Auto merge of rust-lang#12336 - not-elm:fix/issue-12243, r=y21

FIX(12243): redundant_guards

Fixed rust-lang#12243

changelog: Fix[`redundant_guards`]

I have made a correction so that no warning does  appear when y.is_empty() is used within a constant function as follows.

```rust
pub const fn const_fn(x: &str) {
    match x {
        // Shouldn't lint.
        y if y.is_empty() => {},
        _ => {},
    }
}
```

Author: bors

clippy_lints/src/matches/redundant_guards.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::path_to_local;
 use clippy_utils::source::snippet;
 use clippy_utils::visitors::{for_each_expr, is_local_used};
+use clippy_utils::{in_constant, path_to_local};
 use rustc_ast::{BorrowKind, LitKind};
 use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
@@ -123,7 +123,7 @@ fn check_method_calls<'tcx>(
         // `s if s.is_empty()` becomes ""
         // `arr if arr.is_empty()` becomes []
 
-        if ty.is_str() {
+        if ty.is_str() && !in_constant(cx, if_expr.hir_id) {
             r#""""#.into()
         } else if slice_like {
             "[]".into()

tests/ui/redundant_guards.fixed

@@ -1,6 +1,6 @@
 //@aux-build:proc_macros.rs
 #![feature(if_let_guard)]
-#![allow(clippy::no_effect, unused)]
+#![allow(clippy::no_effect, unused, clippy::single_match)]
 #![warn(clippy::redundant_guards)]
 
 #[macro_use]
@@ -16,6 +16,7 @@ struct C(u32, u32);
 
 #[derive(PartialEq)]
 struct FloatWrapper(f32);
+
 fn issue11304() {
     match 0.1 {
         x if x == 0.0 => todo!(),
@@ -258,3 +259,49 @@ fn issue11807() {
         _ => {},
     }
 }
+
+mod issue12243 {
+    pub const fn const_fn(x: &str) {
+        match x {
+            // Shouldn't lint.
+            y if y.is_empty() => {},
+            _ => {},
+        }
+    }
+
+    pub fn non_const_fn(x: &str) {
+        match x {
+            "" => {},
+            //~^ ERROR: redundant guard
+            _ => {},
+        }
+    }
+
+    struct Bar;
+
+    impl Bar {
+        pub const fn const_bar(x: &str) {
+            match x {
+                // Shouldn't lint.
+                y if y.is_empty() => {},
+                _ => {},
+            }
+        }
+
+        pub fn non_const_bar(x: &str) {
+            match x {
+                "" => {},
+                //~^ ERROR: redundant guard
+                _ => {},
+            }
+        }
+    }
+
+    static FOO: () = {
+        match "" {
+            // Shouldn't lint.
+            x if x.is_empty() => {},
+            _ => {},
+        }
+    };
+}

tests/ui/redundant_guards.rs

@@ -1,6 +1,6 @@
 //@aux-build:proc_macros.rs
 #![feature(if_let_guard)]
-#![allow(clippy::no_effect, unused)]
+#![allow(clippy::no_effect, unused, clippy::single_match)]
 #![warn(clippy::redundant_guards)]
 
 #[macro_use]
@@ -16,6 +16,7 @@ struct C(u32, u32);
 
 #[derive(PartialEq)]
 struct FloatWrapper(f32);
+
 fn issue11304() {
     match 0.1 {
         x if x == 0.0 => todo!(),
@@ -258,3 +259,49 @@ fn issue11807() {
         _ => {},
     }
 }
+
+mod issue12243 {
+    pub const fn const_fn(x: &str) {
+        match x {
+            // Shouldn't lint.
+            y if y.is_empty() => {},
+            _ => {},
+        }
+    }
+
+    pub fn non_const_fn(x: &str) {
+        match x {
+            y if y.is_empty() => {},
+            //~^ ERROR: redundant guard
+            _ => {},
+        }
+    }
+
+    struct Bar;
+
+    impl Bar {
+        pub const fn const_bar(x: &str) {
+            match x {
+                // Shouldn't lint.
+                y if y.is_empty() => {},
+                _ => {},
+            }
+        }
+
+        pub fn non_const_bar(x: &str) {
+            match x {
+                y if y.is_empty() => {},
+                //~^ ERROR: redundant guard
+                _ => {},
+            }
+        }
+    }
+
+    static FOO: () = {
+        match "" {
+            // Shouldn't lint.
+            x if x.is_empty() => {},
+            _ => {},
+        }
+    };
+}