Only deny doc_keyword in std and set it as "allow" by default

compiler/rustc_lint/src/internal.rs

@@ -270,7 +270,7 @@ impl EarlyLintPass for LintPassImpl {
 
 declare_tool_lint! {
     pub rustc::EXISTING_DOC_KEYWORD,
-    Deny,
+    Allow,
     "Check that documented keywords in std and core actually exist",
     report_in_external_macro: true
 }

library/std/src/lib.rs

@@ -212,6 +212,7 @@
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)
 )]
+#![deny(rustc::existing_doc_keyword)]
 #![cfg_attr(all(test, target_vendor = "fortanix", target_env = "sgx"), feature(fixed_size_array))]
 // std is implemented with unstable features, many of which are internal
 // compiler details that will never be stable

src/test/ui-fulldeps/internal-lints/existing_doc_keyword.rs

@@ -5,5 +5,7 @@
 
 #![crate_type = "lib"]
 
+#![deny(rustc::existing_doc_keyword)]
+
 #[doc(keyword = "tadam")] //~ ERROR
 mod tadam {}

src/test/ui-fulldeps/internal-lints/existing_doc_keyword.stderr

@@ -1,10 +1,14 @@
 error: Found non-existing keyword `tadam` used in `#[doc(keyword = "...")]`
-  --> $DIR/existing_doc_keyword.rs:8:1
+  --> $DIR/existing_doc_keyword.rs:10:1
    |
 LL | #[doc(keyword = "tadam")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `#[deny(rustc::existing_doc_keyword)]` on by default
+note: the lint level is defined here
+  --> $DIR/existing_doc_keyword.rs:8:9
+   |
+LL | #![deny(rustc::existing_doc_keyword)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: only existing keywords are allowed in core/std
 
 error: aborting due to previous error