Auto merge of #12907 - dtolnay-contrib:featuredevtest, r=epage

Add regression test for issue 6915: features and transitive dev deps

Closes #6915.

That issue was fixed without realizing by #10103. In search of test coverage, I looked through `rg '[a-z0-9_-]+ = \["[a-z0-9_-]+/[a-z0-9_-]+"\]' tests/testsuite/` and did not find any existing test that exercises this situation. The tests added by #10103 are focused on cycles and `cargo tree`, which is substantially different from #6915.

Tested by `cargo test --test testsuite -- features::activating_feature_does_not_activate_transitive_dev_dependency`.

tests/testsuite/features.rs

@@ -1143,6 +1143,61 @@ fn activating_feature_activates_dep() {
     p.cargo("check --features a -v").run();
 }
 
+#[cargo_test]
+fn activating_feature_does_not_activate_transitive_dev_dependency() {
+    let p = project()
+        .no_manifest()
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.0.0"
+                edition = "2021"
+
+                [features]
+                f = ["b/f"]
+
+                [dependencies]
+                b = { path = "../b" }
+            "#,
+        )
+        .file(
+            "b/Cargo.toml",
+            r#"
+                [package]
+                name = "b"
+                version = "0.0.0"
+                edition = "2021"
+
+                [features]
+                f = ["c/f"]
+
+                [dev-dependencies]
+                c = { path = "../c" }
+            "#,
+        )
+        .file(
+            "c/Cargo.toml",
+            r#"
+                [package]
+                name = "c"
+                version = "0.0.0"
+                edition = "2021"
+
+                [features]
+                f = []
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .file("b/src/lib.rs", "")
+        .file("c/src/lib.rs", "compile_error!")
+        .build();
+
+    p.cargo("check --manifest-path a/Cargo.toml --features f")
+        .run();
+}
+
 #[cargo_test]
 fn dep_feature_in_cmd_line() {
     let p = project()