empty_struct_with_brackets: do not lint code coming from macro expansion (#14623)

Do not attempt to fetch a snippet from expansion. Without this change,
the inside of macros could[*] be shown as the source of the problem.

[*] Due to the way the source code is processed and reparsed in this
macro, the declarative macro has to be located outside the current
source file for the bug to appear. Otherwise, the macro call itself will
be (mis)identified as a potential `struct` field definition and the lint
will not trigger.

changelog: [`empty_struct_with_brackets`]: do not lint code coming from
macro expansion

Author: y21

clippy_lints/src/empty_with_brackets.rs

@@ -93,6 +93,7 @@ impl_lint_pass!(EmptyWithBrackets => [EMPTY_STRUCTS_WITH_BRACKETS, EMPTY_ENUM_VA
 impl LateLintPass<'_> for EmptyWithBrackets {
     fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
         if let ItemKind::Struct(ident, var_data, _) = &item.kind
+            && !item.span.from_expansion()
             && has_brackets(var_data)
             && let span_after_ident = item.span.with_lo(ident.span.hi())
             && has_no_fields(cx, var_data, span_after_ident)

tests/ui/empty_structs_with_brackets.fixed

@@ -23,4 +23,12 @@ struct MyTupleStruct(usize, String); // should not trigger lint
 struct MySingleTupleStruct(usize); // should not trigger lint
 struct MyUnitLikeStruct; // should not trigger lint
 
+macro_rules! empty_struct {
+    ($s:ident) => {
+        struct $s {}
+    };
+}
+
+empty_struct!(FromMacro);
+
 fn main() {}

tests/ui/empty_structs_with_brackets.rs

@@ -23,4 +23,12 @@ struct MyTupleStruct(usize, String); // should not trigger lint
 struct MySingleTupleStruct(usize); // should not trigger lint
 struct MyUnitLikeStruct; // should not trigger lint
 
+macro_rules! empty_struct {
+    ($s:ident) => {
+        struct $s {}
+    };
+}
+
+empty_struct!(FromMacro);
+
 fn main() {}