char::is_digit() is const-stable only since Rust 1.87

The `to_digit_is_some()` lint suggests using `char::is_digit()`. It
should not trigger in const contexts before Rust 1.87.

tests/ui/to_digit_is_some.fixed

@@ -9,3 +9,20 @@ fn main() {
     let _ = char::is_digit(c, 8);
     //~^ to_digit_is_some
 }
+
+#[clippy::msrv = "1.86"]
+mod cannot_lint_in_const_context {
+    fn without_const(c: char) -> bool {
+        c.is_digit(8)
+        //~^ to_digit_is_some
+    }
+    const fn with_const(c: char) -> bool {
+        c.to_digit(8).is_some()
+    }
+}
+
+#[clippy::msrv = "1.87"]
+const fn with_const(c: char) -> bool {
+    c.is_digit(8)
+    //~^ to_digit_is_some
+}

tests/ui/to_digit_is_some.rs

@@ -9,3 +9,20 @@ fn main() {
     let _ = char::to_digit(c, 8).is_some();
     //~^ to_digit_is_some
 }
+
+#[clippy::msrv = "1.86"]
+mod cannot_lint_in_const_context {
+    fn without_const(c: char) -> bool {
+        c.to_digit(8).is_some()
+        //~^ to_digit_is_some
+    }
+    const fn with_const(c: char) -> bool {
+        c.to_digit(8).is_some()
+    }
+}
+
+#[clippy::msrv = "1.87"]
+const fn with_const(c: char) -> bool {
+    c.to_digit(8).is_some()
+    //~^ to_digit_is_some
+}

tests/ui/to_digit_is_some.stderr

@@ -13,5 +13,17 @@ error: use of `.to_digit(..).is_some()`
 LL |     let _ = char::to_digit(c, 8).is_some();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `char::is_digit(c, 8)`
 
-error: aborting due to 2 previous errors
+error: use of `.to_digit(..).is_some()`
+  --> tests/ui/to_digit_is_some.rs:16:9
+   |
+LL |         c.to_digit(8).is_some()
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `c.is_digit(8)`
+
+error: use of `.to_digit(..).is_some()`
+  --> tests/ui/to_digit_is_some.rs:26:5
+   |
+LL |     c.to_digit(8).is_some()
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `c.is_digit(8)`
+
+error: aborting due to 4 previous errors