Auto merge of #13556 - linyihai:doc-z-public, r=weihanglo

doc: Add doc for -Zpublic-dependency

### What does this PR try to resolve?

- add doc for -Zpublic-dependency
- optimize log display about -Zpublic-dependency

### How should we test and review this PR?

### Additional information

Author: bors

src/cargo/util/toml/mod.rs

@@ -779,7 +779,7 @@ pub fn to_real_manifest(
                         {
                             d.public = None;
                             manifest_ctx.warnings.push(format!(
-                            "Ignoring `public` on dependency {name}.  Pass `-Zpublic-dependency` to enable support for it", name = &dep.name_in_toml()
+                            "ignoring `public` on dependency {name}, pass `-Zpublic-dependency` to enable support for it", name = &dep.name_in_toml()
                         ))
                         }
                     } else {
@@ -2062,11 +2062,14 @@ fn detailed_dep_to_dependency<P: ResolveToPath + Clone>(
         }
 
         if dep.kind() != DepKind::Normal {
+            let hint = format!(
+                "'public' specifier can only be used on regular dependencies, not {}",
+                dep.kind().kind_table(),
+            );
             match (with_public_feature, with_z_public) {
-                (true, _) => bail!("'public' specifier can only be used on regular dependencies, not {:?} dependencies", dep.kind()),
-                (_, true) => bail!("'public' specifier can only be used on regular dependencies, not {:?} dependencies", dep.kind()),
+                (true, _) | (_, true) => bail!(hint),
                 // If public feature isn't enabled in nightly, we instead warn that.
-                (false, false) =>  manifest_ctx.warnings.push(format!("'public' specifier can only be used on regular dependencies, not {:?} dependencies", dep.kind())),
+                (false, false) => manifest_ctx.warnings.push(hint),
             }
         } else {
             dep.set_public(p);

src/doc/src/reference/unstable.md

@@ -296,7 +296,21 @@ The 'public-dependency' feature allows marking dependencies as 'public'
 or 'private'. When this feature is enabled, additional information is passed to rustc to allow
 the [exported_private_dependencies](../../rustc/lints/listing/warn-by-default.html#exported-private-dependencies) lint to function properly.
 
-This requires the appropriate key to be set in `cargo-features`:
+To enable this feature, you can either use `-Zpublic-dependency`
+
+```sh
+cargo +nightly run -Zpublic-dependency
+```
+
+or `[unstable]` table, for example,
+
+```toml
+# .cargo/config.toml
+[unstable]
+public-dependency = true
+```
+
+`public-dependency` could also be enabled in `cargo-features`, **though this is deprecated and will be removed soon**.
 
 ```toml
 cargo-features = ["public-dependency"]

tests/testsuite/pub_priv.rs

@@ -141,7 +141,7 @@ fn requires_feature() {
         .masquerade_as_nightly_cargo(&["public-dependency"])
         .with_stderr(
             "\
-[WARNING] Ignoring `public` on dependency pub_dep.  Pass `-Zpublic-dependency` to enable support for it
+[WARNING] ignoring `public` on dependency pub_dep, pass `-Zpublic-dependency` to enable support for it
 [UPDATING] `[..]` index
 [DOWNLOADING] crates ...
 [DOWNLOADED] pub_dep v0.1.0 ([..])
@@ -191,7 +191,7 @@ fn pub_dev_dependency() {
 error: failed to parse manifest at `[..]`
 
 Caused by:
-  'public' specifier can only be used on regular dependencies, not Development dependencies
+  'public' specifier can only be used on regular dependencies, not dev-dependencies
 ",
         )
         .run()
@@ -229,7 +229,7 @@ fn pub_dev_dependency_without_feature() {
         .masquerade_as_nightly_cargo(&["public-dependency"])
         .with_stderr(
             "\
-[WARNING] 'public' specifier can only be used on regular dependencies, not Development dependencies
+[WARNING] 'public' specifier can only be used on regular dependencies, not dev-dependencies
 [UPDATING] `[..]` index
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [..]
 ",