+use crate::utils::paths::INTO;
+use crate::utils::{match_def_path, span_lint_and_help};
+use if_chain::if_chain;
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+ /// **What it does:** Searches for implementations of the `Into<..>` trait and suggests to implement `From<..>` instead.
+ ///
+ /// **Why is this bad?** According the std docs implementing `From<..>` is preferred since it gives you `Into<..>` for free where the reverse isn't true.
+ ///
+ /// **Known problems:** None.
+ ///
+ /// **Example:**
+ ///
+ /// ```rust
+ /// struct StringWrapper(String);
+ ///
+ /// impl Into<StringWrapper> for String {
+ /// fn into(self) -> StringWrapper {
+ /// StringWrapper(self)
+ /// }
+ /// }
+ /// ```
+ /// Use instead:
+ /// ```rust
+ /// struct StringWrapper(String);
+ ///
+ /// impl From<String> for StringWrapper {
+ /// fn from(s: String) -> StringWrapper {
+ /// StringWrapper(s)
+ /// }
+ /// }
+ /// ```
+ pub FROM_OVER_INTO,
+ style,
+ "Warns on implementations of `Into<..>` to use `From<..>`"
+}
+
+declare_lint_pass!(FromOverInto => [FROM_OVER_INTO]);
+
+impl LateLintPass<'_> for FromOverInto {
+ fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
+ let impl_def_id = cx.tcx.hir().local_def_id(item.hir_id);
+ if_chain! {
+ if let hir::ItemKind::Impl{ .. } = &item.kind;
+ if let Some(impl_trait_ref) = cx.tcx.impl_trait_ref(impl_def_id);
+ if match_def_path(cx, impl_trait_ref.def_id, &INTO);
+
+ then {
+ span_lint_and_help(
+ cx,
+ FROM_OVER_INTO,
+ item.span,
+ "An implementation of From is preferred since it gives you Into<..> for free where the reverse isn't true.",
+ None,
+ "consider implement From instead",
+ );
+ }
+ }
+ }
+}