Revert "Make PROC_MACRO_DERIVE_RESOLUTION_FALLBACK a hard error"

This reverts commit 7d82cadd97.

I am doing this to buy us some time with respect to issue #106337 w.r.t. the
1.67 release.

compiler/rustc_lint_defs/src/builtin.rs

@@ -2023,6 +2023,73 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `proc_macro_derive_resolution_fallback` lint detects proc macro
+    /// derives using inaccessible names from parent modules.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (proc-macro)
+    /// // foo.rs
+    /// #![crate_type = "proc-macro"]
+    ///
+    /// extern crate proc_macro;
+    ///
+    /// use proc_macro::*;
+    ///
+    /// #[proc_macro_derive(Foo)]
+    /// pub fn foo1(a: TokenStream) -> TokenStream {
+    ///     drop(a);
+    ///     "mod __bar { static mut BAR: Option<Something> = None; }".parse().unwrap()
+    /// }
+    /// ```
+    ///
+    /// ```rust,ignore (needs-dependency)
+    /// // bar.rs
+    /// #[macro_use]
+    /// extern crate foo;
+    ///
+    /// struct Something;
+    ///
+    /// #[derive(Foo)]
+    /// struct Another;
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// warning: cannot find type `Something` in this scope
+    ///  --> src/main.rs:8:10
+    ///   |
+    /// 8 | #[derive(Foo)]
+    ///   |          ^^^ names from parent modules are not accessible without an explicit import
+    ///   |
+    ///   = note: `#[warn(proc_macro_derive_resolution_fallback)]` on by default
+    ///   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    ///   = note: for more information, see issue #50504 <https://github.com/rust-lang/rust/issues/50504>
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// If a proc-macro generates a module, the compiler unintentionally
+    /// allowed items in that module to refer to items in the crate root
+    /// without importing them. This is a [future-incompatible] lint to
+    /// transition this to a hard error in the future. See [issue #50504] for
+    /// more details.
+    ///
+    /// [issue #50504]: https://github.com/rust-lang/rust/issues/50504
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
+    Deny,
+    "detects proc macro derives using inaccessible names from parent modules",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #83583 <https://github.com/rust-lang/rust/issues/83583>",
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
+    };
+}
+
 declare_lint! {
     /// The `macro_use_extern_crate` lint detects the use of the
     /// [`macro_use` attribute].
@@ -3261,6 +3328,7 @@ declare_lint_pass! {
         UNSTABLE_NAME_COLLISIONS,
         IRREFUTABLE_LET_PATTERNS,
         WHERE_CLAUSES_OBJECT_SAFETY,
+        PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
         MACRO_USE_EXTERN_CRATE,
         MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
         ILL_FORMED_ATTRIBUTE_INPUT,