resolve: Use `feature(custom_attribute)` fallback only if the feature is enabled

Normally `#![feature(...)]` shouldn't change behavior, but custom attributes in particular are in the process of retirement, and we should not produce a message telling to enable them.
It also helps with unifying diagnostics for unresolved macros.

4 files changed, 18 insertions, 30 deletions

diff --git a/src/test/ui/attributes/obsolete-attr.rs b/src/test/ui/attributes/obsolete-attr.rs
index 89e2ad2669c..8759344e6f8 100644
--- a/src/test/ui/attributes/obsolete-attr.rs
+++ b/src/test/ui/attributes/obsolete-attr.rs
@@ -1,7 +1,9 @@
 // Obsolete attributes fall back to feature gated custom attributes.
 
-#[ab_isize="stdcall"] extern {} //~ ERROR attribute `ab_isize` is currently unknown
+#[ab_isize="stdcall"] extern {}
+//~^ ERROR cannot find attribute macro `ab_isize` in this scope
 
-#[fixed_stack_segment] fn f() {} //~ ERROR attribute `fixed_stack_segment` is currently unknown
+#[fixed_stack_segment] fn f() {}
+//~^ ERROR cannot find attribute macro `fixed_stack_segment` in this scope
 
 fn main() {}
diff --git a/src/test/ui/attributes/obsolete-attr.stderr b/src/test/ui/attributes/obsolete-attr.stderr
index 6021700dfbb..9c6909f65f3 100644
--- a/src/test/ui/attributes/obsolete-attr.stderr
+++ b/src/test/ui/attributes/obsolete-attr.stderr
@@ -1,21 +1,14 @@
-error[E0658]: The attribute `fixed_stack_segment` is currently unknown to the compiler and may have meaning added to it in the future
-  --> $DIR/obsolete-attr.rs:5:3
+error: cannot find attribute macro `fixed_stack_segment` in this scope
+  --> $DIR/obsolete-attr.rs:6:3
    |
 LL | #[fixed_stack_segment] fn f() {}
    |   ^^^^^^^^^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `ab_isize` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `ab_isize` in this scope
   --> $DIR/obsolete-attr.rs:3:3
    |
 LL | #[ab_isize="stdcall"] extern {}
    |   ^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/attributes/unknown-attr.rs b/src/test/ui/attributes/unknown-attr.rs
index e2a4f3226d5..140a1fc3f93 100644
--- a/src/test/ui/attributes/unknown-attr.rs
+++ b/src/test/ui/attributes/unknown-attr.rs
@@ -2,8 +2,11 @@
 
 #![feature(custom_inner_attributes)]
 
-#![mutable_doc] //~ ERROR attribute `mutable_doc` is currently unknown
+#![mutable_doc]
+//~^ ERROR cannot find attribute macro `mutable_doc` in this scope
 
-#[dance] mod a {} //~ ERROR attribute `dance` is currently unknown
+#[dance] mod a {}
+//~^ ERROR cannot find attribute macro `dance` in this scope
 
-#[dance] fn main() {} //~ ERROR attribute `dance` is currently unknown
+#[dance] fn main() {}
+//~^ ERROR cannot find attribute macro `dance` in this scope
diff --git a/src/test/ui/attributes/unknown-attr.stderr b/src/test/ui/attributes/unknown-attr.stderr
index b46db566293..4d463874d66 100644
--- a/src/test/ui/attributes/unknown-attr.stderr
+++ b/src/test/ui/attributes/unknown-attr.stderr
@@ -1,30 +1,20 @@
-error[E0658]: The attribute `mutable_doc` is currently unknown to the compiler and may have meaning added to it in the future
+error: cannot find attribute macro `mutable_doc` in this scope
   --> $DIR/unknown-attr.rs:5:4
    |
 LL | #![mutable_doc]
    |    ^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `dance` is currently unknown to the compiler and may have meaning added to it in the future
-  --> $DIR/unknown-attr.rs:7:3
+error: cannot find attribute macro `dance` in this scope
+  --> $DIR/unknown-attr.rs:8:3
    |
 LL | #[dance] mod a {}
    |   ^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
-error[E0658]: The attribute `dance` is currently unknown to the compiler and may have meaning added to it in the future
-  --> $DIR/unknown-attr.rs:9:3
+error: cannot find attribute macro `dance` in this scope
+  --> $DIR/unknown-attr.rs:11:3
    |
 LL | #[dance] fn main() {}
    |   ^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
-   = help: add `#![feature(custom_attribute)]` to the crate attributes to enable
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0658`.