lowering: Omit bare trait lint on macro call sites

This commit implements a hacky fix for detecting when a span is pointing
at a macro call site so that bare trait lints are not made incorrectly.

Author: davidtwco

src/librustc/hir/lowering.rs

@@ -5754,13 +5754,21 @@ impl<'a> LoweringContext<'a> {
     }
 
     fn maybe_lint_bare_trait(&self, span: Span, id: NodeId, is_global: bool) {
-        self.sess.buffer_lint_with_diagnostic(
-            builtin::BARE_TRAIT_OBJECTS,
-            id,
-            span,
-            "trait objects without an explicit `dyn` are deprecated",
-            builtin::BuiltinLintDiagnostics::BareTraitObject(span, is_global),
-        )
+        // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
+        // call site which do not have a macro backtrace. See #61963.
+        let is_macro_callsite = self.sess.source_map()
+            .span_to_snippet(span)
+            .map(|snippet| snippet.starts_with("#["))
+            .unwrap_or(true);
+        if !is_macro_callsite {
+            self.sess.buffer_lint_with_diagnostic(
+                builtin::BARE_TRAIT_OBJECTS,
+                id,
+                span,
+                "trait objects without an explicit `dyn` are deprecated",
+                builtin::BuiltinLintDiagnostics::BareTraitObject(span, is_global),
+            )
+        }
     }
 
     fn wrap_in_try_constructor(

src/test/ui/suggestions/issue-61963.rs

@@ -15,7 +15,6 @@ pub trait Bar { }
 pub struct Qux<T>(T);
 
 #[dom_struct]
-//~^ ERROR trait objects without an explicit `dyn` are deprecated [bare_trait_objects]
 pub struct Foo {
     qux: Qux<Qux<Baz>>,
     bar: Box<Bar>,

src/test/ui/suggestions/issue-61963.stderr

@@ -1,5 +1,5 @@
 error: trait objects without an explicit `dyn` are deprecated
-  --> $DIR/issue-61963.rs:21:14
+  --> $DIR/issue-61963.rs:20:14
    |
 LL |     bar: Box<Bar>,
    |              ^^^ help: use `dyn`: `dyn Bar`
@@ -10,11 +10,5 @@ note: lint level defined here
 LL | #![deny(bare_trait_objects)]
    |         ^^^^^^^^^^^^^^^^^^
 
-error: trait objects without an explicit `dyn` are deprecated
-  --> $DIR/issue-61963.rs:17:1
-   |
-LL | #[dom_struct]
-   | ^^^^^^^^^^^^^ help: use `dyn`: `dyn #[dom_struct]`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error