1 files changed, 154 insertions, 22 deletions

diff --git a/src/tools/clippy/clippy_lints/src/from_over_into.rs b/src/tools/clippy/clippy_lints/src/from_over_into.rs
index 5d25c1d06..8b24a4962 100644
--- a/src/tools/clippy/clippy_lints/src/from_over_into.rs
+++ b/src/tools/clippy/clippy_lints/src/from_over_into.rs
@@ -1,11 +1,19 @@
-use clippy_utils::diagnostics::span_lint_and_help;
-use clippy_utils::{meets_msrv, msrvs};
-use if_chain::if_chain;
-use rustc_hir as hir;
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::macros::span_is_local;
+use clippy_utils::source::snippet_opt;
+use clippy_utils::{meets_msrv, msrvs, path_def_id};
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_path, Visitor};
+use rustc_hir::{
+    GenericArg, GenericArgs, HirId, Impl, ImplItemKind, ImplItemRef, Item, ItemKind, PatKind, Path, PathSegment, Ty,
+    TyKind,
+};
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::hir::nested_filter::OnlyBodies;
 use rustc_semver::RustcVersion;
 use rustc_session::{declare_tool_lint, impl_lint_pass};
-use rustc_span::symbol::sym;
+use rustc_span::symbol::{kw, sym};
+use rustc_span::{Span, Symbol};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -54,28 +62,152 @@ impl FromOverInto {
 impl_lint_pass!(FromOverInto => [FROM_OVER_INTO]);
 
 impl<'tcx> LateLintPass<'tcx> for FromOverInto {
-    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'_>) {
-        if !meets_msrv(self.msrv, msrvs::RE_REBALANCING_COHERENCE) {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
+        if !meets_msrv(self.msrv, msrvs::RE_REBALANCING_COHERENCE) || !span_is_local(item.span) {
             return;
         }
 
-        if_chain! {
-            if let hir::ItemKind::Impl{ .. } = &item.kind;
-            if let Some(impl_trait_ref) = cx.tcx.impl_trait_ref(item.def_id);
-            if cx.tcx.is_diagnostic_item(sym::Into, impl_trait_ref.def_id);
-
-            then {
-                span_lint_and_help(
-                    cx,
-                    FROM_OVER_INTO,
-                    cx.tcx.sess.source_map().guess_head_span(item.span),
-                    "an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true",
-                    None,
-                    &format!("consider to implement `From<{}>` instead", impl_trait_ref.self_ty()),
-                );
-            }
+        if let ItemKind::Impl(Impl {
+            of_trait: Some(hir_trait_ref),
+            self_ty,
+            items: [impl_item_ref],
+            ..
+        }) = item.kind
+            && let Some(into_trait_seg) = hir_trait_ref.path.segments.last()
+            // `impl Into<target_ty> for self_ty`
+            && let Some(GenericArgs { args: [GenericArg::Type(target_ty)], .. }) = into_trait_seg.args
+            && let Some(middle_trait_ref) = cx.tcx.impl_trait_ref(item.owner_id)
+            && cx.tcx.is_diagnostic_item(sym::Into, middle_trait_ref.def_id)
+        {
+            span_lint_and_then(
+                cx,
+                FROM_OVER_INTO,
+                cx.tcx.sess.source_map().guess_head_span(item.span),
+                "an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true",
+                |diag| {
+                    // If the target type is likely foreign mention the orphan rules as it's a common source of confusion
+                    if path_def_id(cx, target_ty.peel_refs()).map_or(true, |id| !id.is_local()) {
+                        diag.help(
+                            "`impl From<Local> for Foreign` is allowed by the orphan rules, for more information see\n\
+                            https://doc.rust-lang.org/reference/items/implementations.html#trait-implementation-coherence"
+                        );
+                    }
+
+                    let message = format!("replace the `Into` implentation with `From<{}>`", middle_trait_ref.self_ty());
+                    if let Some(suggestions) = convert_to_from(cx, into_trait_seg, target_ty, self_ty, impl_item_ref) {
+                        diag.multipart_suggestion(message, suggestions, Applicability::MachineApplicable);
+                    } else {
+                        diag.help(message);
+                    }
+                },
+            );
         }
     }
 
     extract_msrv_attr!(LateContext);
 }
+
+/// Finds the occurences of `Self` and `self`
+struct SelfFinder<'a, 'tcx> {
+    cx: &'a LateContext<'tcx>,
+    /// Occurences of `Self`
+    upper: Vec<Span>,
+    /// Occurences of `self`
+    lower: Vec<Span>,
+    /// If any of the `self`/`Self` usages were from an expansion, or the body contained a binding
+    /// already named `val`
+    invalid: bool,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for SelfFinder<'a, 'tcx> {
+    type NestedFilter = OnlyBodies;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.cx.tcx.hir()
+    }
+
+    fn visit_path(&mut self, path: &'tcx Path<'tcx>, _id: HirId) {
+        for segment in path.segments {
+            match segment.ident.name {
+                kw::SelfLower => self.lower.push(segment.ident.span),
+                kw::SelfUpper => self.upper.push(segment.ident.span),
+                _ => continue,
+            }
+        }
+
+        self.invalid |= path.span.from_expansion();
+        if !self.invalid {
+            walk_path(self, path);
+        }
+    }
+
+    fn visit_name(&mut self, name: Symbol) {
+        if name == sym::val {
+            self.invalid = true;
+        }
+    }
+}
+
+fn convert_to_from(
+    cx: &LateContext<'_>,
+    into_trait_seg: &PathSegment<'_>,
+    target_ty: &Ty<'_>,
+    self_ty: &Ty<'_>,
+    impl_item_ref: &ImplItemRef,
+) -> Option<Vec<(Span, String)>> {
+    let impl_item = cx.tcx.hir().impl_item(impl_item_ref.id);
+    let ImplItemKind::Fn(ref sig, body_id) = impl_item.kind else { return None };
+    let body = cx.tcx.hir().body(body_id);
+    let [input] = body.params else { return None };
+    let PatKind::Binding(.., self_ident, None) = input.pat.kind else { return None };
+
+    let from = snippet_opt(cx, self_ty.span)?;
+    let into = snippet_opt(cx, target_ty.span)?;
+
+    let mut suggestions = vec![
+        // impl Into<T> for U  ->  impl From<T> for U
+        //      ~~~~                    ~~~~
+        (into_trait_seg.ident.span, String::from("From")),
+        // impl Into<T> for U  ->  impl Into<U> for U
+        //           ~                       ~
+        (target_ty.span, from.clone()),
+        // impl Into<T> for U  ->  impl Into<T> for T
+        //                  ~                       ~
+        (self_ty.span, into),
+        // fn into(self) -> T  ->  fn from(self) -> T
+        //    ~~~~                    ~~~~
+        (impl_item.ident.span, String::from("from")),
+        // fn into([mut] self) -> T  ->  fn into([mut] v: T) -> T
+        //               ~~~~                          ~~~~
+        (self_ident.span, format!("val: {from}")),
+        // fn into(self) -> T  ->  fn into(self) -> Self
+        //                  ~                       ~~~~
+        (sig.decl.output.span(), String::from("Self")),
+    ];
+
+    let mut finder = SelfFinder {
+        cx,
+        upper: Vec::new(),
+        lower: Vec::new(),
+        invalid: false,
+    };
+    finder.visit_expr(body.value);
+
+    if finder.invalid {
+        return None;
+    }
+
+    // don't try to replace e.g. `Self::default()` with `&[T]::default()`
+    if !finder.upper.is_empty() && !matches!(self_ty.kind, TyKind::Path(_)) {
+        return None;
+    }
+
+    for span in finder.upper {
+        suggestions.push((span, from.clone()));
+    }
+    for span in finder.lower {
+        suggestions.push((span, String::from("val")));
+    }
+
+    Some(suggestions)
+}