Warning on conflicting dev-dependencies keys

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

src/cargo/util/toml/mod.rs

@@ -1264,6 +1264,13 @@ impl TomlManifest {
 
             // Collect the dependencies.
             process_dependencies(&mut cx, me.dependencies.as_ref(), None)?;
+            if me.dev_dependencies.is_some() && me.dev_dependencies2.is_some() {
+                cx.warnings.push(format!(
+                    "found both `dev-dependencies` and `dev_dependencies` are set \
+                     in the `{}` package",
+                    package_name
+                ));
+            }
             let dev_deps = me
                 .dev_dependencies
                 .as_ref()

tests/testsuite/build.rs

@@ -1676,6 +1676,41 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn dev_dependencies_conflicting_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+
+                [dev-dependencies]
+                a = {path = "a"}
+                [dev_dependencies]
+                a = {path = "a"}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.0.1"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .build();
+    p.cargo("build")
+    .with_stderr_contains(
+        "[WARNING] found both `dev-dependencies` and `dev_dependencies` are set in the `foo` package",
+    )
+        .run();
+}
+
 #[cargo_test]
 fn lib_crate_types_conflicting_warning() {
     let p = project()