Fix FP in to_string_in_display

Don't emit a lint when `.to_string()` on anything that is not `self`

clippy_lints/src/to_string_in_display.rs

@@ -1,6 +1,7 @@
-use crate::utils::{match_def_path, match_trait_method, paths, span_lint};
+use crate::utils::{match_def_path, match_trait_method, paths, qpath_res, span_lint};
 use if_chain::if_chain;
-use rustc_hir::{Expr, ExprKind, Item, ItemKind};
+use rustc_hir::def::Res;
+use rustc_hir::{Expr, ExprKind, HirId, ImplItem, ImplItemKind, Item, ItemKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
 
@@ -45,11 +46,15 @@
 #[derive(Default)]
 pub struct ToStringInDisplay {
     in_display_impl: bool,
+    self_hir_id: Option<HirId>,
 }
 
 impl ToStringInDisplay {
     pub fn new() -> Self {
-        Self { in_display_impl: false }
+        Self {
+            in_display_impl: false,
+            self_hir_id: None,
+        }
     }
 }
 
@@ -65,16 +70,33 @@ fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
     fn check_item_post(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
         if is_display_impl(cx, item) {
             self.in_display_impl = false;
+            self.self_hir_id = None;
+        }
+    }
+
+    fn check_impl_item(&mut self, cx: &LateContext<'_>, impl_item: &ImplItem<'_>) {
+        if_chain! {
+            if self.in_display_impl;
+            if let ImplItemKind::Fn(.., body_id) = &impl_item.kind;
+            let body = cx.tcx.hir().body(*body_id);
+            if !body.params.is_empty();
+            then {
+                let self_param = &body.params[0];
+                self.self_hir_id = Some(self_param.pat.hir_id);
+            }
         }
     }
 
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
         if_chain! {
-            if let ExprKind::MethodCall(ref path, _, _, _) = expr.kind;
+            if let ExprKind::MethodCall(ref path, _, args, _) = expr.kind;
             if path.ident.name == sym!(to_string);
             if match_trait_method(cx, expr, &paths::TO_STRING);
             if self.in_display_impl;
-
+            if let ExprKind::Path(ref qpath) = args[0].kind;
+            if let Res::Local(hir_id) = qpath_res(cx, qpath, args[0].hir_id);
+            if let Some(self_hir_id) = self.self_hir_id;
+            if hir_id == self_hir_id;
             then {
                 span_lint(
                     cx,

tests/ui/to_string_in_display.rs

@@ -44,6 +44,20 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     }
 }
 
+enum D {
+    E(String),
+    F,
+}
+
+impl std::fmt::Display for D {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self {
+            Self::E(string) => write!(f, "E {}", string.to_string()),
+            Self::F => write!(f, "F"),
+        }
+    }
+}
+
 fn main() {
     let a = A;
     a.to_string();