rust-clippy/clippy_lints/src/recursive_format_impl.rs

use clippy_utils::diagnostics::span_lint;
use clippy_utils::macros::{is_format_macro, root_macro_call_first_node, FormatArgsArg, FormatArgsExpn};
use clippy_utils::{is_diag_trait_item, path_to_local, peel_ref_operators};
use if_chain::if_chain;
use rustc_hir::{Expr, ExprKind, Impl, Item, ItemKind, QPath};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::{sym, symbol::kw, Symbol};

#[derive(Clone, Copy)]
enum FormatTrait {
    Debug,
    Display,
}

impl FormatTrait {
    fn name(self) -> Symbol {
        match self {
            FormatTrait::Debug => sym::Debug,
            FormatTrait::Display => sym::Display,
        }
    }
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for format trait implementations (e.g. `Display`) with a recursive call to itself
    /// which uses `self` as a parameter.
    /// This is typically done indirectly with the `write!` macro or with `to_string()`.
    ///
    /// ### Why is this bad?
    /// This will lead to infinite recursion and a stack overflow.
    ///
    /// ### 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)
    ///     }
    /// }
    /// ```
    #[clippy::version = "1.48.0"]
    pub RECURSIVE_FORMAT_IMPL,
    correctness,
    "Format trait method called while implementing the same Format trait"
}

#[derive(Default)]
pub struct RecursiveFormatImpl {
    // Whether we are inside Display or Debug trait impl - None for neither
    format_trait_impl: Option<FormatTrait>,
}

impl RecursiveFormatImpl {
    pub fn new() -> Self {
        Self {
            format_trait_impl: None,
        }
    }
}

impl_lint_pass!(RecursiveFormatImpl => [RECURSIVE_FORMAT_IMPL]);

impl<'tcx> LateLintPass<'tcx> for RecursiveFormatImpl {
    fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
        if let Some(format_trait_impl) = is_format_trait_impl(cx, item) {
            self.format_trait_impl = Some(format_trait_impl);
        }
    }

    fn check_item_post(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
        // Assume no nested Impl of Debug and Display within eachother
        if is_format_trait_impl(cx, item).is_some() {
            self.format_trait_impl = None;
        }
    }

    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        match self.format_trait_impl {
            Some(FormatTrait::Display) => {
                check_to_string_in_display(cx, expr);
                check_self_in_format_args(cx, expr, FormatTrait::Display);
            },
            Some(FormatTrait::Debug) => {
                check_self_in_format_args(cx, expr, FormatTrait::Debug);
            },
            None => {},
        }
    }
}

fn check_to_string_in_display(cx: &LateContext<'_>, expr: &Expr<'_>) {
    if_chain! {
        // Get the hir_id of the object we are calling the method on
        if let ExprKind::MethodCall(path, [ref self_arg, ..], _) = expr.kind;
        // Is the method to_string() ?
        if path.ident.name == sym!(to_string);
        // Is the method a part of the ToString trait? (i.e. not to_string() implemented
        // separately)
        if let Some(expr_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
        if is_diag_trait_item(cx, expr_def_id, sym::ToString);
        // Is the method is called on self
        if let ExprKind::Path(QPath::Resolved(_, path)) = self_arg.kind;
        if let [segment] = path.segments;
        if segment.ident.name == kw::SelfLower;
        then {
            span_lint(
                cx,
                RECURSIVE_FORMAT_IMPL,
                expr.span,
                "using `self.to_string` in `fmt::Display` implementation will cause infinite recursion",
            );
        }
    }
}

fn check_self_in_format_args<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, impl_trait: FormatTrait) {
    // Check each arg in format calls - do we ever use Display on self (directly or via deref)?
    if_chain! {
        if let Some(outer_macro) = root_macro_call_first_node(cx, expr);
        if let macro_def_id = outer_macro.def_id;
        if let Some(format_args) = FormatArgsExpn::find_nested(cx, expr, outer_macro.expn);
        if is_format_macro(cx, macro_def_id);
        if let Some(args) = format_args.args();
        then {
            for arg in args {
                if arg.format_trait != impl_trait.name() {
                    continue;
                }
                check_format_arg_self(cx, expr, &arg, impl_trait);
            }
        }
    }
}

fn check_format_arg_self(cx: &LateContext<'_>, expr: &Expr<'_>, arg: &FormatArgsArg<'_>, impl_trait: FormatTrait) {
    // Handle multiple dereferencing of references e.g. &&self
    // Handle dereference of &self -> self that is equivalent (i.e. via *self in fmt() impl)
    // Since the argument to fmt is itself a reference: &self
    let reference = peel_ref_operators(cx, arg.value);
    let map = cx.tcx.hir();
    // Is the reference self?
    let symbol_ident = impl_trait.name().to_ident_string();
    if path_to_local(reference).map(|x| map.name(x)) == Some(kw::SelfLower) {
        span_lint(
            cx,
            RECURSIVE_FORMAT_IMPL,
            expr.span,
            &format!(
                "using `self` as `{}` in `impl {}` will cause infinite recursion",
                &symbol_ident, &symbol_ident
            ),
        );
    }
}

fn is_format_trait_impl(cx: &LateContext<'_>, item: &Item<'_>) -> Option<FormatTrait> {
    if_chain! {
        // Are we at an Impl?
        if let ItemKind::Impl(Impl { of_trait: Some(trait_ref), .. }) = &item.kind;
        if let Some(did) = trait_ref.trait_def_id();
        if let Some(name) = cx.tcx.get_diagnostic_name(did);
        then {
            // Is Impl for Debug or Display?
            match name {
                sym::Debug => Some(FormatTrait::Debug),
                sym::Display => Some(FormatTrait::Display),
                _ => None,
            }
        } else {
            None
        }
    }
}