fix!: Disallow empty registry names

This was allowed when switching from `toml` v0.5 to `toml_edit` which
started allowing empty keys when parsing TOML.
This mirrors the change
we made for disallowing empty feature names in rust-lang#12928.

src/cargo/util/command_prelude.rs

@@ -833,6 +833,9 @@ Run `{cmd}` to see possible targets."
             (None, None) => config.default_registry()?.map(RegistryOrIndex::Registry),
             (None, Some(i)) => Some(RegistryOrIndex::Index(i.into_url()?)),
             (Some(r), None) => {
+                if r.is_empty() {
+                    bail!("registry name cannot be empty");
+                }
                 restricted_names::validate_package_name(r, "registry name", "")?;
                 Some(RegistryOrIndex::Registry(r.to_string()))
             }
@@ -848,6 +851,9 @@ Run `{cmd}` to see possible targets."
         match self._value_of("registry").map(|s| s.to_string()) {
             None => config.default_registry(),
             Some(registry) => {
+                if registry.is_empty() {
+                    bail!("registry name cannot be empty");
+                }
                 restricted_names::validate_package_name(&registry, "registry name", "")?;
                 Ok(Some(registry))
             }

src/cargo/util/config/mod.rs

@@ -1551,6 +1551,9 @@ impl Config {
 
     /// Gets the index for a registry.
     pub fn get_registry_index(&self, registry: &str) -> CargoResult<Url> {
+        if registry.is_empty() {
+            bail!("registry name cannot be empty");
+        }
         validate_package_name(registry, "registry name", "")?;
         if let Some(index) = self.get_string(&format!("registries.{}.index", registry))? {
             self.resolve_registry_index(&index).with_context(|| {

tests/testsuite/alt_registry.rs

@@ -1568,7 +1568,7 @@ fn config_empty_registry_name() {
         .with_status(101)
         .with_stderr(
             "\
-[ERROR] registry index was not found in any configuration: ``",
+[ERROR] registry name cannot be empty",
         )
         .run();
 }
@@ -1583,7 +1583,7 @@ fn empty_registry_flag() {
         .with_status(101)
         .with_stderr(
             "\
-[ERROR] registry index was not found in any configuration: ``",
+[ERROR] registry name cannot be empty",
         )
         .run();
 }
@@ -1618,7 +1618,7 @@ fn empty_dependency_registry() {
 [ERROR] failed to parse manifest at `[CWD]/Cargo.toml`
 
 Caused by:
-  registry index was not found in any configuration: ``",
+  registry name cannot be empty",
         )
         .run();
 }