Warn when Cargo is run directly. Fix #3957.

.gitignore

@@ -1,4 +1,6 @@
 /.cargo/config
+/Cargo.lock
+/target
 /components/servo/target
 /ports/cef/target
 /ports/android/bin

Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+
+name = "not-the-toml-you-re-looking-for"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+
+build = "support/not-the-toml-you-re-looking-for.rs"
+
+[lib]
+name = "not-the-toml-you-re-looking-for"

support/not-the-toml-you-re-looking-for.rs

@@ -0,0 +1,9 @@
+fn main() {
+    ::std::os::set_exit_status(1);
+    let _ = ::std::io::stderr().write(br"
+
+    This is not the `Cargo.toml` file you're looking for.
+    Invoke Cargo through mach instead, e.g. `./mach build`.
+
+");
+}