rust/clippy_lints/src/misc.rs

use clippy_utils::diagnostics::{span_lint, span_lint_and_then, span_lint_hir_and_then};
use clippy_utils::source::{snippet, snippet_with_context};
use clippy_utils::sugg::Sugg;
use clippy_utils::{
    any_parent_is_automatically_derived, fulfill_or_allowed, get_parent_expr, is_lint_allowed, iter_input_pats,
    last_path_segment, SpanlessEq,
};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::def::Res;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{
    BinOpKind, BindingAnnotation, Body, ByRef, Expr, ExprKind, FnDecl, Mutability, PatKind, QPath, Stmt, StmtKind,
};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::def_id::LocalDefId;
use rustc_span::source_map::Span;

use crate::ref_patterns::REF_PATTERNS;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for function arguments and let bindings denoted as
    /// `ref`.
    ///
    /// ### Why is this bad?
    /// The `ref` declaration makes the function take an owned
    /// value, but turns the argument into a reference (which means that the value
    /// is destroyed when exiting the function). This adds not much value: either
    /// take a reference type, or take an owned value and create references in the
    /// body.
    ///
    /// For let bindings, `let x = &foo;` is preferred over `let ref x = foo`. The
    /// type of `x` is more obvious with the former.
    ///
    /// ### Known problems
    /// If the argument is dereferenced within the function,
    /// removing the `ref` will lead to errors. This can be fixed by removing the
    /// dereferences, e.g., changing `*x` to `x` within the function.
    ///
    /// ### Example
    /// ```no_run
    /// fn foo(ref _x: u8) {}
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// fn foo(_x: &u8) {}
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub TOPLEVEL_REF_ARG,
    style,
    "an entire binding declared as `ref`, in a function argument or a `let` statement"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for the use of bindings with a single leading
    /// underscore.
    ///
    /// ### Why is this bad?
    /// A single leading underscore is usually used to indicate
    /// that a binding will not be used. Using such a binding breaks this
    /// expectation.
    ///
    /// ### Known problems
    /// The lint does not work properly with desugaring and
    /// macro, it has been allowed in the mean time.
    ///
    /// ### Example
    /// ```no_run
    /// let _x = 0;
    /// let y = _x + 1; // Here we are using `_x`, even though it has a leading
    ///                 // underscore. We should rename `_x` to `x`
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub USED_UNDERSCORE_BINDING,
    pedantic,
    "using a binding which is prefixed with an underscore"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for the use of short circuit boolean conditions as
    /// a
    /// statement.
    ///
    /// ### Why is this bad?
    /// Using a short circuit boolean condition as a statement
    /// may hide the fact that the second part is executed or not depending on the
    /// outcome of the first part.
    ///
    /// ### Example
    /// ```rust,ignore
    /// f() && g(); // We should write `if f() { g(); }`.
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub SHORT_CIRCUIT_STATEMENT,
    complexity,
    "using a short circuit boolean condition as a statement"
}

declare_lint_pass!(LintPass => [
    TOPLEVEL_REF_ARG,
    USED_UNDERSCORE_BINDING,
    SHORT_CIRCUIT_STATEMENT,
]);

impl<'tcx> LateLintPass<'tcx> for LintPass {
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        k: FnKind<'tcx>,
        decl: &'tcx FnDecl<'_>,
        body: &'tcx Body<'_>,
        span: Span,
        _: LocalDefId,
    ) {
        if let FnKind::Closure = k {
            // Does not apply to closures
            return;
        }
        if in_external_macro(cx.tcx.sess, span) {
            return;
        }
        for arg in iter_input_pats(decl, body) {
            // Do not emit if clippy::ref_patterns is not allowed to avoid having two lints for the same issue.
            if !is_lint_allowed(cx, REF_PATTERNS, arg.pat.hir_id) {
                return;
            }
            if let PatKind::Binding(BindingAnnotation(ByRef::Yes, _), ..) = arg.pat.kind {
                span_lint(
                    cx,
                    TOPLEVEL_REF_ARG,
                    arg.pat.span,
                    "`ref` directly on a function argument is ignored. \
                    Consider using a reference type instead",
                );
            }
        }
    }

    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
        if_chain! {
            if !in_external_macro(cx.tcx.sess, stmt.span);
            if let StmtKind::Local(local) = stmt.kind;
            if let PatKind::Binding(BindingAnnotation(ByRef::Yes, mutabl), .., name, None) = local.pat.kind;
            if let Some(init) = local.init;
            // Do not emit if clippy::ref_patterns is not allowed to avoid having two lints for the same issue.
            if is_lint_allowed(cx, REF_PATTERNS, local.pat.hir_id);
            then {
                let ctxt = local.span.ctxt();
                let mut app = Applicability::MachineApplicable;
                let sugg_init = Sugg::hir_with_context(cx, init, ctxt, "..", &mut app);
                let (mutopt, initref) = if mutabl == Mutability::Mut {
                    ("mut ", sugg_init.mut_addr())
                } else {
                    ("", sugg_init.addr())
                };
                let tyopt = if let Some(ty) = local.ty {
                    let ty_snip = snippet_with_context(cx, ty.span, ctxt, "_", &mut app).0;
                    format!(": &{mutopt}{ty_snip}")
                } else {
                    String::new()
                };
                span_lint_hir_and_then(
                    cx,
                    TOPLEVEL_REF_ARG,
                    init.hir_id,
                    local.pat.span,
                    "`ref` on an entire `let` pattern is discouraged, take a reference with `&` instead",
                    |diag| {
                        diag.span_suggestion(
                            stmt.span,
                            "try",
                            format!(
                                "let {name}{tyopt} = {initref};",
                                name=snippet(cx, name.span, ".."),
                            ),
                            app,
                        );
                    }
                );
            }
        };
        if_chain! {
            if let StmtKind::Semi(expr) = stmt.kind;
            if let ExprKind::Binary(ref binop, a, b) = expr.kind;
            if binop.node == BinOpKind::And || binop.node == BinOpKind::Or;
            if let Some(sugg) = Sugg::hir_opt(cx, a);
            then {
                span_lint_hir_and_then(
                    cx,
                    SHORT_CIRCUIT_STATEMENT,
                    expr.hir_id,
                    stmt.span,
                    "boolean short circuit operator in statement may be clearer using an explicit test",
                    |diag| {
                        let sugg = if binop.node == BinOpKind::Or { !sugg } else { sugg };
                        diag.span_suggestion(
                            stmt.span,
                            "replace it with",
                            format!(
                                "if {sugg} {{ {}; }}",
                                &snippet(cx, b.span, ".."),
                            ),
                            Applicability::MachineApplicable, // snippet
                        );
                    });
            }
        };
    }

    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if in_external_macro(cx.sess(), expr.span)
            || expr.span.desugaring_kind().is_some()
            || any_parent_is_automatically_derived(cx.tcx, expr.hir_id)
        {
            return;
        }
        let (definition_hir_id, ident) = match expr.kind {
            ExprKind::Path(ref qpath) => {
                if let QPath::Resolved(None, path) = qpath
                    && let Res::Local(id) = path.res
                    && is_used(cx, expr)
                {
                    (id, last_path_segment(qpath).ident)
                } else {
                    return;
                }
            },
            ExprKind::Field(recv, ident) => {
                if let Some(adt_def) = cx.typeck_results().expr_ty_adjusted(recv).ty_adt_def()
                    && let Some(field) = adt_def.all_fields().find(|field| field.name == ident.name)
                    && let Some(local_did) = field.did.as_local()
                    && let Some(hir_id) = cx.tcx.opt_local_def_id_to_hir_id(local_did)
                    && !cx.tcx.type_of(field.did).skip_binder().is_phantom_data()
                {
                    (hir_id, ident)
                } else {
                    return;
                }
            },
            _ => return,
        };

        let name = ident.name.as_str();
        if name.starts_with('_')
            && !name.starts_with("__")
            && let definition_span = cx.tcx.hir().span(definition_hir_id)
            && !definition_span.from_expansion()
            && !fulfill_or_allowed(cx, USED_UNDERSCORE_BINDING, [expr.hir_id, definition_hir_id])
        {
            span_lint_and_then(
                cx,
                USED_UNDERSCORE_BINDING,
                expr.span,
                &format!(
                    "used binding `{name}` which is prefixed with an underscore. A leading \
                     underscore signals that a binding will not be used"
                ),
                |diag| {
                    diag.span_note(definition_span, format!("`{name}` is defined here"));
                }
            );
        }
    }
}

/// Heuristic to see if an expression is used. Should be compatible with
/// `unused_variables`'s idea
/// of what it means for an expression to be "used".
fn is_used(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    get_parent_expr(cx, expr).map_or(true, |parent| match parent.kind {
        ExprKind::Assign(_, rhs, _) | ExprKind::AssignOp(_, _, rhs) => SpanlessEq::new(cx).eq_expr(rhs, expr),
        _ => is_used(cx, parent),
    })
}