src/tools/clippy/clippy_lints/src/reference.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::{snippet_opt, snippet_with_applicability};
use if_chain::if_chain;
use rustc_ast::ast::{Expr, ExprKind, Mutability, UnOp};
use rustc_errors::Applicability;
use rustc_lint::{EarlyContext, EarlyLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::BytePos;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of `*&` and `*&mut` in expressions.
    ///
    /// ### Why is this bad?
    /// Immediately dereferencing a reference is no-op and
    /// makes the code less clear.
    ///
    /// ### Known problems
    /// Multiple dereference/addrof pairs are not handled so
    /// the suggested fix for `x = **&&y` is `x = *&y`, which is still incorrect.
    ///
    /// ### Example
    /// ```rust,ignore
    /// let a = f(*&mut b);
    /// let c = *&d;
    /// ```
    ///
    /// Use instead:
    /// ```rust,ignore
    /// let a = f(b);
    /// let c = d;
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub DEREF_ADDROF,
    complexity,
    "use of `*&` or `*&mut` in an expression"
}

declare_lint_pass!(DerefAddrOf => [DEREF_ADDROF]);

fn without_parens(mut e: &Expr) -> &Expr {
    while let ExprKind::Paren(ref child_e) = e.kind {
        e = child_e;
    }
    e
}

impl EarlyLintPass for DerefAddrOf {
    fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &Expr) {
        if_chain! {
            if let ExprKind::Unary(UnOp::Deref, ref deref_target) = e.kind;
            if let ExprKind::AddrOf(_, ref mutability, ref addrof_target) = without_parens(deref_target).kind;
            if deref_target.span.ctxt() == e.span.ctxt();
            if !addrof_target.span.from_expansion();
            then {
                let mut applicability = Applicability::MachineApplicable;
                let sugg = if e.span.from_expansion() {
                    if let Some(macro_source) = snippet_opt(cx, e.span) {
                        // Remove leading whitespace from the given span
                        // e.g: ` $visitor` turns into `$visitor`
                        let trim_leading_whitespaces = |span| {
                            snippet_opt(cx, span).and_then(|snip| {
                                #[expect(clippy::cast_possible_truncation)]
                                snip.find(|c: char| !c.is_whitespace()).map(|pos| {
                                    span.lo() + BytePos(pos as u32)
                                })
                            }).map_or(span, |start_no_whitespace| e.span.with_lo(start_no_whitespace))
                        };

                        let mut generate_snippet = |pattern: &str| {
                            #[expect(clippy::cast_possible_truncation)]
                            macro_source.rfind(pattern).map(|pattern_pos| {
                                let rpos = pattern_pos + pattern.len();
                                let span_after_ref = e.span.with_lo(BytePos(e.span.lo().0 + rpos as u32));
                                let span = trim_leading_whitespaces(span_after_ref);
                                snippet_with_applicability(cx, span, "_", &mut applicability)
                            })
                        };

                        if *mutability == Mutability::Mut {
                            generate_snippet("mut")
                        } else {
                            generate_snippet("&")
                        }
                    } else {
                        Some(snippet_with_applicability(cx, e.span, "_", &mut applicability))
                    }
                } else {
                    Some(snippet_with_applicability(cx, addrof_target.span, "_", &mut applicability))
                };
                if let Some(sugg) = sugg {
                    span_lint_and_sugg(
                        cx,
                        DEREF_ADDROF,
                        e.span,
                        "immediately dereferencing a reference",
                        "try",
                        sugg.to_string(),
                        applicability,
                    );
                }
            }
        }
    }
}