raise ambiguity error on attribute macro that could be tool attribute

compiler/rustc_resolve/src/diagnostics/mod.rs

@@ -1774,3 +1774,14 @@ pub(crate) enum UnusedImportsSugg {
         num_to_remove: usize,
     },
 }
+
+#[derive(Diagnostic)]
+#[diag("attribute macro `{$attr_path}` is ambiguous")]
+#[note("ambiguous because of a name conflict with a tool namespace")]
+#[note("`{$tool}` could refer to a tool namespace")]
+pub(crate) struct ToolAttrAmbiguity {
+    #[primary_span]
+    pub path_span: Span,
+    pub attr_path: String,
+    pub tool: Symbol,
+}

compiler/rustc_resolve/src/macros.rs

@@ -692,6 +692,16 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         });
                     }
                 }
+                if kind == MacroKind::Attr
+                    && let [tool, _second, ..] = path.segments.as_slice()
+                    && self.registered_attr_tool_decls.contains_key(&IdentKey::new(tool.ident))
+                {
+                    self.dcx().emit_err(diagnostics::ToolAttrAmbiguity {
+                        path_span: path.span,
+                        attr_path: pprust::path_to_string(path),
+                        tool: tool.ident.name,
+                    });
+                }
             }
             Res::NonMacroAttr(..) | Res::Err => {}
             _ => panic!("expected `DefKind::Macro` or `Res::NonMacroAttr`"),

tests/ui/diagnostic_namespace/existing_proc_macros.rs

@@ -1,4 +1,3 @@
-//@ check-pass
 //@ proc-macro: proc-macro-helper.rs
 
 extern crate proc_macro_helper;
@@ -17,6 +16,7 @@ mod test2 {
     }
 
     #[diagnostic::on_unimplemented]
+    //~^ ERROR attribute macro `diagnostic::on_unimplemented` is ambiguous
     trait Foo {}
 }
 

tests/ui/diagnostic_namespace/existing_proc_macros.stderr

@@ -0,0 +1,11 @@
+error: attribute macro `diagnostic::on_unimplemented` is ambiguous
+  --> $DIR/existing_proc_macros.rs:18:7
+   |
+LL |     #[diagnostic::on_unimplemented]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ambiguous because of a name conflict with a tool namespace
+   = note: `diagnostic` could refer to a tool namespace
+
+error: aborting due to 1 previous error
+

tests/ui/tool-attributes/no_implicit_prelude.rs

@@ -0,0 +1,27 @@
+#![no_implicit_prelude]
+#![feature(register_tool)]
+#![register_tool(my_tool)]
+
+mod test1 {
+    mod my_tool {
+        pub use ::std::derive as attr;
+    }
+
+    #[my_tool::attr(Debug)]
+    //~^ ERROR attribute macro `my_tool::attr` is ambiguous
+    struct Foo;
+}
+
+mod test2 {
+    mod clippy {
+        pub use ::std::derive as format_args;
+    }
+
+    #[rustfmt::skip]
+    //~^ ERROR cannot find module or crate `rustfmt` in this scope
+    #[clippy::format_args(Debug)]
+    //~^ ERROR attribute macro `clippy::format_args` is ambiguous
+    struct Foo;
+}
+
+fn main() {}

tests/ui/tool-attributes/no_implicit_prelude.stderr

@@ -0,0 +1,27 @@
+error: attribute macro `my_tool::attr` is ambiguous
+  --> $DIR/no_implicit_prelude.rs:10:7
+   |
+LL |     #[my_tool::attr(Debug)]
+   |       ^^^^^^^^^^^^^
+   |
+   = note: ambiguous because of a name conflict with a tool namespace
+   = note: `my_tool` could refer to a tool namespace
+
+error: attribute macro `clippy::format_args` is ambiguous
+  --> $DIR/no_implicit_prelude.rs:22:7
+   |
+LL |     #[clippy::format_args(Debug)]
+   |       ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ambiguous because of a name conflict with a tool namespace
+   = note: `clippy` could refer to a tool namespace
+
+error[E0433]: cannot find module or crate `rustfmt` in this scope
+  --> $DIR/no_implicit_prelude.rs:20:7
+   |
+LL |     #[rustfmt::skip]
+   |       ^^^^^^^ use of unresolved module or unlinked crate `rustfmt`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0433`.