--- /dev/null
+//! Utilities for evaluating whether eagerly evaluated expressions can be made lazy and vice versa.
+//!
+//! Things to consider:
+//! - has the expression side-effects?
+//! - is the expression computationally expensive?
+//!
+//! See lints:
+//! - unnecessary-lazy-evaluations
+//! - or-fun-call
+//! - option-if-let-else
+
+use crate::{is_ctor_or_promotable_const_function, is_type_diagnostic_item};
+use rustc_hir::def::{DefKind, Res};
+
+use rustc_hir::intravisit;
+use rustc_hir::intravisit::{NestedVisitorMap, Visitor};
+
+use rustc_hir::{Block, Expr, ExprKind, Path, QPath};
+use rustc_lint::LateContext;
+use rustc_middle::hir::map::Map;
+use rustc_span::sym;
+
+/// Is the expr pure (is it free from side-effects)?
+/// This function is named so to stress that it isn't exhaustive and returns FNs.
+fn identify_some_pure_patterns(expr: &Expr<'_>) -> bool {
+ match expr.kind {
+ ExprKind::Lit(..) | ExprKind::ConstBlock(..) | ExprKind::Path(..) | ExprKind::Field(..) => true,
+ ExprKind::AddrOf(_, _, addr_of_expr) => identify_some_pure_patterns(addr_of_expr),
+ ExprKind::Tup(tup_exprs) => tup_exprs.iter().all(|expr| identify_some_pure_patterns(expr)),
+ ExprKind::Struct(_, fields, expr) => {
+ fields.iter().all(|f| identify_some_pure_patterns(f.expr))
+ && expr.map_or(true, |e| identify_some_pure_patterns(e))
+ },
+ ExprKind::Call(
+ &Expr {
+ kind:
+ ExprKind::Path(QPath::Resolved(
+ _,
+ Path {
+ res: Res::Def(DefKind::Ctor(..) | DefKind::Variant, ..),
+ ..
+ },
+ )),
+ ..
+ },
+ args,
+ ) => args.iter().all(|expr| identify_some_pure_patterns(expr)),
+ ExprKind::Block(
+ &Block {
+ stmts,
+ expr: Some(expr),
+ ..
+ },
+ _,
+ ) => stmts.is_empty() && identify_some_pure_patterns(expr),
+ ExprKind::Box(..)
+ | ExprKind::Array(..)
+ | ExprKind::Call(..)
+ | ExprKind::MethodCall(..)
+ | ExprKind::Binary(..)
+ | ExprKind::Unary(..)
+ | ExprKind::Cast(..)
+ | ExprKind::Type(..)
+ | ExprKind::DropTemps(..)
+ | ExprKind::Loop(..)
+ | ExprKind::If(..)
+ | ExprKind::Match(..)
+ | ExprKind::Closure(..)
+ | ExprKind::Block(..)
+ | ExprKind::Assign(..)
+ | ExprKind::AssignOp(..)
+ | ExprKind::Index(..)
+ | ExprKind::Break(..)
+ | ExprKind::Continue(..)
+ | ExprKind::Ret(..)
+ | ExprKind::InlineAsm(..)
+ | ExprKind::LlvmInlineAsm(..)
+ | ExprKind::Repeat(..)
+ | ExprKind::Yield(..)
+ | ExprKind::Err => false,
+ }
+}
+
+/// Identify some potentially computationally expensive patterns.
+/// This function is named so to stress that its implementation is non-exhaustive.
+/// It returns FNs and FPs.
+fn identify_some_potentially_expensive_patterns<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+ // Searches an expression for method calls or function calls that aren't ctors
+ struct FunCallFinder<'a, 'tcx> {
+ cx: &'a LateContext<'tcx>,
+ found: bool,
+ }
+
+ impl<'a, 'tcx> intravisit::Visitor<'tcx> for FunCallFinder<'a, 'tcx> {
+ type Map = Map<'tcx>;
+
+ fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+ let call_found = match &expr.kind {
+ // ignore enum and struct constructors
+ ExprKind::Call(..) => !is_ctor_or_promotable_const_function(self.cx, expr),
+ ExprKind::Index(obj, _) => {
+ let ty = self.cx.typeck_results().expr_ty(obj);
+ is_type_diagnostic_item(self.cx, ty, sym::hashmap_type)
+ || is_type_diagnostic_item(self.cx, ty, sym::BTreeMap)
+ },
+ ExprKind::MethodCall(..) => true,
+ _ => false,
+ };
+
+ if call_found {
+ self.found |= true;
+ }
+
+ if !self.found {
+ intravisit::walk_expr(self, expr);
+ }
+ }
+
+ fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+ NestedVisitorMap::None
+ }
+ }
+
+ let mut finder = FunCallFinder { cx, found: false };
+ finder.visit_expr(expr);
+ finder.found
+}
+
+pub fn is_eagerness_candidate<'a, 'tcx>(cx: &'a LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+ !identify_some_potentially_expensive_patterns(cx, expr) && identify_some_pure_patterns(expr)
+}
+
+pub fn is_lazyness_candidate<'a, 'tcx>(cx: &'a LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+ identify_some_potentially_expensive_patterns(cx, expr)
+}