i32 }
  52     /// let mut a: A = Default::default();
  53     /// a.i = 42;
  54     /// ```
  55     /// Use instead:
  56     /// ```
  57     /// # #[derive(Default)]
  58     /// # struct A { i: i32 }
  59     /// let a = A {
  60     ///     i: 42,
  61     ///     .. Default::default()
  62     /// };
  63     /// ```
  64     pub FIELD_REASSIGN_WITH_DEFAULT,
  65     style,
  66     "binding initialized with Default should have its fields set in the initializer"
  67 }
  68
  69 #[derive(Default)]
  70 pub struct Default {
  71     // Spans linted by `field_reassign_with_default`.
  72     reassigned_linted: FxHashSet<Span>,
  73 }
  74
  75 impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);
  76
  77 impl LateLintPass<'_> for Default {
  78     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  79         if_chain! {
  80             if !in_macro(expr.span);
  81             // Avoid cases already linted by `field_reassign_with_default`
  82             if !self.reassigned_linted.contains(&expr.span);
  83             if let ExprKind::Call(path, ..) = expr.kind;
  84             if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
  85             if let ExprKind::Path(ref qpath) = path.kind;
  86             if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
  87             if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
  88             // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
  89             if let QPath::Resolved(None, _path) = qpath;
  90             let expr_ty = cx.typeck_results().expr_ty(expr);
  91             if let ty::Adt(def, ..) = expr_ty.kind();
  92             then {
  93                 // TODO: Work out a way to put "whatever the imported way of referencing
  94                 // this type in this file" rather than a fully-qualified type.
  95                 let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did));
  96                 span_lint_and_sugg(
  97                     cx,
  98                     DEFAULT_TRAIT_ACCESS,
  99                     expr.span,
 100                     &format!("calling `{}` is more clear than this expression", replacement),
 101                     "try",
 102                     replacement,
 103                     Applicability::Unspecified, // First resolve the TODO above
 104                 );
 105             }
 106         }
 107     }
 108
 109     #[allow(clippy::too_many_lines)]
 110     fn check_block<'tcx>(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
 111         // start from the `let mut _ = _::default();` and look at all the following
 112         // statements, see if they re-assign the fields of the binding
 113         let stmts_head = match block.stmts {
 114             // Skip the last statement since there cannot possibly be any following statements that re-assign fields.
 115             [head @ .., _] if !head.is_empty() => head,
 116             _ => return,
 117         };
 118         for (stmt_idx, stmt) in stmts_head.iter().enumerate() {
 119             // find all binding statements like `let mut _ = T::default()` where `T::default()` is the
 120             // `default` method of the `Default` trait, and store statement index in current block being
 121             // checked and the name of the bound variable
 122             let (local, variant, binding_name, binding_type, span) = if_chain! {
 123                 // only take `let ...` statements
 124                 if let StmtKind::Local(local) = stmt.kind;
 125                 if let Some(expr) = local.init;
 126                 if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
 127                 if !in_macro(expr.span);
 128                 // only take bindings to identifiers
 129                 if let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind;
 130                 // only when assigning `... = Default::default()`
 131                 if is_expr_default(expr, cx);
 132                 let binding_type = cx.typeck_results().node_type(binding_id);
 133                 if let Some(adt) = binding_type.ty_adt_def();
 134                 if adt.is_struct();
 135                 let variant = adt.non_enum_variant();
 136                 if adt.did.is_local() || !variant.is_field_list_non_exhaustive();
 137                 let module_did = cx.tcx.parent_module(stmt.hir_id).to_def_id();
 138                 if variant
 139                     .fields
 140                     .iter()
 141                     .all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
 142                 then {
 143                     (local, variant, ident.name, binding_type, expr.span)
 144                 } else {
 145                     continue;
 146                 }
 147             };
 148
 149             // find all "later statement"'s where the fields of the binding set as
 150             // Default::default() get reassigned, unless the reassignment refers to the original binding
 151             let mut first_assign = None;
 152             let mut assigned_fields = Vec::new();
 153             let mut cancel_lint = false;
 154             for consecutive_statement in &block.stmts[stmt_idx + 1..] {
 155                 // find out if and which field was set by this `consecutive_statement`
 156                 if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consecutive_statement, binding_name) {
 157                     // interrupt and cancel lint if assign_rhs references the original binding
 158                     if contains_name(binding_name, assign_rhs) {
 159                         cancel_lint = true;
 160                         break;
 161                     }
 162
 163                     // if the field was previously assigned, replace the assignment, otherwise insert the assignment
 164                     if let Some(prev) = assigned_fields
 165                         .iter_mut()
 166                         .find(|(field_name, _)| field_name == &field_ident.name)
 167                     {
 168                         *prev = (field_ident.name, assign_rhs);
 169                     } else {
 170                         assigned_fields.push((field_ident.name, assign_rhs));
 171                     }
 172
 173                     // also set first instance of error for help message
 174                     if first_assign.is_none() {
 175                         first_assign = Some(consecutive_statement);
 176                     }
 177                 }
 178                 // interrupt if no field was assigned, since we only want to look at consecutive statements
 179                 else {
 180                     break;
 181                 }
 182             }
 183
 184             // if there are incorrectly assigned fields, do a span_lint_and_note to suggest
 185             // construction using `Ty { fields, ..Default::default() }`
 186             if !assigned_fields.is_empty() && !cancel_lint {
 187                 // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
 188                 let ext_with_default = !variant
 189                     .fields
 190                     .iter()
 191                     .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.ident.name));
 192
 193                 let field_list = assigned_fields
 194                     .into_iter()
 195                     .map(|(field, rhs)| {
 196                         // extract and store the assigned value for help message
 197                         let value_snippet = snippet_with_macro_callsite(cx, rhs.span, "..");
 198                         format!("{}: {}", field, value_snippet)
 199                     })
 200                     .collect::<Vec<String>>()
 201                     .join(", ");
 202
 203                 // give correct suggestion if generics are involved (see #6944)
 204                 let binding_type = if_chain! {
 205                     if let ty::Adt(adt_def, substs) = binding_type.kind();
 206                     if !substs.is_empty();
 207                     then {
 208                         let adt_def_ty_name = cx.tcx.item_name(adt_def.did);
 209                         let generic_args = substs.iter().collect::<Vec<_>>();
 210                         let tys_str = generic_args
 211                             .iter()
 212                             .map(ToString::to_string)
 213                             .collect::<Vec<_>>()
 214                             .join(", ");
 215                         format!("{}::<{}>", adt_def_ty_name, &tys_str)
 216                     } else {
 217                         binding_type.to_string()
 218                     }
 219                 };
 220
 221                 let sugg = if ext_with_default {
 222                     if field_list.is_empty() {
 223                         format!("{}::default()", binding_type)
 224                     } else {
 225                         format!("{} {{ {}, ..Default::default() }}", binding_type, field_list)
 226                     }
 227                 } else {
 228                     format!("{} {{ {} }}", binding_type, field_list)
 229                 };
 230
 231                 // span lint once per statement that binds default
 232                 span_lint_and_note(
 233                     cx,
 234                     FIELD_REASSIGN_WITH_DEFAULT,
 235                     first_assign.unwrap().span,
 236                     "field assignment outside of initializer for an instance created with Default::default()",
 237                     Some(local.span),
 238                     &format!(
 239                         "consider initializing the variable with `{}` and removing relevant reassignments",
 240                         sugg
 241                     ),
 242                 );
 243                 self.reassigned_linted.insert(span);
 244             }
 245         }
 246     }
 247 }
 248
 249 /// Checks if the given expression is the `default` method belonging to the `Default` trait.
 250 fn is_expr_default<'tcx>(expr: &'tcx Expr<'tcx>, cx: &LateContext<'tcx>) -> bool {
 251     if_chain! {
 252         if let ExprKind::Call(fn_expr, _) = &expr.kind;
 253         if let ExprKind::Path(qpath) = &fn_expr.kind;
 254         if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
 255         then {
 256             // right hand side of assignment is `Default::default`
 257             match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)
 258         } else {
 259             false
 260         }
 261     }
 262 }
 263
 264 /// Returns the reassigned field and the assigning expression (right-hand side of assign).
 265 fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Option<(Ident, &'tcx Expr<'tcx>)> {
 266     if_chain! {
 267         // only take assignments
 268         if let StmtKind::Semi(later_expr) = this.kind;
 269         if let ExprKind::Assign(assign_lhs, assign_rhs, _) = later_expr.kind;
 270         // only take assignments to fields where the left-hand side field is a field of
 271         // the same binding as the previous statement
 272         if let ExprKind::Field(binding, field_ident) = assign_lhs.kind;
 273         if let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind;
 274         if let Some(second_binding_name) = path.segments.last();
 275         if second_binding_name.ident.name == binding_name;
 276         then {
 277             Some((field_ident, assign_rhs))
 278         } else {
 279             None
 280         }
 281     }
 282 }