refactor: Warn not Error on unsupported lint tool

src/cargo/util/toml/mod.rs

@@ -2231,8 +2231,15 @@ fn verify_lints(
     for (tool, lints) in lints {
         let supported = ["cargo", "clippy", "rust", "rustdoc"];
         if !supported.contains(&tool.as_str()) {
-            let supported = supported.join(", ");
-            anyhow::bail!("unsupported `{tool}` in `[lints]`, must be one of {supported}")
+            let message = format!(
+                "unrecognized lint tool `lints.{tool}`
+
+specifying an unrecognized lint tool could lead to future breakage, as `cargo`
+reserves all of the `lints` namespace for itself and provides no compatibility
+guarantees when encountering unrecognized tools"
+            );
+            warnings.push(message);
+            continue;
         }
         if tool == "cargo" && !gctx.cli_unstable().cargo_lints {
             warn_for_cargo_lint_feature(gctx, warnings);

tests/testsuite/lints_table.rs

@@ -105,13 +105,15 @@ fn fail_on_invalid_tool() {
         .build();
 
     foo.cargo("check")
-        .with_status(101)
         .with_stderr(
             "\
-[..]
+[WARNING] [CWD]/Cargo.toml: unrecognized lint tool `lints.super-awesome-linter`
 
-Caused by:
-  unsupported `super-awesome-linter` in `[lints]`, must be one of cargo, clippy, rust, rustdoc
+specifying an unrecognized lint tool could lead to future breakage, as `cargo`
+reserves all of the `lints` namespace for itself and provides no compatibility
+guarantees when encountering unrecognized tools
+[CHECKING] foo v0.0.1 ([CWD])
+[FINISHED] [..]
 ",
         )
         .run();