Change error about unknown doc attributes to a warning

This prevents breakage across the ecosystem, since the error was just introduced recently without first having a warning period.

Author: jyn514

compiler/rustc_passes/src/check_attr.rs

@@ -567,16 +567,23 @@ impl CheckAttrVisitor<'tcx> {
  .iter()
  .any(|m| i_meta.has_name(*m))
  {
- self.tcx
- .sess
- .struct_span_err(
- meta.span(),
- &format!(
+ self.tcx.struct_span_lint_hir(
+ UNUSED_ATTRIBUTES,
+ hir_id,
+ i_meta.span,
+ |lint| {
+ lint.build(&format!(
  "unknown `doc` attribute `{}`",
- i_meta.name_or_empty(),
- ),
- )
- .emit();
+ i_meta.name_or_empty()
+ ))
+ .warn(
+ "this was previously accepted by the compiler but is \
+ being phased out; it will become a hard error in \
+ a future release!",
+ )
+ .emit();
+ },
+ );
  return false;
  }
  }

src/test/rustdoc-ui/doc-attr.rs

@@ -1,5 +1,11 @@
 #![crate_type = "lib"]
-#![doc(as_ptr)] //~ ERROR
+#![deny(unused_attributes)]
+//~^ NOTE lint level is defined here
+#![doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 
-#[doc(as_ptr)] //~ ERROR
+#[doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 pub fn foo() {}

src/test/rustdoc-ui/doc-attr.stderr

@@ -1,14 +1,23 @@
 error: unknown `doc` attribute `as_ptr`
- --> $DIR/doc-attr.rs:4:7
+ --> $DIR/doc-attr.rs:8:7
  |
 LL | #[doc(as_ptr)]
  | ^^^^^^
+ |
+note: the lint level is defined here
+ --> $DIR/doc-attr.rs:2:9
+ |
+LL | #![deny(unused_attributes)]
+ | ^^^^^^^^^^^^^^^^^
+ = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unknown `doc` attribute `as_ptr`
- --> $DIR/doc-attr.rs:2:8
+ --> $DIR/doc-attr.rs:4:8
  |
 LL | #![doc(as_ptr)]
  | ^^^^^^
+ |
+ = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: aborting due to 2 previous errors
 

src/test/ui/attributes/doc-attr.rs

@@ -1,5 +1,11 @@
 #![crate_type = "lib"]
-#![doc(as_ptr)] //~ ERROR
+#![deny(unused_attributes)]
+//~^ NOTE lint level is defined here
+#![doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 
-#[doc(as_ptr)] //~ ERROR
+#[doc(as_ptr)]
+//~^ ERROR unknown `doc` attribute
+//~| WARNING will become a hard error in a future release
 pub fn foo() {}

src/test/ui/attributes/doc-attr.stderr

@@ -1,14 +1,23 @@
 error: unknown `doc` attribute `as_ptr`
- --> $DIR/doc-attr.rs:4:7
+ --> $DIR/doc-attr.rs:8:7
  |
 LL | #[doc(as_ptr)]
  | ^^^^^^
+ |
+note: the lint level is defined here
+ --> $DIR/doc-attr.rs:2:9
+ |
+LL | #![deny(unused_attributes)]
+ | ^^^^^^^^^^^^^^^^^
+ = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: unknown `doc` attribute `as_ptr`
- --> $DIR/doc-attr.rs:2:8
+ --> $DIR/doc-attr.rs:4:8
  |
 LL | #![doc(as_ptr)]
  | ^^^^^^
+ |
+ = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 
 error: aborting due to 2 previous errors