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