Check that no profile is present in Cargo.toml files

tests/no-profile-in-cargo-toml.rs

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+// Check that we do not have `profile.*` sections in our `Cargo.toml` files,
+// as this causes warnings when run from the compiler repository which includes
+// Clippy in a workspace.
+//
+// Those sections can be put into `.cargo/config.toml` which will be read
+// when commands are issued from the top-level Clippy directory, outside of
+// a workspace.
+
+use std::fs::File;
+use std::io::{self, BufRead as _};
+use walkdir::WalkDir;
+
+#[test]
+fn no_profile_in_cargo_toml() {
+    // This check could parse `Cargo.toml` using a TOML deserializer, but in practice
+    // profile sections would be added at the beginning of a line as `[profile.*]`, so
+    // keep it fast and simple.
+    for entry in WalkDir::new(".")
+        .into_iter()
+        .filter_map(Result::ok)
+        .filter(|e| e.file_name().to_str() == Some("Cargo.toml"))
+    {
+        for line in io::BufReader::new(File::open(entry.path()).unwrap())
+            .lines()
+            .map(Result::unwrap)
+        {
+            if line.starts_with("[profile.") {
+                eprintln!("Profile section `{line}` found in file `{}`.", entry.path().display());
+                eprintln!("Use `.cargo/config.toml` for profiles specific to the standalone Clippy repository.");
+                panic!("Profile section found in `Cargo.toml`");
+            }
+        }
+    }
+}