1 files changed, 103 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/equatable_if_let.rs b/src/tools/clippy/clippy_lints/src/equatable_if_let.rs
new file mode 100644
index 000000000..fdfb821ac
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/equatable_if_let.rs
@@ -0,0 +1,103 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_context;
+use clippy_utils::ty::implements_trait;
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, Pat, PatKind};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::Ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for pattern matchings that can be expressed using equality.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// * It reads better and has less cognitive load because equality won't cause binding.
+    /// * It is a [Yoda condition](https://en.wikipedia.org/wiki/Yoda_conditions). Yoda conditions are widely
+    /// criticized for increasing the cognitive load of reading the code.
+    /// * Equality is a simple bool expression and can be merged with `&&` and `||` and
+    /// reuse if blocks
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// if let Some(2) = x {
+    ///     do_thing();
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust,ignore
+    /// if x == Some(2) {
+    ///     do_thing();
+    /// }
+    /// ```
+    #[clippy::version = "1.57.0"]
+    pub EQUATABLE_IF_LET,
+    nursery,
+    "using pattern matching instead of equality"
+}
+
+declare_lint_pass!(PatternEquality => [EQUATABLE_IF_LET]);
+
+/// detects if pattern matches just one thing
+fn unary_pattern(pat: &Pat<'_>) -> bool {
+    fn array_rec(pats: &[Pat<'_>]) -> bool {
+        pats.iter().all(unary_pattern)
+    }
+    match &pat.kind {
+        PatKind::Slice(_, _, _) | PatKind::Range(_, _, _) | PatKind::Binding(..) | PatKind::Wild | PatKind::Or(_) => {
+            false
+        },
+        PatKind::Struct(_, a, etc) => !etc && a.iter().all(|x| unary_pattern(x.pat)),
+        PatKind::Tuple(a, etc) | PatKind::TupleStruct(_, a, etc) => !etc.is_some() && array_rec(a),
+        PatKind::Ref(x, _) | PatKind::Box(x) => unary_pattern(x),
+        PatKind::Path(_) | PatKind::Lit(_) => true,
+    }
+}
+
+fn is_structural_partial_eq<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, other: Ty<'tcx>) -> bool {
+    if let Some(def_id) = cx.tcx.lang_items().eq_trait() {
+        implements_trait(cx, ty, def_id, &[other.into()])
+    } else {
+        false
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for PatternEquality {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if_chain! {
+            if !in_external_macro(cx.sess(), expr.span);
+            if let ExprKind::Let(let_expr) = expr.kind;
+            if unary_pattern(let_expr.pat);
+            let exp_ty = cx.typeck_results().expr_ty(let_expr.init);
+            let pat_ty = cx.typeck_results().pat_ty(let_expr.pat);
+            if is_structural_partial_eq(cx, exp_ty, pat_ty);
+            then {
+
+                let mut applicability = Applicability::MachineApplicable;
+                let pat_str = match let_expr.pat.kind {
+                    PatKind::Struct(..) => format!(
+                        "({})",
+                        snippet_with_context(cx, let_expr.pat.span, expr.span.ctxt(), "..", &mut applicability).0,
+                    ),
+                    _ => snippet_with_context(cx, let_expr.pat.span, expr.span.ctxt(), "..", &mut applicability).0.to_string(),
+                };
+                span_lint_and_sugg(
+                    cx,
+                    EQUATABLE_IF_LET,
+                    expr.span,
+                    "this pattern matching can be expressed using equality",
+                    "try",
+                    format!(
+                        "{} == {}",
+                        snippet_with_context(cx, let_expr.init.span, expr.span.ctxt(), "..", &mut applicability).0,
+                        pat_str,
+                    ),
+                    applicability,
+                );
+            }
+        }
+    }
+}