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