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

compiler/rustc_hir_typeck/src/cast.rs

@@ -31,7 +31,9 @@
 use super::FnCtxt;
 
 use crate::type_error_struct;
-use rustc_errors::{struct_span_err, Applicability, DelayDm, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{
+    struct_span_err, Applicability, DelayDm, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+};
 use rustc_hir as hir;
 use rustc_macros::{TypeFoldable, TypeVisitable};
 use rustc_middle::mir::Mutability;
@@ -270,6 +272,9 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                         }
                     ));
                 }
+
+                self.try_suggest_collection_to_bool(fcx, &mut err);
+
                 err.emit();
             }
             CastError::NeedViaInt => {
@@ -517,6 +522,9 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 } else {
                     err.span_label(self.span, "invalid cast");
                 }
+
+                self.try_suggest_collection_to_bool(fcx, &mut err);
+
                 err.emit();
             }
             CastError::SizedUnsizedCast => {
@@ -1080,4 +1088,40 @@ impl<'a, 'tcx> CastCheck<'tcx> {
             },
         );
     }
+
+    /// Attempt to suggest using `.is_empty` when trying to cast from a
+    /// collection type to a boolean.
+    fn try_suggest_collection_to_bool(&self, fcx: &FnCtxt<'a, 'tcx>, err: &mut Diagnostic) {
+        if self.cast_ty.is_bool() {
+            let derefed = fcx
+                .autoderef(self.expr_span, self.expr_ty)
+                .silence_errors()
+                .find(|t| matches!(t.0.kind(), ty::Str | ty::Slice(..)));
+
+            if let Some((deref_ty, _)) = derefed {
+                // Give a note about what the expr derefs to.
+                if deref_ty != self.expr_ty.peel_refs() {
+                    err.span_note(
+                        self.expr_span,
+                        format!(
+                            "this expression `Deref`s to `{}` which implements `is_empty`",
+                            fcx.ty_to_string(deref_ty)
+                        ),
+                    );
+                }
+
+                // Create a multipart suggestion: add `!` and `.is_empty()` in
+                // place of the cast.
+                let suggestion = vec![
+                    (self.expr_span.shrink_to_lo(), "!".to_string()),
+                    (self.span.with_lo(self.expr_span.hi()), ".is_empty()".to_string()),
+                ];
+
+                err.multipart_suggestion_verbose(format!(
+                    "consider using the `is_empty` method on `{}` to determine if it contains anything",
+                    fcx.ty_to_string(self.expr_ty),
+                ),  suggestion, Applicability::MaybeIncorrect);
+            }
+        }
+    }
 }

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
 }

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
 

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]
+}

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`.