suggest `is_empty` for collections when casting to `bool`

4 files changed, 97 insertions, 3 deletions

diff --git a/tests/ui/cast/cast-as-bool.rs b/tests/ui/cast/cast-as-bool.rs
index 1aed218aeb4..fbebc80d91c 100644
--- a/tests/ui/cast/cast-as-bool.rs
+++ b/tests/ui/cast/cast-as-bool.rs
@@ -2,8 +2,12 @@ fn main() {
     let u = 5 as bool; //~ ERROR cannot cast as `bool`
                        //~| HELP compare with zero instead
                        //~| SUGGESTION 5 != 0
+
     let t = (1 + 2) as bool; //~ ERROR cannot cast as `bool`
                              //~| HELP compare with zero instead
                              //~| SUGGESTION (1 + 2) != 0
-    let v = "hello" as bool; //~ ERROR casting `&'static str` as `bool` is invalid
+
+    let v = "hello" as bool;
+    //~^ ERROR casting `&'static str` as `bool` is invalid
+    //~| HELP consider using the `is_empty` method on `&'static str` to determine if it contains anything
 }
diff --git a/tests/ui/cast/cast-as-bool.stderr b/tests/ui/cast/cast-as-bool.stderr
index 15d94ab69d8..19ac8f10fec 100644
--- a/tests/ui/cast/cast-as-bool.stderr
+++ b/tests/ui/cast/cast-as-bool.stderr
@@ -5,16 +5,21 @@ LL |     let u = 5 as bool;
    |             ^^^^^^^^^ help: compare with zero instead: `5 != 0`
 
 error[E0054]: cannot cast as `bool`
-  --> $DIR/cast-as-bool.rs:5:13
+  --> $DIR/cast-as-bool.rs:6:13
    |
 LL |     let t = (1 + 2) as bool;
    |             ^^^^^^^^^^^^^^^ help: compare with zero instead: `(1 + 2) != 0`
 
 error[E0606]: casting `&'static str` as `bool` is invalid
-  --> $DIR/cast-as-bool.rs:8:13
+  --> $DIR/cast-as-bool.rs:10:13
    |
 LL |     let v = "hello" as bool;
    |             ^^^^^^^^^^^^^^^
+   |
+help: consider using the `is_empty` method on `&'static str` to determine if it contains anything
+   |
+LL |     let v = !"hello".is_empty();
+   |             +       ~~~~~~~~~~~
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/cast/issue-106883-is-empty.rs b/tests/ui/cast/issue-106883-is-empty.rs
new file mode 100644
index 00000000000..27e0816dd1c
--- /dev/null
+++ b/tests/ui/cast/issue-106883-is-empty.rs
@@ -0,0 +1,27 @@
+use std::ops::Deref;
+
+struct Foo;
+
+impl Deref for Foo {
+    type Target = [u8];
+
+    fn deref(&self) -> &Self::Target {
+        &[]
+    }
+}
+
+fn main() {
+    let _ = "foo" as bool;
+    //~^ ERROR casting `&'static str` as `bool` is invalid [E0606]
+
+    let _ = String::from("foo") as bool;
+    //~^ ERROR non-primitive cast: `String` as `bool` [E0605]
+
+    let _ = Foo as bool;
+    //~^ ERROR non-primitive cast: `Foo` as `bool` [E0605]
+}
+
+fn _slice(bar: &[i32]) -> bool {
+    bar as bool
+    //~^ ERROR casting `&[i32]` as `bool` is invalid [E0606]
+}
diff --git a/tests/ui/cast/issue-106883-is-empty.stderr b/tests/ui/cast/issue-106883-is-empty.stderr
new file mode 100644
index 00000000000..7115c7704ca
--- /dev/null
+++ b/tests/ui/cast/issue-106883-is-empty.stderr
@@ -0,0 +1,58 @@
+error[E0606]: casting `&'static str` as `bool` is invalid
+  --> $DIR/issue-106883-is-empty.rs:14:13
+   |
+LL |     let _ = "foo" as bool;
+   |             ^^^^^^^^^^^^^
+   |
+help: consider using the `is_empty` method on `&'static str` to determine if it contains anything
+   |
+LL |     let _ = !"foo".is_empty();
+   |             +     ~~~~~~~~~~~
+
+error[E0605]: non-primitive cast: `String` as `bool`
+  --> $DIR/issue-106883-is-empty.rs:17:13
+   |
+LL |     let _ = String::from("foo") as bool;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |
+note: this expression `Deref`s to `str` which implements `is_empty`
+  --> $DIR/issue-106883-is-empty.rs:17:13
+   |
+LL |     let _ = String::from("foo") as bool;
+   |             ^^^^^^^^^^^^^^^^^^^
+help: consider using the `is_empty` method on `String` to determine if it contains anything
+   |
+LL |     let _ = !String::from("foo").is_empty();
+   |             +                   ~~~~~~~~~~~
+
+error[E0605]: non-primitive cast: `Foo` as `bool`
+  --> $DIR/issue-106883-is-empty.rs:20:13
+   |
+LL |     let _ = Foo as bool;
+   |             ^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |
+note: this expression `Deref`s to `[u8]` which implements `is_empty`
+  --> $DIR/issue-106883-is-empty.rs:20:13
+   |
+LL |     let _ = Foo as bool;
+   |             ^^^
+help: consider using the `is_empty` method on `Foo` to determine if it contains anything
+   |
+LL |     let _ = !Foo.is_empty();
+   |             +   ~~~~~~~~~~~
+
+error[E0606]: casting `&[i32]` as `bool` is invalid
+  --> $DIR/issue-106883-is-empty.rs:25:5
+   |
+LL |     bar as bool
+   |     ^^^^^^^^^^^
+   |
+help: consider using the `is_empty` method on `&[i32]` to determine if it contains anything
+   |
+LL |     !bar.is_empty()
+   |     +   ~~~~~~~~~~~
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0605, E0606.
+For more information about an error, try `rustc --explain E0605`.