src/tools/clippy/clippy_lints/src/shadow.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::snippet;
   3 use clippy_utils::{contains_name, higher, iter_input_pats};
   4 use rustc_hir::intravisit::FnKind;
   5 use rustc_hir::{
   6     Block, Body, Expr, ExprKind, FnDecl, Guard, HirId, Local, MutTy, Pat, PatKind, Path, QPath, StmtKind, Ty, TyKind,
   7     UnOp,
   8 };
   9 use rustc_lint::{LateContext, LateLintPass, LintContext};
  10 use rustc_middle::lint::in_external_macro;
  11 use rustc_middle::ty;
  12 use rustc_session::{declare_lint_pass, declare_tool_lint};
  13 use rustc_span::source_map::Span;
  14 use rustc_span::symbol::Symbol;
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for bindings that shadow other bindings already in
  18     /// scope, while just changing reference level or mutability.
  19     ///
  20     /// **Why is this bad?** Not much, in fact it's a very common pattern in Rust
  21     /// code. Still, some may opt to avoid it in their code base, they can set this
  22     /// lint to `Warn`.
  23     ///
  24     /// **Known problems:** This lint, as the other shadowing related lints,
  25     /// currently only catches very simple patterns.
  26     ///
  27     /// **Example:**
  28     /// ```rust
  29     /// # let x = 1;
  30     /// // Bad
  31     /// let x = &x;
  32     ///
  33     /// // Good
  34     /// let y = &x; // use different variable name
  35     /// ```
  36     pub SHADOW_SAME,
  37     restriction,
  38     "rebinding a name to itself, e.g., `let mut x = &mut x`"
  39 }
  40
  41 declare_clippy_lint! {
  42     /// **What it does:** Checks for bindings that shadow other bindings already in
  43     /// scope, while reusing the original value.
  44     ///
  45     /// **Why is this bad?** Not too much, in fact it's a common pattern in Rust
  46     /// code. Still, some argue that name shadowing like this hurts readability,
  47     /// because a value may be bound to different things depending on position in
  48     /// the code.
  49     ///
  50     /// **Known problems:** This lint, as the other shadowing related lints,
  51     /// currently only catches very simple patterns.
  52     ///
  53     /// **Example:**
  54     /// ```rust
  55     /// let x = 2;
  56     /// let x = x + 1;
  57     /// ```
  58     /// use different variable name:
  59     /// ```rust
  60     /// let x = 2;
  61     /// let y = x + 1;
  62     /// ```
  63     pub SHADOW_REUSE,
  64     restriction,
  65     "rebinding a name to an expression that re-uses the original value, e.g., `let x = x + 1`"
  66 }
  67
  68 declare_clippy_lint! {
  69     /// **What it does:** Checks for bindings that shadow other bindings already in
  70     /// scope, either without a initialization or with one that does not even use
  71     /// the original value.
  72     ///
  73     /// **Why is this bad?** Name shadowing can hurt readability, especially in
  74     /// large code bases, because it is easy to lose track of the active binding at
  75     /// any place in the code. This can be alleviated by either giving more specific
  76     /// names to bindings or introducing more scopes to contain the bindings.
  77     ///
  78     /// **Known problems:** This lint, as the other shadowing related lints,
  79     /// currently only catches very simple patterns. Note that
  80     /// `allow`/`warn`/`deny`/`forbid` attributes only work on the function level
  81     /// for this lint.
  82     ///
  83     /// **Example:**
  84     /// ```rust
  85     /// # let y = 1;
  86     /// # let z = 2;
  87     /// let x = y;
  88     ///
  89     /// // Bad
  90     /// let x = z; // shadows the earlier binding
  91     ///
  92     /// // Good
  93     /// let w = z; // use different variable name
  94     /// ```
  95     pub SHADOW_UNRELATED,
  96     pedantic,
  97     "rebinding a name without even using the original value"
  98 }
  99
 100 declare_lint_pass!(Shadow => [SHADOW_SAME, SHADOW_REUSE, SHADOW_UNRELATED]);
 101
 102 impl<'tcx> LateLintPass<'tcx> for Shadow {
 103     fn check_fn(
 104         &mut self,
 105         cx: &LateContext<'tcx>,
 106         _: FnKind<'tcx>,
 107         decl: &'tcx FnDecl<'_>,
 108         body: &'tcx Body<'_>,
 109         _: Span,
 110         _: HirId,
 111     ) {
 112         if in_external_macro(cx.sess(), body.value.span) {
 113             return;
 114         }
 115         check_fn(cx, decl, body);
 116     }
 117 }
 118
 119 fn check_fn<'tcx>(cx: &LateContext<'tcx>, decl: &'tcx FnDecl<'_>, body: &'tcx Body<'_>) {
 120     let mut bindings = Vec::with_capacity(decl.inputs.len());
 121     for arg in iter_input_pats(decl, body) {
 122         if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 123             bindings.push((ident.name, ident.span))
 124         }
 125     }
 126     check_expr(cx, &body.value, &mut bindings);
 127 }
 128
 129 fn check_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'_>, bindings: &mut Vec<(Symbol, Span)>) {
 130     let len = bindings.len();
 131     for stmt in block.stmts {
 132         match stmt.kind {
 133             StmtKind::Local(local) => check_local(cx, local, bindings),
 134             StmtKind::Expr(e) | StmtKind::Semi(e) => check_expr(cx, e, bindings),
 135             StmtKind::Item(..) => {},
 136         }
 137     }
 138     if let Some(o) = block.expr {
 139         check_expr(cx, o, bindings);
 140     }
 141     bindings.truncate(len);
 142 }
 143
 144 fn check_local<'tcx>(cx: &LateContext<'tcx>, local: &'tcx Local<'_>, bindings: &mut Vec<(Symbol, Span)>) {
 145     if in_external_macro(cx.sess(), local.span) {
 146         return;
 147     }
 148     if higher::is_from_for_desugar(local) {
 149         return;
 150     }
 151     let Local {
 152         pat,
 153         ref ty,
 154         ref init,
 155         span,
 156         ..
 157     } = *local;
 158     if let Some(t) = *ty {
 159         check_ty(cx, t, bindings)
 160     }
 161     if let Some(o) = *init {
 162         check_expr(cx, o, bindings);
 163         check_pat(cx, pat, Some(o), span, bindings);
 164     } else {
 165         check_pat(cx, pat, None, span, bindings);
 166     }
 167 }
 168
 169 fn is_binding(cx: &LateContext<'_>, pat_id: HirId) -> bool {
 170     let var_ty = cx.typeck_results().node_type_opt(pat_id);
 171     var_ty.map_or(false, |var_ty| !matches!(var_ty.kind(), ty::Adt(..)))
 172 }
 173
 174 fn check_pat<'tcx>(
 175     cx: &LateContext<'tcx>,
 176     pat: &'tcx Pat<'_>,
 177     init: Option<&'tcx Expr<'_>>,
 178     span: Span,
 179     bindings: &mut Vec<(Symbol, Span)>,
 180 ) {
 181     // TODO: match more stuff / destructuring
 182     match pat.kind {
 183         PatKind::Binding(.., ident, ref inner) => {
 184             let name = ident.name;
 185             if is_binding(cx, pat.hir_id) {
 186                 let mut new_binding = true;
 187                 for tup in bindings.iter_mut() {
 188                     if tup.0 == name {
 189                         lint_shadow(cx, name, span, pat.span, init, tup.1);
 190                         tup.1 = ident.span;
 191                         new_binding = false;
 192                         break;
 193                     }
 194                 }
 195                 if new_binding {
 196                     bindings.push((name, ident.span));
 197                 }
 198             }
 199             if let Some(p) = *inner {
 200                 check_pat(cx, p, init, span, bindings);
 201             }
 202         },
 203         PatKind::Struct(_, pfields, _) => {
 204             if let Some(init_struct) = init {
 205                 if let ExprKind::Struct(_, efields, _) = init_struct.kind {
 206                     for field in pfields {
 207                         let name = field.ident.name;
 208                         let efield = efields
 209                             .iter()
 210                             .find_map(|f| if f.ident.name == name { Some(&*f.expr) } else { None });
 211                         check_pat(cx, field.pat, efield, span, bindings);
 212                     }
 213                 } else {
 214                     for field in pfields {
 215                         check_pat(cx, field.pat, init, span, bindings);
 216                     }
 217                 }
 218             } else {
 219                 for field in pfields {
 220                     check_pat(cx, field.pat, None, span, bindings);
 221                 }
 222             }
 223         },
 224         PatKind::Tuple(inner, _) => {
 225             if let Some(init_tup) = init {
 226                 if let ExprKind::Tup(tup) = init_tup.kind {
 227                     for (i, p) in inner.iter().enumerate() {
 228                         check_pat(cx, p, Some(&tup[i]), p.span, bindings);
 229                     }
 230                 } else {
 231                     for p in inner {
 232                         check_pat(cx, p, init, span, bindings);
 233                     }
 234                 }
 235             } else {
 236                 for p in inner {
 237                     check_pat(cx, p, None, span, bindings);
 238                 }
 239             }
 240         },
 241         PatKind::Box(inner) => {
 242             if let Some(initp) = init {
 243                 if let ExprKind::Box(inner_init) = initp.kind {
 244                     check_pat(cx, inner, Some(inner_init), span, bindings);
 245                 } else {
 246                     check_pat(cx, inner, init, span, bindings);
 247                 }
 248             } else {
 249                 check_pat(cx, inner, init, span, bindings);
 250             }
 251         },
 252         PatKind::Ref(inner, _) => check_pat(cx, inner, init, span, bindings),
 253         // PatVec(Vec<P<Pat>>, Option<P<Pat>>, Vec<P<Pat>>),
 254         _ => (),
 255     }
 256 }
 257
 258 fn lint_shadow<'tcx>(
 259     cx: &LateContext<'tcx>,
 260     name: Symbol,
 261     span: Span,
 262     pattern_span: Span,
 263     init: Option<&'tcx Expr<'_>>,
 264     prev_span: Span,
 265 ) {
 266     if let Some(expr) = init {
 267         if is_self_shadow(name, expr) {
 268             span_lint_and_then(
 269                 cx,
 270                 SHADOW_SAME,
 271                 span,
 272                 &format!(
 273                     "`{}` is shadowed by itself in `{}`",
 274                     snippet(cx, pattern_span, "_"),
 275                     snippet(cx, expr.span, "..")
 276                 ),
 277                 |diag| {
 278                     diag.span_note(prev_span, "previous binding is here");
 279                 },
 280             );
 281         } else if contains_name(name, expr) {
 282             span_lint_and_then(
 283                 cx,
 284                 SHADOW_REUSE,
 285                 pattern_span,
 286                 &format!(
 287                     "`{}` is shadowed by `{}` which reuses the original value",
 288                     snippet(cx, pattern_span, "_"),
 289                     snippet(cx, expr.span, "..")
 290                 ),
 291                 |diag| {
 292                     diag.span_note(expr.span, "initialization happens here");
 293                     diag.span_note(prev_span, "previous binding is here");
 294                 },
 295             );
 296         } else {
 297             span_lint_and_then(
 298                 cx,
 299                 SHADOW_UNRELATED,
 300                 pattern_span,
 301                 &format!("`{}` is being shadowed", snippet(cx, pattern_span, "_")),
 302                 |diag| {
 303                     diag.span_note(expr.span, "initialization happens here");
 304                     diag.span_note(prev_span, "previous binding is here");
 305                 },
 306             );
 307         }
 308     } else {
 309         span_lint_and_then(
 310             cx,
 311             SHADOW_UNRELATED,
 312             span,
 313             &format!("`{}` shadows a previous declaration", snippet(cx, pattern_span, "_")),
 314             |diag| {
 315                 diag.span_note(prev_span, "previous binding is here");
 316             },
 317         );
 318     }
 319 }
 320
 321 fn check_expr<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, bindings: &mut Vec<(Symbol, Span)>) {
 322     if in_external_macro(cx.sess(), expr.span) {
 323         return;
 324     }
 325     match expr.kind {
 326         ExprKind::Unary(_, e) | ExprKind::Field(e, _) | ExprKind::AddrOf(_, _, e) | ExprKind::Box(e) => {
 327             check_expr(cx, e, bindings)
 328         },
 329         ExprKind::Block(block, _) | ExprKind::Loop(block, ..) => check_block(cx, block, bindings),
 330         // ExprKind::Call
 331         // ExprKind::MethodCall
 332         ExprKind::Array(v) | ExprKind::Tup(v) => {
 333             for e in v {
 334                 check_expr(cx, e, bindings)
 335             }
 336         },
 337         ExprKind::If(cond, then, ref otherwise) => {
 338             check_expr(cx, cond, bindings);
 339             check_expr(cx, then, bindings);
 340             if let Some(o) = *otherwise {
 341                 check_expr(cx, o, bindings);
 342             }
 343         },
 344         ExprKind::Match(init, arms, _) => {
 345             check_expr(cx, init, bindings);
 346             let len = bindings.len();
 347             for arm in arms {
 348                 check_pat(cx, arm.pat, Some(init), arm.pat.span, bindings);
 349                 // This is ugly, but needed to get the right type
 350                 if let Some(ref guard) = arm.guard {
 351                     match guard {
 352                         Guard::If(if_expr) => check_expr(cx, if_expr, bindings),
 353                         Guard::IfLet(guard_pat, guard_expr) => {
 354                             check_pat(cx, guard_pat, Some(*guard_expr), guard_pat.span, bindings);
 355                             check_expr(cx, guard_expr, bindings);
 356                         },
 357                     }
 358                 }
 359                 check_expr(cx, arm.body, bindings);
 360                 bindings.truncate(len);
 361             }
 362         },
 363         _ => (),
 364     }
 365 }
 366
 367 fn check_ty<'tcx>(cx: &LateContext<'tcx>, ty: &'tcx Ty<'_>, bindings: &mut Vec<(Symbol, Span)>) {
 368     match ty.kind {
 369         TyKind::Slice(sty) => check_ty(cx, sty, bindings),
 370         TyKind::Array(fty, ref anon_const) => {
 371             check_ty(cx, fty, bindings);
 372             check_expr(cx, &cx.tcx.hir().body(anon_const.body).value, bindings);
 373         },
 374         TyKind::Ptr(MutTy { ty: mty, .. }) | TyKind::Rptr(_, MutTy { ty: mty, .. }) => check_ty(cx, mty, bindings),
 375         TyKind::Tup(tup) => {
 376             for t in tup {
 377                 check_ty(cx, t, bindings)
 378             }
 379         },
 380         TyKind::Typeof(ref anon_const) => check_expr(cx, &cx.tcx.hir().body(anon_const.body).value, bindings),
 381         _ => (),
 382     }
 383 }
 384
 385 fn is_self_shadow(name: Symbol, expr: &Expr<'_>) -> bool {
 386     match expr.kind {
 387         ExprKind::Box(inner) | ExprKind::AddrOf(_, _, inner) => is_self_shadow(name, inner),
 388         ExprKind::Block(block, _) => {
 389             block.stmts.is_empty() && block.expr.as_ref().map_or(false, |e| is_self_shadow(name, e))
 390         },
 391         ExprKind::Unary(op, inner) => (UnOp::Deref == op) && is_self_shadow(name, inner),
 392         ExprKind::Path(QPath::Resolved(_, path)) => path_eq_name(name, path),
 393         _ => false,
 394     }
 395 }
 396
 397 fn path_eq_name(name: Symbol, path: &Path<'_>) -> bool {
 398     !path.is_global() && path.segments.len() == 1 && path.segments[0].ident.name == name
 399 }