No to_string in display

CHANGELOG.md

@@ -1579,6 +1579,7 @@ Released 2018-09-13
 [`temporary_assignment`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_assignment
 [`temporary_cstring_as_ptr`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_cstring_as_ptr
 [`to_digit_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_digit_is_some
+[`to_string_in_display`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_display
 [`todo`]: https://rust-lang.github.io/rust-clippy/master/index.html#todo
 [`too_many_arguments`]: https://rust-lang.github.io/rust-clippy/master/index.html#too_many_arguments
 [`too_many_lines`]: https://rust-lang.github.io/rust-clippy/master/index.html#too_many_lines

clippy_lints/src/lib.rs

@@ -310,6 +310,7 @@ mod swap;
 mod tabs_in_doc_comments;
 mod temporary_assignment;
 mod to_digit_is_some;
+mod to_string_in_display;
 mod trait_bounds;
 mod transmute;
 mod transmuting_null;
@@ -797,6 +798,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &tabs_in_doc_comments::TABS_IN_DOC_COMMENTS,
         &temporary_assignment::TEMPORARY_ASSIGNMENT,
         &to_digit_is_some::TO_DIGIT_IS_SOME,
+        &to_string_in_display::TO_STRING_IN_DISPLAY,
         &trait_bounds::TYPE_REPETITION_IN_BOUNDS,
         &transmute::CROSSPOINTER_TRANSMUTE,
         &transmute::TRANSMUTE_BYTES_TO_STR,
@@ -1017,6 +1019,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| box reference::DerefAddrOf);
     store.register_early_pass(|| box reference::RefInDeref);
     store.register_early_pass(|| box double_parens::DoubleParens);
+    store.register_late_pass(|| box to_string_in_display::ToStringInDisplay);
     store.register_early_pass(|| box unsafe_removed_from_name::UnsafeNameRemoval);
     store.register_early_pass(|| box if_not_else::IfNotElse);
     store.register_early_pass(|| box else_if_without_else::ElseIfWithoutElse);
@@ -1402,6 +1405,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&tabs_in_doc_comments::TABS_IN_DOC_COMMENTS),
         LintId::of(&temporary_assignment::TEMPORARY_ASSIGNMENT),
         LintId::of(&to_digit_is_some::TO_DIGIT_IS_SOME),
+        LintId::of(&to_string_in_display::TO_STRING_IN_DISPLAY),
         LintId::of(&transmute::CROSSPOINTER_TRANSMUTE),
         LintId::of(&transmute::TRANSMUTE_BYTES_TO_STR),
         LintId::of(&transmute::TRANSMUTE_FLOAT_TO_INT),
@@ -1674,6 +1678,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL),
         LintId::of(&suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL),
         LintId::of(&swap::ALMOST_SWAPPED),
+        LintId::of(&to_string_in_display::TO_STRING_IN_DISPLAY),
         LintId::of(&transmute::UNSOUND_COLLECTION_TRANSMUTE),
         LintId::of(&transmute::WRONG_TRANSMUTE),
         LintId::of(&transmuting_null::TRANSMUTING_NULL),

clippy_lints/src/to_string_in_display.rs

@@ -0,0 +1,77 @@
+use crate::utils::{match_def_path, paths, span_lint};
+use if_chain::if_chain;
+use rustc_hir::{def, Expr, ExprKind, FnDecl, FnSig, ImplItem, ImplItemKind, MutTy, Node, Path, QPath, Ty, TyKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for uses of `to_string()` when implementing
+    /// `Display` traits.
+    ///
+    /// **Why is this bad?** Usually `to_string` is implemented indirectly
+    /// via `Display`. Hence using it while implementing `Display` would
+    /// lead to infinite recursion.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// use std::fmt;
+    ///
+    /// struct Structure(i32);
+    /// impl fmt::Display for Structure {
+    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    ///         write!(f, "{}", self.to_string())
+    ///     }
+    /// }
+    ///
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::fmt;
+    ///
+    /// struct Structure(i32);
+    /// impl fmt::Display for Structure {
+    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    ///         write!(f, "{}", self.0)
+    ///     }
+    /// }
+    /// ```
+    pub TO_STRING_IN_DISPLAY,
+    correctness,
+    "to_string method used while implementing Display trait"
+}
+
+declare_lint_pass!(ToStringInDisplay => [TO_STRING_IN_DISPLAY]);
+
+impl LateLintPass<'_, '_> for ToStringInDisplay {
+    fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr<'_>) {
+        let parent_id = cx.tcx.hir().get_parent_item(expr.hir_id);
+        let parent_node = cx.tcx.hir().find(parent_id);
+
+        if_chain! {
+            if let ExprKind::MethodCall(ref path, _, _) = expr.kind;
+            if path.ident.as_str() == "to_string";
+            if let Some(Node::ImplItem(ImplItem {ident, kind, ..})) = parent_node;
+            if ident.as_str() == "fmt";
+            if let ImplItemKind::Fn(FnSig {decl, ..}, _) = kind;
+            if let FnDecl{inputs, ..} = decl;
+            if let Ty {kind, ..} = &inputs[0];
+            if let TyKind::Rptr(_, MutTy {ty, ..}) = kind;
+            if let Ty {kind, ..} = ty;
+            if let TyKind::Path(QPath::Resolved(_, path)) = kind;
+            if let Path{res, ..} = path;
+            if let def::Res::SelfTy(Some(did), _) = res;
+            if match_def_path(cx, *did, &paths::DISPLAY_TRAIT);
+            then {
+                span_lint(
+                    cx,
+                    TO_STRING_IN_DISPLAY,
+                    expr.span,
+                    "Using to_string in fmt::Display implementation might lead to infinite recursion",
+                );
+            }
+        }
+    }
+}

src/lintlist/mod.rs

@@ -2138,6 +2138,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "to_digit_is_some",
     },
+    Lint {
+        name: "to_string_in_display",
+        group: "correctness",
+        desc: "to_string method used while implementing Display trait",
+        deprecation: None,
+        module: "to_string_in_display",
+    },
     Lint {
         name: "todo",
         group: "restriction",

tests/ui/to_string_in_display.rs

@@ -0,0 +1,37 @@
+#![warn(clippy::to_string_in_display)]
+
+use std::fmt;
+
+struct A;
+impl A {
+    fn fmt(&self) {
+        self.to_string();
+    }
+}
+
+trait B {
+    fn fmt(&self) {}
+}
+
+impl B for A {
+    fn fmt(&self) {
+        self.to_string();
+    }
+}
+
+impl fmt::Display for A {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.to_string())
+    }
+}
+
+fn fmt(a: A) {
+    a.to_string();
+}
+
+fn main() {
+    let a = A;
+    a.to_string();
+    a.fmt();
+    fmt(a);
+}

tests/ui/to_string_in_display.stderr

@@ -0,0 +1,10 @@
+error: Using to_string in fmt::Display implementation might lead to infinite recursion
+  --> $DIR/to_string_in_display.rs:24:25
+   |
+LL |         write!(f, "{}", self.to_string())
+   |                         ^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::to-string-in-display` implied by `-D warnings`
+
+error: aborting due to previous error
+