4 files changed, 196 insertions, 1 deletions
diff --git a/clippy_lints/src/utils/eager_or_lazy.rs b/clippy_lints/src/utils/eager_or_lazy.rs
new file mode 100644
index 00000000000..6938d9971d9
--- /dev/null
+++ b/clippy_lints/src/utils/eager_or_lazy.rs
@@ -0,0 +1,128 @@
+//! 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::utils::is_ctor_or_promotable_const_function;
+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;
+
+/// 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::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::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<'a, 'tcx>(cx: &'a 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::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)
+}
diff --git a/clippy_lints/src/utils/mod.rs b/clippy_lints/src/utils/mod.rs
index 8db7e693e62..95eedf88178 100644
--- a/clippy_lints/src/utils/mod.rs
+++ b/clippy_lints/src/utils/mod.rs
@@ -10,6 +10,7 @@ pub mod comparisons;
 pub mod conf;
 pub mod constants;
 mod diagnostics;
+pub mod eager_or_lazy;
 pub mod higher;
 mod hir_utils;
 pub mod inspector;
diff --git a/clippy_lints/src/utils/sugg.rs b/clippy_lints/src/utils/sugg.rs
index 811fde388d1..ec8b7e59b59 100644
--- a/clippy_lints/src/utils/sugg.rs
+++ b/clippy_lints/src/utils/sugg.rs
@@ -49,7 +49,7 @@ impl<'a> Sugg<'a> {
     /// Convenience function around `hir_opt` for suggestions with a default
     /// text.
     pub fn hir(cx: &LateContext<'_>, expr: &hir::Expr<'_>, default: &'a str) -> Self {
-        Self::hir_opt(cx, expr).unwrap_or_else(|| Sugg::NonParen(Cow::Borrowed(default)))
+        Self::hir_opt(cx, expr).unwrap_or(Sugg::NonParen(Cow::Borrowed(default)))
     }
 
     /// Same as `hir`, but it adapts the applicability level by following rules:
diff --git a/clippy_lints/src/utils/usage.rs b/clippy_lints/src/utils/usage.rs
index 4a64b935ac9..ea1dc3be29b 100644
--- a/clippy_lints/src/utils/usage.rs
+++ b/clippy_lints/src/utils/usage.rs
@@ -1,6 +1,8 @@
 use crate::utils::match_var;
 use rustc_data_structures::fx::FxHashSet;
+use rustc_hir as hir;
 use rustc_hir::def::Res;
+use rustc_hir::intravisit;
 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
 use rustc_hir::{Expr, HirId, Path};
 use rustc_infer::infer::TyCtxtInferExt;
@@ -108,3 +110,67 @@ pub fn is_unused<'tcx>(ident: &'tcx Ident, body: &'tcx Expr<'_>) -> bool {
     walk_expr(&mut visitor, body);
     !visitor.used
 }
+
+pub struct ParamBindingIdCollector {
+    binding_hir_ids: Vec<hir::HirId>,
+}
+impl<'tcx> ParamBindingIdCollector {
+    fn collect_binding_hir_ids(body: &'tcx hir::Body<'tcx>) -> Vec<hir::HirId> {
+        let mut finder = ParamBindingIdCollector {
+            binding_hir_ids: Vec::new(),
+        };
+        finder.visit_body(body);
+        finder.binding_hir_ids
+    }
+}
+impl<'tcx> intravisit::Visitor<'tcx> for ParamBindingIdCollector {
+    type Map = Map<'tcx>;
+
+    fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
+        if let hir::PatKind::Binding(_, hir_id, ..) = param.pat.kind {
+            self.binding_hir_ids.push(hir_id);
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
+        intravisit::NestedVisitorMap::None
+    }
+}
+
+pub struct BindingUsageFinder<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    binding_ids: Vec<hir::HirId>,
+    usage_found: bool,
+}
+impl<'a, 'tcx> BindingUsageFinder<'a, 'tcx> {
+    pub fn are_params_used(cx: &'a LateContext<'tcx>, body: &'tcx hir::Body<'tcx>) -> bool {
+        let mut finder = BindingUsageFinder {
+            cx,
+            binding_ids: ParamBindingIdCollector::collect_binding_hir_ids(body),
+            usage_found: false,
+        };
+        finder.visit_body(body);
+        finder.usage_found
+    }
+}
+impl<'a, 'tcx> intravisit::Visitor<'tcx> for BindingUsageFinder<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
+        if !self.usage_found {
+            intravisit::walk_expr(self, expr);
+        }
+    }
+
+    fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
+        if let hir::def::Res::Local(id) = path.res {
+            if self.binding_ids.contains(&id) {
+                self.usage_found = true;
+            }
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
+        intravisit::NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+    }
+}