clippy_utils/src/eager_or_lazy.rs

   1 //! Utilities for evaluating whether eagerly evaluated expressions can be made lazy and vice versa.
   2 //!
   3 //! Things to consider:
   4 //!  - has the expression side-effects?
   5 //!  - is the expression computationally expensive?
   6 //!
   7 //! See lints:
   8 //!  - unnecessary-lazy-evaluations
   9 //!  - or-fun-call
  10 //!  - option-if-let-else
  11
  12 use crate::ty::{all_predicates_of, is_copy};
  13 use crate::visitors::is_const_evaluatable;
  14 use rustc_hir::def::{DefKind, Res};
  15 use rustc_hir::intravisit::{walk_expr, ErasedMap, NestedVisitorMap, Visitor};
  16 use rustc_hir::{def_id::DefId, Block, Expr, ExprKind, QPath, UnOp};
  17 use rustc_lint::LateContext;
  18 use rustc_middle::ty::{self, PredicateKind};
  19 use rustc_span::{sym, Symbol};
  20 use std::cmp;
  21 use std::ops;
  22
  23 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
  24 enum EagernessSuggestion {
  25     // The expression is cheap and should be evaluated eagerly
  26     Eager,
  27     // The expression may be cheap, so don't suggested lazy evaluation; or the expression may not be safe to switch to
  28     // eager evaluation.
  29     NoChange,
  30     // The expression is likely expensive and should be evaluated lazily.
  31     Lazy,
  32     // The expression cannot be placed into a closure.
  33     ForceNoChange,
  34 }
  35 impl ops::BitOr for EagernessSuggestion {
  36     type Output = Self;
  37     fn bitor(self, rhs: Self) -> Self {
  38         cmp::max(self, rhs)
  39     }
  40 }
  41 impl ops::BitOrAssign for EagernessSuggestion {
  42     fn bitor_assign(&mut self, rhs: Self) {
  43         *self = *self | rhs;
  44     }
  45 }
  46
  47 /// Determine the eagerness of the given function call.
  48 fn fn_eagerness<'tcx>(
  49     cx: &LateContext<'tcx>,
  50     fn_id: DefId,
  51     name: Symbol,
  52     args: &'tcx [Expr<'_>],
  53 ) -> EagernessSuggestion {
  54     use EagernessSuggestion::{Eager, Lazy, NoChange};
  55     let name = name.as_str();
  56
  57     let ty = match cx.tcx.impl_of_method(fn_id) {
  58         Some(id) => cx.tcx.type_of(id),
  59         None => return Lazy,
  60     };
  61
  62     if (name.starts_with("as_") || name == "len" || name == "is_empty") && args.len() == 1 {
  63         if matches!(
  64             cx.tcx.crate_name(fn_id.krate),
  65             sym::std | sym::core | sym::alloc | sym::proc_macro
  66         ) {
  67             Eager
  68         } else {
  69             NoChange
  70         }
  71     } else if let ty::Adt(def, subs) = ty.kind() {
  72         // Types where the only fields are generic types (or references to) with no trait bounds other
  73         // than marker traits.
  74         // Due to the limited operations on these types functions should be fairly cheap.
  75         if def
  76             .variants
  77             .iter()
  78             .flat_map(|v| v.fields.iter())
  79             .any(|x| matches!(cx.tcx.type_of(x.did).peel_refs().kind(), ty::Param(_)))
  80             && all_predicates_of(cx.tcx, fn_id).all(|(pred, _)| match pred.kind().skip_binder() {
  81                 PredicateKind::Trait(pred) => cx.tcx.trait_def(pred.trait_ref.def_id).is_marker,
  82                 _ => true,
  83             })
  84             && subs.types().all(|x| matches!(x.peel_refs().kind(), ty::Param(_)))
  85         {
  86             // Limit the function to either `(self) -> bool` or `(&self) -> bool`
  87             match &**cx.tcx.fn_sig(fn_id).skip_binder().inputs_and_output {
  88                 [arg, res] if !arg.is_mutable_ptr() && arg.peel_refs() == ty && res.is_bool() => NoChange,
  89                 _ => Lazy,
  90             }
  91         } else {
  92             Lazy
  93         }
  94     } else {
  95         Lazy
  96     }
  97 }
  98
  99 #[allow(clippy::too_many_lines)]
 100 fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessSuggestion {
 101     struct V<'cx, 'tcx> {
 102         cx: &'cx LateContext<'tcx>,
 103         eagerness: EagernessSuggestion,
 104     }
 105
 106     impl<'cx, 'tcx> Visitor<'tcx> for V<'cx, 'tcx> {
 107         type Map = ErasedMap<'tcx>;
 108         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 109             NestedVisitorMap::None
 110         }
 111
 112         fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
 113             use EagernessSuggestion::{ForceNoChange, Lazy, NoChange};
 114             if self.eagerness == ForceNoChange {
 115                 return;
 116             }
 117             match e.kind {
 118                 ExprKind::Call(
 119                     &Expr {
 120                         kind: ExprKind::Path(ref path),
 121                         hir_id,
 122                         ..
 123                     },
 124                     args,
 125                 ) => match self.cx.qpath_res(path, hir_id) {
 126                     Res::Def(DefKind::Ctor(..) | DefKind::Variant, _) | Res::SelfCtor(_) => (),
 127                     Res::Def(_, id) if self.cx.tcx.is_promotable_const_fn(id) => (),
 128                     // No need to walk the arguments here, `is_const_evaluatable` already did
 129                     Res::Def(..) if is_const_evaluatable(self.cx, e) => {
 130                         self.eagerness |= NoChange;
 131                         return;
 132                     },
 133                     Res::Def(_, id) => match path {
 134                         QPath::Resolved(_, p) => {
 135                             self.eagerness |= fn_eagerness(self.cx, id, p.segments.last().unwrap().ident.name, args);
 136                         },
 137                         QPath::TypeRelative(_, name) => {
 138                             self.eagerness |= fn_eagerness(self.cx, id, name.ident.name, args);
 139                         },
 140                         QPath::LangItem(..) => self.eagerness = Lazy,
 141                     },
 142                     _ => self.eagerness = Lazy,
 143                 },
 144                 // No need to walk the arguments here, `is_const_evaluatable` already did
 145                 ExprKind::MethodCall(..) if is_const_evaluatable(self.cx, e) => {
 146                     self.eagerness |= NoChange;
 147                     return;
 148                 },
 149                 ExprKind::MethodCall(name, _, args, _) => {
 150                     self.eagerness |= self
 151                         .cx
 152                         .typeck_results()
 153                         .type_dependent_def_id(e.hir_id)
 154                         .map_or(Lazy, |id| fn_eagerness(self.cx, id, name.ident.name, args));
 155                 },
 156                 ExprKind::Index(_, e) => {
 157                     let ty = self.cx.typeck_results().expr_ty_adjusted(e);
 158                     if is_copy(self.cx, ty) && !ty.is_ref() {
 159                         self.eagerness |= NoChange;
 160                     } else {
 161                         self.eagerness = Lazy;
 162                     }
 163                 },
 164
 165                 // Dereferences should be cheap, but dereferencing a raw pointer earlier may not be safe.
 166                 ExprKind::Unary(UnOp::Deref, e) if !self.cx.typeck_results().expr_ty(e).is_unsafe_ptr() => (),
 167                 ExprKind::Unary(UnOp::Deref, _) => self.eagerness |= NoChange,
 168
 169                 ExprKind::Unary(_, e)
 170                     if matches!(
 171                         self.cx.typeck_results().expr_ty(e).kind(),
 172                         ty::Bool | ty::Int(_) | ty::Uint(_),
 173                     ) => {},
 174                 ExprKind::Binary(_, lhs, rhs)
 175                     if self.cx.typeck_results().expr_ty(lhs).is_primitive()
 176                         && self.cx.typeck_results().expr_ty(rhs).is_primitive() => {},
 177
 178                 // Can't be moved into a closure
 179                 ExprKind::Break(..)
 180                 | ExprKind::Continue(_)
 181                 | ExprKind::Ret(_)
 182                 | ExprKind::InlineAsm(_)
 183                 | ExprKind::Yield(..)
 184                 | ExprKind::Err => {
 185                     self.eagerness = ForceNoChange;
 186                     return;
 187                 },
 188
 189                 // Memory allocation, custom operator, loop, or call to an unknown function
 190                 ExprKind::Box(_)
 191                 | ExprKind::Unary(..)
 192                 | ExprKind::Binary(..)
 193                 | ExprKind::Loop(..)
 194                 | ExprKind::Call(..) => self.eagerness = Lazy,
 195
 196                 ExprKind::ConstBlock(_)
 197                 | ExprKind::Array(_)
 198                 | ExprKind::Tup(_)
 199                 | ExprKind::Lit(_)
 200                 | ExprKind::Cast(..)
 201                 | ExprKind::Type(..)
 202                 | ExprKind::DropTemps(_)
 203                 | ExprKind::Let(..)
 204                 | ExprKind::If(..)
 205                 | ExprKind::Match(..)
 206                 | ExprKind::Closure(..)
 207                 | ExprKind::Field(..)
 208                 | ExprKind::Path(_)
 209                 | ExprKind::AddrOf(..)
 210                 | ExprKind::Struct(..)
 211                 | ExprKind::Repeat(..)
 212                 | ExprKind::Block(Block { stmts: [], .. }, _) => (),
 213
 214                 // Assignment might be to a local defined earlier, so don't eagerly evaluate.
 215                 // Blocks with multiple statements might be expensive, so don't eagerly evaluate.
 216                 // TODO: Actually check if either of these are true here.
 217                 ExprKind::Assign(..) | ExprKind::AssignOp(..) | ExprKind::Block(..) => self.eagerness |= NoChange,
 218             }
 219             walk_expr(self, e);
 220         }
 221     }
 222
 223     let mut v = V {
 224         cx,
 225         eagerness: EagernessSuggestion::Eager,
 226     };
 227     v.visit_expr(e);
 228     v.eagerness
 229 }
 230
 231 /// Whether the given expression should be changed to evaluate eagerly
 232 pub fn switch_to_eager_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
 233     expr_eagerness(cx, expr) == EagernessSuggestion::Eager
 234 }
 235
 236 /// Whether the given expression should be changed to evaluate lazily
 237 pub fn switch_to_lazy_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
 238     expr_eagerness(cx, expr) == EagernessSuggestion::Lazy
 239 }