Rollup merge of rust-lang#72780 - GuillaumeGomez:enforce-doc-alias-check, r=ollie27

Enforce doc alias check

Part of rust-lang#50146.

r? @ollie27

Author: Manishearth

src/librustdoc/clean/types.rs

@@ -486,6 +486,33 @@ impl Attributes {
         })
     }
 
+    /// Enforce the format of attributes inside `#[doc(...)]`.
+    pub fn check_doc_attributes(
+        diagnostic: &::rustc_errors::Handler,
+        mi: &ast::MetaItem,
+    ) -> Option<(String, String)> {
+        mi.meta_item_list().and_then(|list| {
+            for meta in list {
+                if meta.check_name(sym::alias) {
+                    if !meta.is_value_str()
+                        || meta
+                            .value_str()
+                            .map(|s| s.to_string())
+                            .unwrap_or_else(String::new)
+                            .is_empty()
+                    {
+                        diagnostic.span_err(
+                            meta.span(),
+                            "doc alias attribute expects a string: #[doc(alias = \"0\")]",
+                        );
+                    }
+                }
+            }
+
+            None
+        })
+    }
+
     pub fn has_doc_flag(&self, flag: Symbol) -> bool {
         for attr in &self.other_attrs {
             if !attr.check_name(sym::doc) {
@@ -529,6 +556,7 @@ impl Attributes {
                 } else {
                     if attr.check_name(sym::doc) {
                         if let Some(mi) = attr.meta() {
+                            Attributes::check_doc_attributes(&diagnostic, &mi);
                             if let Some(cfg_mi) = Attributes::extract_cfg(&mi) {
                                 // Extracted #[doc(cfg(...))]
                                 match Cfg::parse(cfg_mi) {

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

@@ -0,0 +1,9 @@
+#![feature(doc_alias)]
+
+#[doc(alias = "foo")] // ok!
+pub struct Bar;
+
+#[doc(alias)] //~ ERROR
+#[doc(alias = 0)] //~ ERROR
+#[doc(alias("bar"))] //~ ERROR
+pub struct Foo;

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

@@ -0,0 +1,20 @@
+error: doc alias attribute expects a string: #[doc(alias = "0")]
+  --> $DIR/check-doc-alias-attr.rs:6:7
+   |
+LL | #[doc(alias)]
+   |       ^^^^^
+
+error: doc alias attribute expects a string: #[doc(alias = "0")]
+  --> $DIR/check-doc-alias-attr.rs:7:7
+   |
+LL | #[doc(alias = 0)]
+   |       ^^^^^^^^^
+
+error: doc alias attribute expects a string: #[doc(alias = "0")]
+  --> $DIR/check-doc-alias-attr.rs:8:7
+   |
+LL | #[doc(alias("bar"))]
+   |       ^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+