Report that doc(plugins) doesn't work using diagnostics instead of println!

This also adds a test for the output and fixes `rustc_attr` to properly
know that `plugins` is a valid attribute.

Author: jyn514

compiler/rustc_passes/src/check_attr.rs

@@ -559,7 +559,8 @@ impl CheckAttrVisitor<'tcx> {
                             sym::masked,
                             sym::no_default_passes, // deprecated
                             sym::no_inline,
-                            sym::passes, // deprecated
+                            sym::passes,  // deprecated
+                            sym::plugins, // removed, but rustdoc warns about it itself
                             sym::primitive,
                             sym::spotlight,
                             sym::test,

src/librustdoc/core.rs

@@ -572,6 +572,8 @@ crate fn run_global_ctxt(
 
         if name == "no_default_passes" {
             msg.help("you may want to use `#![doc(document_private_items)]`");
+        } else if name.starts_with("plugins") {
+            msg.warn("`#![doc(plugins = \"...\")]` no longer functions; see CVE-2018-1000622 <https://nvd.nist.gov/vuln/detail/CVE-2018-1000622>");
         }
 
         msg.emit();
@@ -618,10 +620,6 @@ crate fn run_global_ctxt(
                 }
                 sym::plugins => {
                     report_deprecated_attr("plugins = \"...\"", diag, attr.span());
-                    eprintln!(
-                        "WARNING: `#![doc(plugins = \"...\")]` \
-                         no longer functions; see CVE-2018-1000622"
-                    );
                     continue;
                 }
                 _ => continue,

src/test/rustdoc-ui/deprecated-attrs.rs

@@ -11,3 +11,7 @@
 //~| NOTE see issue #44136
 //~| WARNING ignoring unknown pass
 //~| NOTE `collapse-docs` pass was removed
+#![doc(plugins = "xxx")]
+//~^ WARNING attribute is deprecated
+//~| NOTE see issue #44136
+//~| WARNING no longer functions; see CVE

src/test/rustdoc-ui/deprecated-attrs.stderr

@@ -29,5 +29,14 @@ LL | #![doc(passes = "collapse-docs unindent-comments")]
    |
    = note: the `collapse-docs` pass was removed in #80261 <https://github.com/rust-lang/rust/pull/80261>
 
-warning: 4 warnings emitted
+warning: the `#![doc(plugins = "...")]` attribute is deprecated
+  --> $DIR/deprecated-attrs.rs:14:8
+   |
+LL | #![doc(plugins = "xxx")]
+   |        ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #44136 <https://github.com/rust-lang/rust/issues/44136> for more information
+   = warning: `#![doc(plugins = "...")]` no longer functions; see CVE-2018-1000622 <https://nvd.nist.gov/vuln/detail/CVE-2018-1000622>
+
+warning: 5 warnings emitted