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

use crate::utils::{in_macro, snippet_opt, snippet_with_applicability, span_lint_and_sugg};
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
    /// // Bad
    /// let a = f(*&mut b);
    /// let c = *&d;
    ///
    /// // Good
    /// let a = f(b);
    /// let c = d;
    /// ```
    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 !in_macro(addrof_target.span);
            then {
                let mut applicability = Applicability::MachineApplicable;
                let sugg = if e.span.from_expansion() {
                    if let Ok(macro_source) = cx.sess.source_map().span_to_snippet(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| {
                                #[allow(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| {
                            #[allow(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 this",
                        sugg.to_string(),
                        applicability,
                    );
                }
            }
        }
    }
}

declare_clippy_lint! {
    /// **What it does:** Checks for references in expressions that use
    /// auto dereference.
    ///
    /// **Why is this bad?** The reference is a no-op and is automatically
    /// dereferenced by the compiler and makes the code less clear.
    ///
    /// **Example:**
    /// ```rust
    /// struct Point(u32, u32);
    /// let point = Point(30, 20);
    /// let x = (&point).0;
    /// ```
    pub REF_IN_DEREF,
    complexity,
    "Use of reference in auto dereference expression."
}

declare_lint_pass!(RefInDeref => [REF_IN_DEREF]);

impl EarlyLintPass for RefInDeref {
    fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &Expr) {
        if_chain! {
            if let ExprKind::Field(ref object, _) = e.kind;
            if let ExprKind::Paren(ref parened) = object.kind;
            if let ExprKind::AddrOf(_, _, ref inner) = parened.kind;
            then {
                let mut applicability = Applicability::MachineApplicable;
                span_lint_and_sugg(
                    cx,
                    REF_IN_DEREF,
                    object.span,
                    "creating a reference that is immediately dereferenced",
                    "try this",
                    snippet_with_applicability(cx, inner.span, "_", &mut applicability).to_string(),
                    applicability,
                );
            }
        }
    }
}