Auto merge of #82918 - Manishearth:edition-2015-warn, r=oli-obk

Turn old edition lint (anonymous-parameters) into warn-by-default on 2015

This makes `anonymous_parameters` <s>and `keyword_idents` </s>warn-by-default on the 2015 edition. I would also like to do this for `absolute_paths_not_starting_with_crate`, but I feel that case is slightly less clear-cut.

Note that this only affects code on the 2015 edition, such code is illegal in future editions anyway.

This was spurred by https://github.com/dtolnay/syn/issues/972: old edition syntax breaks tooling (like syn), and while the tooling should be free to find its balance on how much to support prior editions, it does seem like we should be nudging such code towards the newer edition, and we can do that by turning this Allow lint into a Warn.

In general, I feel like migration lints from an old edition should be made Warn after a year or so, and idiom lints for the new edition should be made Warn after a couple months.

cc `@m-ou-se,` this is for stuff from the 2015-2018 migration but you might be interested.

1 files changed, 7 insertions, 4 deletions

diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index 0b49e6562f9..c4acaef21dd 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -857,11 +857,10 @@ declare_lint! {
     /// ```
     ///
     /// This syntax is now a hard error in the 2018 edition. In the 2015
-    /// edition, this lint is "allow" by default, because the old code is
-    /// still valid, and warning for all old code can be noisy. This lint
+    /// edition, this lint is "warn" by default. This lint
     /// enables the [`cargo fix`] tool with the `--edition` flag to
     /// automatically transition old code from the 2015 edition to 2018. The
-    /// tool will switch this lint to "warn" and will automatically apply the
+    /// tool will run this lint and automatically apply the
     /// suggested fix from the compiler (which is to add `_` to each
     /// parameter). This provides a completely automated way to update old
     /// code for a new edition. See [issue #41686] for more details.
@@ -869,7 +868,7 @@ declare_lint! {
     /// [issue #41686]: https://github.com/rust-lang/rust/issues/41686
     /// [`cargo fix`]: https://doc.rust-lang.org/cargo/commands/cargo-fix.html
     pub ANONYMOUS_PARAMETERS,
-    Allow,
+    Warn,
     "detects anonymous parameters",
     @future_incompatible = FutureIncompatibleInfo {
         reference: "issue #41686 <https://github.com/rust-lang/rust/issues/41686>",
@@ -884,6 +883,10 @@ declare_lint_pass!(
 
 impl EarlyLintPass for AnonymousParameters {
     fn check_trait_item(&mut self, cx: &EarlyContext<'_>, it: &ast::AssocItem) {
+        if cx.sess.edition() != Edition::Edition2015 {
+            // This is a hard error in future editions; avoid linting and erroring
+            return;
+        }
         if let ast::AssocItemKind::Fn(box FnKind(_, ref sig, _, _)) = it.kind {
             for arg in sig.decl.inputs.iter() {
                 if let ast::PatKind::Ident(_, ident, None) = arg.pat.kind {