Validate feature names are crates.io friendly.

Closes rust-lang#5554
Relates to rust-lang/crates-io-cargo-teams#41

src/cargo/ops/cargo_package.rs

@@ -56,6 +56,7 @@ pub fn package(ws: &Workspace<'_>, opts: &PackageOpts<'_>) -> CargoResult<Option
 
     if opts.check_metadata {
         check_metadata(pkg, config)?;
+        verify_features(pkg)?;
     }
 
     verify_dependencies(pkg)?;
@@ -218,6 +219,31 @@ fn verify_dependencies(pkg: &Package) -> CargoResult<()> {
     Ok(())
 }
 
+// Checks that the package features are rusty / can be published to crates.io.
+// Intended to avoid this suprise after a successful cargo publish --dry-run:
+// invalid upload request: invalid value: string "futures-0.1", expected a
+// valid feature name containing only letters, numbers, hyphens, or underscores
+fn verify_features(pkg: &Package) -> CargoResult<()> {
+    for (feature, _) in pkg.summary().features() {
+        if feature.is_empty() || feature.as_str() == "_" {
+            failure::bail!(
+                "expected a valid (not empty nor \"_\") feature name containing \
+                 only letters, numbers, hyphens, or underscores.\n\
+                 feature `{}` is not valid.",
+                feature
+            )
+        } else if !feature.chars().all(|ch| ch.is_ascii_alphanumeric() || ch == '-' || ch == '_') {
+            failure::bail!(
+                "expected a valid (not empty nor \"_\") feature name containing \
+                 only letters, numbers, hyphens, or underscores.\n\
+                 feature `{}` contains other characters.",
+                feature
+            )
+        }
+    }
+    Ok(())
+}
+
 // Checks if the package source is in a *git* DVCS repository. If *git*, and
 // the source is *dirty* (e.g., has uncommitted changes) and not `allow_dirty`
 // then `bail!` with an informative message. Otherwise return the sha1 hash of

tests/testsuite/publish.rs

@@ -898,6 +898,36 @@ fn publish_with_no_default_features() {
         .run();
 }
 
+#[cargo_test]
+fn publish_with_unrusty_features() {
+    registry::init();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.0.1"
+            authors = []
+            license = "MIT"
+            description = "foo"
+
+            [features]
+            "invalid::characters" = []
+        "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("publish --no-default-features --dry-run --index")
+        .arg(registry_url().to_string())
+        .with_stderr_contains("error: expected a valid (not empty nor \"_\") feature name containing only letters, numbers, hyphens, or underscores.")
+        .with_stderr_contains("feature `invalid::characters` contains other characters.")
+        .with_status(101)
+        .run();
+}
+
 #[cargo_test]
 fn publish_with_patch() {
     Package::new("bar", "1.0.0").publish();