Migrate 'is_empty' diagnostics

Author: clubby789

compiler/rustc_hir_typeck/messages.ftl

@@ -29,6 +29,8 @@ hir_typeck_convert_using_method = try using `{$sugg}` to convert `{$found}` to `
 
 hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
 
+hir_typeck_deref_is_empty = this expression `Deref`s to `{$deref_ty}` which implements `is_empty`
+
 hir_typeck_expected_default_return_type = expected `()` because of default return type
 
 hir_typeck_expected_return_type = expected `{$expected}` because of return type
@@ -126,5 +128,9 @@ hir_typeck_suggest_ptr_null_mut = consider using `core::ptr::null_mut` instead
 hir_typeck_union_pat_dotdot = `..` cannot be used in union patterns
 
 hir_typeck_union_pat_multiple_fields = union patterns should have exactly one field
+
+hir_typeck_use_is_empty =
+    consider using the `is_empty` method on `{$expr_ty}` to determine if it contains anything
+
 hir_typeck_yield_expr_outside_of_generator =
     yield expression outside of generator literal

compiler/rustc_hir_typeck/src/cast.rs

@@ -1068,26 +1068,19 @@ impl<'a, 'tcx> CastCheck<'tcx> {
             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)
-                        ),
-                    );
+                    err.subdiagnostic(errors::DerefImplsIsEmpty {
+                        span: self.expr_span,
+                        deref_ty: 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);
+                err.subdiagnostic(errors::UseIsEmpty {
+                    lo: self.expr_span.shrink_to_lo(),
+                    hi: self.span.with_lo(self.expr_span.hi()),
+                    expr_ty: fcx.ty_to_string(self.expr_ty),
+                });
             }
         }
     }

compiler/rustc_hir_typeck/src/errors.rs

@@ -459,6 +459,20 @@ pub struct UnionPatDotDot {
     pub span: Span,
 }
 
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    hir_typeck_use_is_empty,
+    applicability = "maybe-incorrect",
+    style = "verbose"
+)]
+pub struct UseIsEmpty {
+    #[suggestion_part(code = "!")]
+    pub lo: Span,
+    #[suggestion_part(code = ".is_empty()")]
+    pub hi: Span,
+    pub expr_ty: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_typeck_arg_mismatch_indeterminate)]
 pub struct ArgMismatchIndeterminate {
@@ -535,6 +549,14 @@ pub struct CtorIsPrivate {
     pub def: String,
 }
 
+#[derive(Subdiagnostic)]
+#[note(hir_typeck_deref_is_empty)]
+pub struct DerefImplsIsEmpty {
+    #[primary_span]
+    pub span: Span,
+    pub deref_ty: String,
+}
+
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(
     hir_typeck_convert_using_method,