Default cargo publish to the alt registry if it's the only allowed one

src/cargo/core/package.rs

@@ -210,7 +210,10 @@ impl Package {
     pub fn authors(&self) -> &Vec<String> {
         &self.manifest().metadata().authors
     }
-    /// Returns `true` if the package is set to publish.
+
+    /// Returns `None` if the package is set to publish.
+    /// Returns `Some(allowed_registries)` if publishing is limited to specified
+    /// registries or if package is set to not publish.
     pub fn publish(&self) -> &Option<Vec<String>> {
         self.manifest().publish()
     }

src/cargo/ops/registry.rs

@@ -52,10 +52,25 @@ pub struct PublishOpts<'cfg> {
 
 pub fn publish(ws: &Workspace<'_>, opts: &PublishOpts<'_>) -> CargoResult<()> {
     let pkg = ws.current()?;
+    let mut publish_registry = opts.registry.clone();
 
     if let Some(ref allowed_registries) = *pkg.publish() {
-        let reg_name = opts
-            .registry
+        if publish_registry.is_none() && allowed_registries.len() == 1 {
+            // If there is only one allowed registry, push to that one directly,
+            // even though there is no registry specified in the command.
+            let default_registry = &allowed_registries[0];
+            if default_registry != CRATES_IO_REGISTRY {
+                // Don't change the registry for crates.io and don't warn the user.
+                // crates.io will be defaulted even without this.
+                opts.config.shell().note(&format!(
+                    "Found `{}` as only allowed registry. Publishing to it automatically.",
+                    default_registry
+                ))?;
+                publish_registry = Some(default_registry.clone());
+            }
+        }
+
+        let reg_name = publish_registry
             .clone()
             .unwrap_or_else(|| CRATES_IO_REGISTRY.to_string());
         if !allowed_registries.contains(&reg_name) {
@@ -72,7 +87,7 @@ pub fn publish(ws: &Workspace<'_>, opts: &PublishOpts<'_>) -> CargoResult<()> {
         opts.config,
         opts.token.clone(),
         opts.index.clone(),
-        opts.registry.clone(),
+        publish_registry,
         true,
         !opts.dry_run,
     )?;

src/doc/man/cargo-publish.md

@@ -51,7 +51,15 @@ Allow working directories with uncommitted VCS changes to be packaged.
 
 {{> options-index }}
 
-{{> options-registry }}
+{{#option "`--registry` _registry_"}}
+Name of the registry to publish to. Registry names are defined in [Cargo
+config files](../reference/config.html). If not specified, and there is a
+[`package.publish`](../reference/manifest.html#the-publish-field) field in
+`Cargo.toml` with a single registry, then it will publish to that registry.
+Otherwise it will use the default registry, which is defined by the
+[`registry.default`](../reference/config.html#registry-default) config key
+which defaults to `crates-io`.
+{{/option}}
 
 {{/options}}
 

src/doc/man/generated_txt/cargo-publish.txt

@@ -56,11 +56,16 @@ OPTIONS
            The URL of the registry index to use.
 
        --registry registry
-           Name of the registry to use. Registry names are defined in Cargo
-           config files
+           Name of the registry to publish to. Registry names are defined in
+           Cargo config files
            <https://doc.rust-lang.org/cargo/reference/config.html>. If not
-           specified, the default registry is used, which is defined by the
-           registry.default config key which defaults to crates-io.
+           specified, and there is a package.publish
+           <https://doc.rust-lang.org/cargo/reference/manifest.html#the-publish-field>
+           field in Cargo.toml with a single registry, then it will publish to
+           that registry. Otherwise it will use the default registry, which is
+           defined by the registry.default
+           <https://doc.rust-lang.org/cargo/reference/config.html#registry-default>
+           config key which defaults to crates-io.
 
    Compilation Options
        --target triple

src/doc/src/commands/cargo-publish.md

@@ -65,11 +65,13 @@ of the registry in all capital letters.</dd>
 
 
 <dt class="option-term" id="option-cargo-publish---registry"><a class="option-anchor" href="#option-cargo-publish---registry"></a><code>--registry</code> <em>registry</em></dt>
-<dd class="option-desc">Name of the registry to use. Registry names are defined in <a href="https://doc.rust-lang.org/cargo/reference/config.html">Cargo config
-files</a>. If not specified, the default registry is used,
-which is defined by the <code>registry.default</code> config key which defaults to
-<code>crates-io</code>.</dd>
-
+<dd class="option-desc">Name of the registry to publish to. Registry names are defined in <a href="https://doc.rust-lang.org/cargo/reference/config.html">Cargo
+config files</a>. If not specified, and there is a
+<a href="https://doc.rust-lang.org/cargo/reference/manifest.html#the-publish-field"><code>package.publish</code></a> field in
+<code>Cargo.toml</code> with a single registry, then it will publish to that registry.
+Otherwise it will use the default registry, which is defined by the
+<a href="https://doc.rust-lang.org/cargo/reference/config.html#registry-default"><code>registry.default</code></a> config key
+which defaults to <code>crates-io</code>.</dd>
 
 
 </dl>

src/doc/src/reference/manifest.md

@@ -406,6 +406,9 @@ allowed to be published to.
 publish = ["some-registry-name"]
 ```
 
+If publish array contains a single registry, `cargo publish` command will use
+it when `--registry` flag is not specified.
+
 <a id="the-metadata-table-optional"></a>
 #### The `metadata` table
 

src/etc/man/cargo-publish.1

@@ -74,10 +74,13 @@ The URL of the registry index to use.
 .sp
 \fB\-\-registry\fR \fIregistry\fR
 .RS 4
-Name of the registry to use. Registry names are defined in \fICargo config
-files\fR <https://doc.rust\-lang.org/cargo/reference/config.html>\&. If not specified, the default registry is used,
-which is defined by the \fBregistry.default\fR config key which defaults to
-\fBcrates\-io\fR\&.
+Name of the registry to publish to. Registry names are defined in \fICargo
+config files\fR <https://doc.rust\-lang.org/cargo/reference/config.html>\&. If not specified, and there is a
+\fI\f(BIpackage.publish\fI\fR <https://doc.rust\-lang.org/cargo/reference/manifest.html#the\-publish\-field> field in
+\fBCargo.toml\fR with a single registry, then it will publish to that registry.
+Otherwise it will use the default registry, which is defined by the
+\fI\f(BIregistry.default\fI\fR <https://doc.rust\-lang.org/cargo/reference/config.html#registry\-default> config key
+which defaults to \fBcrates\-io\fR\&.
 .RE
 .SS "Compilation Options"
 .sp

tests/testsuite/publish.rs

@@ -715,6 +715,82 @@ fn publish_allowed_registry() {
     );
 }
 
+#[cargo_test]
+fn publish_implicitly_to_only_allowed_registry() {
+    registry::init();
+
+    let p = project().build();
+
+    let _ = repo(&paths::root().join("foo"))
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+            license = "MIT"
+            description = "foo"
+            documentation = "foo"
+            homepage = "foo"
+            repository = "foo"
+            publish = ["alternative"]
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("publish").run();
+
+    publish::validate_alt_upload(
+        CLEAN_FOO_JSON,
+        "foo-0.0.1.crate",
+        &[
+            "Cargo.lock",
+            "Cargo.toml",
+            "Cargo.toml.orig",
+            "src/main.rs",
+            ".cargo_vcs_info.json",
+        ],
+    );
+}
+
+#[cargo_test]
+fn publish_fail_with_no_registry_specified() {
+    registry::init();
+
+    let p = project().build();
+
+    let _ = repo(&paths::root().join("foo"))
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+            license = "MIT"
+            description = "foo"
+            documentation = "foo"
+            homepage = "foo"
+            repository = "foo"
+            publish = ["alternative", "test"]
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("publish")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] `foo` cannot be published.
+The registry `crates-io` is not listed in the `publish` value in Cargo.toml.
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn block_publish_no_registry() {
     registry::init();