Fix handling of macro arguments within the dropping_copy_types lint

compiler/rustc_lint/src/drop_forget_useless.rs

@@ -151,10 +151,11 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
                     && let Node::Stmt(stmt) = node
                     && let StmtKind::Semi(e) = stmt.kind
                     && e.hir_id == expr.hir_id
+                    && let Some(arg_span) = arg.span.find_ancestor_inside(expr.span)
                 {
                     UseLetUnderscoreIgnoreSuggestion::Suggestion {
-                        start_span: expr.span.shrink_to_lo().until(arg.span),
-                        end_span: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
+                        start_span: expr.span.shrink_to_lo().until(arg_span),
+                        end_span: arg_span.shrink_to_hi().until(expr.span.shrink_to_hi()),
                     }
                 } else {
                     UseLetUnderscoreIgnoreSuggestion::Note

tests/ui/lint/dropping_copy_types-macros.fixed

@@ -0,0 +1,12 @@
+//@ check-fail
+//@ run-rustfix
+
+#![deny(dropping_copy_types)]
+
+use std::fmt::Write;
+
+fn main() {
+    let mut msg = String::new();
+    let _ = writeln!(&mut msg, "test");
+    //~^ ERROR calls to `std::mem::drop`
+}

tests/ui/lint/dropping_copy_types-macros.rs

@@ -0,0 +1,12 @@
+//@ check-fail
+//@ run-rustfix
+
+#![deny(dropping_copy_types)]
+
+use std::fmt::Write;
+
+fn main() {
+    let mut msg = String::new();
+    drop(writeln!(&mut msg, "test"));
+    //~^ ERROR calls to `std::mem::drop`
+}

tests/ui/lint/dropping_copy_types-macros.stderr

@@ -0,0 +1,21 @@
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-macros.rs:10:5
+   |
+LL |     drop(writeln!(&mut msg, "test"));
+   |     ^^^^^--------------------------^
+   |          |
+   |          argument has type `Result<(), std::fmt::Error>`
+   |
+note: the lint level is defined here
+  --> $DIR/dropping_copy_types-macros.rs:4:9
+   |
+LL | #![deny(dropping_copy_types)]
+   |         ^^^^^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     drop(writeln!(&mut msg, "test"));
+LL +     let _ = writeln!(&mut msg, "test");
+   |
+
+error: aborting due to 1 previous error
+