diff --git a/src/cargo/core/compiler/rustdoc.rs b/src/cargo/core/compiler/rustdoc.rs
index 37e70fde0b4..7cab50a7070 100644
--- a/src/cargo/core/compiler/rustdoc.rs
+++ b/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 {
-    registries: HashMap<String, String>,
+    #[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);
@@ -80,10 +103,6 @@ pub fn add_root_urls(
         return Ok(());
     }
     let map = config.doc_extern_map()?;
-    if map.registries.is_empty() && map.std.is_none() {
-        // Skip doing unnecessary work.
-        return Ok(());
-    }
     let mut unstable_opts = false;
     // Collect mapping of registry name -> index url.
     let name2url: HashMap<&String, Url> = map
diff --git a/tests/testsuite/rustdoc_extern_html.rs b/tests/testsuite/rustdoc_extern_html.rs
index 368794785fd..b5b970320dd 100644
--- a/tests/testsuite/rustdoc_extern_html.rs
+++ b/tests/testsuite/rustdoc_extern_html.rs
@@ -32,21 +32,10 @@ fn basic_project() -> Project {
         .build()
 }
 
-fn docs_rs(p: &Project) {
-    p.change_file(
-        ".cargo/config",
-        r#"
-            [doc.extern-map.registries]
-            crates-io = "https://docs.rs/"
-        "#,
-    );
-}
-
 #[cargo_test]
 fn ignores_on_stable() {
     // Requires -Zrustdoc-map to use.
     let p = basic_project();
-    docs_rs(&p);
     p.cargo("doc -v --no-deps")
         .with_stderr_does_not_contain("[..]--extern-html-root-url[..]")
         .run();
@@ -60,7 +49,6 @@ fn simple() {
         return;
     }
     let p = basic_project();
-    docs_rs(&p);
     p.cargo("doc -v --no-deps -Zrustdoc-map")
         .masquerade_as_nightly_cargo()
         .with_stderr_contains(
@@ -157,7 +145,6 @@ fn renamed_dep() {
             "#,
         )
         .build();
-    docs_rs(&p);
     p.cargo("doc -v --no-deps -Zrustdoc-map")
         .masquerade_as_nightly_cargo()
         .with_stderr_contains(
@@ -211,7 +198,6 @@ fn lib_name() {
             "#,
         )
         .build();
-    docs_rs(&p);
     p.cargo("doc -v --no-deps -Zrustdoc-map")
         .masquerade_as_nightly_cargo()
         .with_stderr_contains(
@@ -338,7 +324,6 @@ fn multiple_versions() {
             ",
         )
         .build();
-    docs_rs(&p);
     p.cargo("doc -v --no-deps -Zrustdoc-map")
         .masquerade_as_nightly_cargo()
         .with_stderr_contains(
@@ -364,12 +349,21 @@ fn rebuilds_when_changing() {
     let p = basic_project();
     p.cargo("doc -v --no-deps -Zrustdoc-map")
         .masquerade_as_nightly_cargo()
-        .with_stderr_does_not_contain("[..]--extern-html-root-url[..]")
+        .with_stderr_contains("[..]--extern-html-root-url[..]")
         .run();
 
-    docs_rs(&p);
+    // This also tests that the map for docs.rs can be overridden.
+    p.change_file(
+        ".cargo/config",
+        r#"
+            [doc.extern-map.registries]
+            crates-io = "https://example.com/"
+        "#,
+    );
     p.cargo("doc -v --no-deps -Zrustdoc-map")
         .masquerade_as_nightly_cargo()
-        .with_stderr_contains("[..]--extern-html-root-url[..]")
+        .with_stderr_contains(
+            "[RUNNING] `rustdoc [..]--extern-html-root-url [..]bar=https://example.com/bar/1.0.0/[..]",
+        )
         .run();
 }