Use deserialize_with instead of specializing doc_extern_map

src/cargo/core/compiler/rustdoc.rs

@@ -52,13 +52,36 @@ impl<'de> serde::de::Deserialize<'de> for RustdocExternMode {
     }
 }
 
-#[derive(serde::Deserialize, Debug, Default)]
+#[derive(serde::Deserialize, Debug)]
 #[serde(default)]
 pub struct RustdocExternMap {
+    #[serde(deserialize_with = "default_crates_io_to_docs_rs")]
     pub(crate) registries: HashMap<String, String>,
     std: Option<RustdocExternMode>,
 }
 
+impl Default for RustdocExternMap {
+    fn default() -> Self {
+        let mut registries = HashMap::new();
+        registries.insert("crates-io".into(), "https://docs.rs/".into());
+        Self {
+            registries,
+            std: None,
+        }
+    }
+}
+
+fn default_crates_io_to_docs_rs<'de, D: serde::Deserializer<'de>>(
+    de: D,
+) -> Result<HashMap<String, String>, D::Error> {
+    use serde::Deserialize;
+    let mut registries = HashMap::deserialize(de)?;
+    if !registries.contains_key("crates-io") {
+        registries.insert("crates-io".into(), "https://docs.rs/".into());
+    }
+    Ok(registries)
+}
+
 impl hash::Hash for RustdocExternMap {
     fn hash<H: hash::Hasher>(&self, into: &mut H) {
         self.std.hash(into);

src/cargo/util/config/mod.rs

@@ -1217,15 +1217,8 @@ impl Config {
         // Note: This does not support environment variables. The `Unit`
         // fundamentally does not have access to the registry name, so there is
         // nothing to query. Plumbing the name into SourceId is quite challenging.
-        self.doc_extern_map.try_borrow_with(|| {
-            let mut extern_map = self.get::<RustdocExternMap>("doc.extern-map");
-            if let Ok(map) = &mut extern_map {
-                map.registries
-                    .entry("crates-io".into())
-                    .or_insert("https://docs.rs/".into());
-            }
-            extern_map
-        })
+        self.doc_extern_map
+            .try_borrow_with(|| self.get::<RustdocExternMap>("doc.extern-map"))
     }
 
     /// Returns the `[target]` table definition for the given target triple.