fix(equatable_if_let): don't suggest = in const context

clippy_lints/src/equatable_if_let.rs

@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_in_const_context;
 use clippy_utils::source::snippet_with_context;
 use clippy_utils::ty::implements_trait;
 use rustc_errors::Applicability;
@@ -110,7 +111,16 @@ impl<'tcx> LateLintPass<'tcx> for PatternEquality {
             let pat_ty = cx.typeck_results().pat_ty(let_expr.pat);
             let mut applicability = Applicability::MachineApplicable;
 
-            if is_structural_partial_eq(cx, exp_ty, pat_ty) && !contains_type_mismatch(cx, let_expr.pat) {
+            if is_structural_partial_eq(cx, exp_ty, pat_ty)
+                && !contains_type_mismatch(cx, let_expr.pat)
+                // Calls to trait methods (`PartialEq::eq` in this case) aren't stable yet. We could _technically_
+                // try looking at whether:
+                // 1) features `const_trait_impl` and `const_cmp` are enabled
+                // 2) implementation of `PartialEq<Rhs=PatTy> for ExpTy` has `fn eq` that is `const`
+                //
+                // but that didn't quite work out (see #15482), so we just reject outright in this case
+                && !is_in_const_context(cx)
+            {
                 let pat_str = match let_expr.pat.kind {
                     PatKind::Struct(..) => format!(
                         "({})",

tests/ui/equatable_if_let.fixed

@@ -139,3 +139,24 @@ mod issue8710 {
         }
     }
 }
+
+// PartialEq is not stable in consts yet
+fn issue15376() {
+    enum NonConstEq {
+        A,
+        B,
+    }
+    impl PartialEq for NonConstEq {
+        fn eq(&self, _other: &Self) -> bool {
+            true
+        }
+    }
+
+    const N: NonConstEq = NonConstEq::A;
+
+    // `impl PartialEq` is not const, suggest `matches!`
+    const _: u32 = if matches!(N, NonConstEq::A) { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+    const _: u32 = if matches!(Some(N), Some(NonConstEq::A)) { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+}

tests/ui/equatable_if_let.rs

@@ -139,3 +139,24 @@ mod issue8710 {
         }
     }
 }
+
+// PartialEq is not stable in consts yet
+fn issue15376() {
+    enum NonConstEq {
+        A,
+        B,
+    }
+    impl PartialEq for NonConstEq {
+        fn eq(&self, _other: &Self) -> bool {
+            true
+        }
+    }
+
+    const N: NonConstEq = NonConstEq::A;
+
+    // `impl PartialEq` is not const, suggest `matches!`
+    const _: u32 = if let NonConstEq::A = N { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+    const _: u32 = if let Some(NonConstEq::A) = Some(N) { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+}

tests/ui/equatable_if_let.stderr

@@ -103,5 +103,17 @@ error: this pattern matching can be expressed using `matches!`
 LL |         if let Some(MyEnum::B) = get_enum() {
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `matches!(get_enum(), Some(MyEnum::B))`
 
-error: aborting due to 17 previous errors
+error: this pattern matching can be expressed using `matches!`
+  --> tests/ui/equatable_if_let.rs:158:23
+   |
+LL |     const _: u32 = if let NonConstEq::A = N { 0 } else { 1 };
+   |                       ^^^^^^^^^^^^^^^^^^^^^ help: try: `matches!(N, NonConstEq::A)`
+
+error: this pattern matching can be expressed using `matches!`
+  --> tests/ui/equatable_if_let.rs:160:23
+   |
+LL |     const _: u32 = if let Some(NonConstEq::A) = Some(N) { 0 } else { 1 };
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `matches!(Some(N), Some(NonConstEq::A))`
+
+error: aborting due to 19 previous errors
 

tests/ui/equatable_if_let_const_cmp.fixed

@@ -0,0 +1,24 @@
+#![warn(clippy::equatable_if_let)]
+#![allow(clippy::eq_op)]
+#![feature(const_trait_impl, const_cmp)]
+
+fn issue15376() {
+    enum ConstEq {
+        A,
+        B,
+    }
+    impl const PartialEq for ConstEq {
+        fn eq(&self, _other: &Self) -> bool {
+            true
+        }
+    }
+
+    const C: ConstEq = ConstEq::A;
+
+    // `impl PartialEq` is const... but we still suggest `matches!` for now
+    // TODO: detect this and suggest `=`
+    const _: u32 = if matches!(C, ConstEq::A) { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+    const _: u32 = if matches!(Some(C), Some(ConstEq::A)) { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+}

tests/ui/equatable_if_let_const_cmp.rs

@@ -0,0 +1,24 @@
+#![warn(clippy::equatable_if_let)]
+#![allow(clippy::eq_op)]
+#![feature(const_trait_impl, const_cmp)]
+
+fn issue15376() {
+    enum ConstEq {
+        A,
+        B,
+    }
+    impl const PartialEq for ConstEq {
+        fn eq(&self, _other: &Self) -> bool {
+            true
+        }
+    }
+
+    const C: ConstEq = ConstEq::A;
+
+    // `impl PartialEq` is const... but we still suggest `matches!` for now
+    // TODO: detect this and suggest `=`
+    const _: u32 = if let ConstEq::A = C { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+    const _: u32 = if let Some(ConstEq::A) = Some(C) { 0 } else { 1 };
+    //~^ ERROR: this pattern matching can be expressed using `matches!`
+}

tests/ui/equatable_if_let_const_cmp.stderr

@@ -0,0 +1,17 @@
+error: this pattern matching can be expressed using `matches!`
+  --> tests/ui/equatable_if_let_const_cmp.rs:20:23
+   |
+LL |     const _: u32 = if let ConstEq::A = C { 0 } else { 1 };
+   |                       ^^^^^^^^^^^^^^^^^^ help: try: `matches!(C, ConstEq::A)`
+   |
+   = note: `-D clippy::equatable-if-let` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::equatable_if_let)]`
+
+error: this pattern matching can be expressed using `matches!`
+  --> tests/ui/equatable_if_let_const_cmp.rs:22:23
+   |
+LL |     const _: u32 = if let Some(ConstEq::A) = Some(C) { 0 } else { 1 };
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `matches!(Some(C), Some(ConstEq::A))`
+
+error: aborting due to 2 previous errors
+