Stop bare trait lint applying to macro call sites

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/auxiliary/issue-61963-1.rs

@@ -0,0 +1,40 @@
+// force-host
+// no-prefer-dynamic
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::{Group, TokenStream, TokenTree};
+
+// This macro exists as part of a reproduction of #61963 but without using quote/syn/proc_macro2.
+
+#[proc_macro_derive(DomObject)]
+pub fn expand_token_stream(input: TokenStream) -> TokenStream {
+    // Construct a dummy span - `#0 bytes(0..0)` - which is present in the input because
+    // of the specially crafted generated tokens in the `attribute-crate` proc-macro.
+    let dummy_span = input.clone().into_iter().nth(0).unwrap().span();
+
+    // Define what the macro would output if constructed properly from the source using syn/quote.
+    let output: TokenStream = "impl Bar for ((), Qux<Qux<Baz> >) { }
+    impl Bar for ((), Box<Bar>) { }".parse().unwrap();
+
+    let mut tokens: Vec<_> = output.into_iter().collect();
+    // Adjust token spans to match the original crate (which would use `quote`). Some of the
+    // generated tokens point to the dummy span.
+    for token in tokens.iter_mut() {
+        if let TokenTree::Group(group) = token {
+            let mut tokens: Vec<_> = group.stream().into_iter().collect();
+            for token in tokens.iter_mut().skip(2) {
+                token.set_span(dummy_span);
+            }
+
+            let mut stream = TokenStream::new();
+            stream.extend(tokens);
+            *group = Group::new(group.delimiter(), stream);
+        }
+    }
+
+    let mut output = TokenStream::new();
+    output.extend(tokens);
+    output
+}

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

@@ -0,0 +1,41 @@
+// force-host
+// no-prefer-dynamic
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::{Group, Spacing, Punct, TokenTree, TokenStream};
+
+// This macro exists as part of a reproduction of #61963 but without using quote/syn/proc_macro2.
+
+#[proc_macro_attribute]
+pub fn dom_struct(_: TokenStream, input: TokenStream) -> TokenStream {
+    // Construct the expected output tokens - the input but with a `#[derive(DomObject)]` applied.
+    let attributes: TokenStream =
+        "#[derive(DomObject)]".to_string().parse().unwrap();
+    let output: TokenStream = attributes.into_iter()
+        .chain(input.into_iter()).collect();
+
+    let mut tokens: Vec<_> = output.into_iter().collect();
+    // Adjust the spacing of `>` tokens to match what `quote` would produce.
+    for token in tokens.iter_mut() {
+        if let TokenTree::Group(group) = token {
+            let mut tokens: Vec<_> = group.stream().into_iter().collect();
+            for token in tokens.iter_mut() {
+                if let TokenTree::Punct(p) = token {
+                    if p.as_char() == '>' {
+                        *p = Punct::new('>', Spacing::Alone);
+                    }
+                }
+            }
+
+            let mut stream = TokenStream::new();
+            stream.extend(tokens);
+            *group = Group::new(group.delimiter(), stream);
+        }
+    }
+
+    let mut output = TokenStream::new();
+    output.extend(tokens);
+    output
+}

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

@@ -0,0 +1,24 @@
+// aux-build:issue-61963.rs
+// aux-build:issue-61963-1.rs
+#![deny(bare_trait_objects)]
+
+#[macro_use]
+extern crate issue_61963;
+#[macro_use]
+extern crate issue_61963_1;
+
+// This test checks that the bare trait object lint does not trigger on macro attributes that
+// generate code which would trigger the lint.
+
+pub struct Baz;
+pub trait Bar { }
+pub struct Qux<T>(T);
+
+#[dom_struct]
+pub struct Foo {
+    qux: Qux<Qux<Baz>>,
+    bar: Box<Bar>,
+    //~^ ERROR trait objects without an explicit `dyn` are deprecated [bare_trait_objects]
+}
+
+fn main() {}

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

@@ -0,0 +1,14 @@
+error: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/issue-61963.rs:20:14
+   |
+LL |     bar: Box<Bar>,
+   |              ^^^ help: use `dyn`: `dyn Bar`
+   |
+note: lint level defined here
+  --> $DIR/issue-61963.rs:3:9
+   |
+LL | #![deny(bare_trait_objects)]
+   |         ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+