diff --git a/src/cargo/core/source/source_id.rs b/src/cargo/core/source/source_id.rs
index affb130cdaa9..653685b8beca 100644
--- a/src/cargo/core/source/source_id.rs
+++ b/src/cargo/core/source/source_id.rs
@@ -1,4 +1,5 @@
 use crate::core::PackageId;
+use crate::sources::registry::CRATES_IO_HTTP_INDEX;
 use crate::sources::{DirectorySource, CRATES_IO_DOMAIN, CRATES_IO_INDEX, CRATES_IO_REGISTRY};
 use crate::sources::{GitSource, PathSource, RegistrySource};
 use crate::util::{CanonicalUrl, CargoResult, Config, IntoUrl};
@@ -206,6 +207,18 @@ impl SourceId {
         })
     }
 
+    /// Returns the `SourceId` corresponding to the main repository, using the
+    /// http index if allowed.
+    pub fn crates_io_maybe_http(config: &Config) -> CargoResult<SourceId> {
+        if config.cli_unstable().http_registry {
+            config.check_registry_index_not_set()?;
+            let url = CRATES_IO_HTTP_INDEX.into_url().unwrap();
+            SourceId::new(SourceKind::Registry, url, Some(CRATES_IO_REGISTRY))
+        } else {
+            Self::crates_io(config)
+        }
+    }
+
     /// Gets the `SourceId` associated with given name of the remote registry.
     pub fn alt_registry(config: &Config, key: &str) -> CargoResult<SourceId> {
         let url = config.get_registry_index(key)?;
@@ -356,7 +369,8 @@ impl SourceId {
             SourceKind::Registry => {}
             _ => return false,
         }
-        self.inner.url.as_str() == CRATES_IO_INDEX
+        let url = self.inner.url.as_str();
+        url == CRATES_IO_INDEX || url == CRATES_IO_HTTP_INDEX
     }
 
     /// Hashes `self`.
diff --git a/src/cargo/ops/registry.rs b/src/cargo/ops/registry.rs
index 5bd8decb1b82..0a3928e33f13 100644
--- a/src/cargo/ops/registry.rs
+++ b/src/cargo/ops/registry.rs
@@ -196,6 +196,10 @@ fn verify_dependencies(
         if super::check_dep_has_version(dep, true)? {
             continue;
         }
+        // Allow publishing to crates.io with index.crates.io as a source replacement.
+        if registry_src.is_default_registry() && dep.source_id().is_default_registry() {
+            continue;
+        }
         // TomlManifest::prepare_for_publish will rewrite the dependency
         // to be just the `version` field.
         if dep.source_id() != registry_src {
diff --git a/src/cargo/sources/config.rs b/src/cargo/sources/config.rs
index 0e2b24efb647..ff3b9ccb9578 100644
--- a/src/cargo/sources/config.rs
+++ b/src/cargo/sources/config.rs
@@ -91,6 +91,15 @@ impl<'cfg> SourceConfigMap<'cfg> {
                 replace_with: None,
             },
         )?;
+        if config.cli_unstable().http_registry {
+            base.add(
+                CRATES_IO_REGISTRY,
+                SourceConfig {
+                    id: SourceId::crates_io_maybe_http(config)?,
+                    replace_with: None,
+                },
+            )?;
+        }
         Ok(base)
     }
 
@@ -248,7 +257,7 @@ restore the source replacement configuration to continue the build
             check_not_set("rev", def.rev)?;
         }
         if name == CRATES_IO_REGISTRY && srcs.is_empty() {
-            srcs.push(SourceId::crates_io(self.config)?);
+            srcs.push(SourceId::crates_io_maybe_http(self.config)?);
         }
 
         match srcs.len() {
diff --git a/src/cargo/sources/registry/mod.rs b/src/cargo/sources/registry/mod.rs
index 95510e313b65..413734e10ecb 100644
--- a/src/cargo/sources/registry/mod.rs
+++ b/src/cargo/sources/registry/mod.rs
@@ -186,6 +186,7 @@ use crate::util::{restricted_names, CargoResult, Config, Filesystem, OptVersionR
 
 const PACKAGE_SOURCE_LOCK: &str = ".cargo-ok";
 pub const CRATES_IO_INDEX: &str = "https://github.com/rust-lang/crates.io-index";
+pub const CRATES_IO_HTTP_INDEX: &str = "sparse+https://index.crates.io/";
 pub const CRATES_IO_REGISTRY: &str = "crates-io";
 pub const CRATES_IO_DOMAIN: &str = "crates.io";
 const CRATE_TEMPLATE: &str = "{crate}";