tests: Add minimal reproduction of #61963.

This commit adds a reproduction of the error reported in servo which
demonstrates the current, incorrect behaviour.

Co-authored-by: Rémy Rakić <remy.rakic@gmail.com>

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 +1,25 @@
+// 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]
+//~^ ERROR trait objects without an explicit `dyn` are deprecated [bare_trait_objects]
+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 +1,20 @@
+error: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/issue-61963.rs:21:14
+   |
+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: 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
+