Avoid `kw::Empty` when dealing with `rustc_allowed_through_unstable_modules`.

The existing code produces `Some(kw::Empty)` for these invalid forms:

- a non-name-value, e.g. `#[rustc_allowed_through_unstable_modules]`

- a non-string arg, e.g. `#[rustc_allowed_through_unstable_modules = 3]`

The new code avoids the `kw::Empty` and is a little shorter. It will
produce `None` in those cases, which means E0789 won't be produced if
the `stable` attribute is missing for these invalid forms. This doesn't
matter, because these invalid forms will trigger an "malformed
`rustc_allowed_through_unstable_modules` attribute" anyway.

2 files changed, 3 insertions, 7 deletions

diff --git a/compiler/rustc_attr_parsing/src/attributes/stability.rs b/compiler/rustc_attr_parsing/src/attributes/stability.rs
index 6d76456e83c..bdad6b50186 100644
--- a/compiler/rustc_attr_parsing/src/attributes/stability.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/stability.rs
@@ -5,7 +5,7 @@ use rustc_attr_data_structures::{
     StableSince, UnstableReason, VERSION_PLACEHOLDER,
 };
 use rustc_errors::ErrorGuaranteed;
-use rustc_span::{Span, Symbol, kw, sym};
+use rustc_span::{Span, Symbol, sym};
 
 use super::util::parse_version;
 use super::{AcceptMapping, AttributeParser, SingleAttributeParser};
@@ -61,11 +61,7 @@ impl AttributeParser for StabilityParser {
         }),
         (&[sym::rustc_allowed_through_unstable_modules], |this, cx, args| {
             reject_outside_std!(cx);
-            this.allowed_through_unstable_modules =
-                Some(match args.name_value().and_then(|i| i.value_as_str()) {
-                    Some(msg) => msg,
-                    None => kw::Empty,
-                });
+            this.allowed_through_unstable_modules = args.name_value().and_then(|i| i.value_as_str())
         }),
     ];
 
diff --git a/compiler/rustc_error_codes/src/error_codes/E0789.md b/compiler/rustc_error_codes/src/error_codes/E0789.md
index 2c0018cc799..c7bc6cfde51 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0789.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0789.md
@@ -14,7 +14,7 @@ Erroneous code example:
 
 #![unstable(feature = "foo_module", reason = "...", issue = "123")]
 
-#[rustc_allowed_through_unstable_modules]
+#[rustc_allowed_through_unstable_modules = "deprecation message"]
 // #[stable(feature = "foo", since = "1.0")]
 struct Foo;
 // ^^^ error: `rustc_allowed_through_unstable_modules` attribute must be