rust-clippy/clippy_lints/src/non_copy_const.rs

//! Checks for uses of const which the type is not Freeze (Cell-free).
//!
//! This lint is **deny** by default.

use crate::utils::{in_constant, in_macro, is_copy, span_lint_and_then};
use rustc::hir::def::Def;
use rustc::hir::*;
use rustc::lint::{LateContext, LateLintPass, Lint, LintArray, LintPass};
use rustc::ty::adjustment::Adjust;
use rustc::ty::{self, TypeFlags};
use rustc::{declare_tool_lint, lint_array};
use rustc_errors::Applicability;
use rustc_typeck::hir_ty_to_ty;
use std::ptr;
use syntax_pos::{Span, DUMMY_SP};

/// **What it does:** Checks for declaration of `const` items which is interior
/// mutable (e.g. contains a `Cell`, `Mutex`, `AtomicXxxx` etc).
///
/// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.
/// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
/// or `AtomicXxxx` will be created, which defeats the whole purpose of using
/// these types in the first place.
///
/// The `const` should better be replaced by a `static` item if a global
/// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
///
/// **Known problems:** A "non-constant" const item is a legacy way to supply an
/// initialized value to downstream `static` items (e.g. the
/// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
/// and this lint should be suppressed.
///
/// **Example:**
/// ```rust
/// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
///
/// // Bad.
/// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
/// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
/// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
///
/// // Good.
/// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
/// STATIC_ATOM.store(9, SeqCst);
/// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
/// ```
declare_clippy_lint! {
    pub DECLARE_INTERIOR_MUTABLE_CONST,
    correctness,
    "declaring const with interior mutability"
}

/// **What it does:** Checks if `const` items which is interior mutable (e.g.
/// contains a `Cell`, `Mutex`, `AtomicXxxx` etc) has been borrowed directly.
///
/// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.
/// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
/// or `AtomicXxxx` will be created, which defeats the whole purpose of using
/// these types in the first place.
///
/// The `const` value should be stored inside a `static` item.
///
/// **Known problems:** None
///
/// **Example:**
/// ```rust
/// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
/// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
///
/// // Bad.
/// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
/// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
///
/// // Good.
/// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
/// STATIC_ATOM.store(9, SeqCst);
/// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
/// ```
declare_clippy_lint! {
    pub BORROW_INTERIOR_MUTABLE_CONST,
    correctness,
    "referencing const with interior mutability"
}

#[derive(Copy, Clone)]
enum Source {
    Item { item: Span },
    Assoc { item: Span, ty: Span },
    Expr { expr: Span },
}

impl Source {
    fn lint(&self) -> (&'static Lint, &'static str, Span) {
        match self {
            Source::Item { item } | Source::Assoc { item, .. } => (
                DECLARE_INTERIOR_MUTABLE_CONST,
                "a const item should never be interior mutable",
                *item,
            ),
            Source::Expr { expr } => (
                BORROW_INTERIOR_MUTABLE_CONST,
                "a const item with interior mutability should not be borrowed",
                *expr,
            ),
        }
    }
}

fn verify_ty_bound<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, ty: ty::Ty<'tcx>, source: Source) {
    if ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP) || is_copy(cx, ty) {
        // an UnsafeCell is !Copy, and an UnsafeCell is also the only type which
        // is !Freeze, thus if our type is Copy we can be sure it must be Freeze
        // as well.
        return;
    }

    let (lint, msg, span) = source.lint();
    span_lint_and_then(cx, lint, span, msg, |db| {
        if in_macro(span) {
            return; // Don't give suggestions into macros.
        }
        match source {
            Source::Item { .. } => {
                let const_kw_span = span.from_inner_byte_pos(0, 5);
                db.span_suggestion(
                    const_kw_span,
                    "make this a static item",
                    "static".to_string(),
                    Applicability::MachineApplicable,
                );
            },
            Source::Assoc { ty: ty_span, .. } => {
                if ty.flags.contains(TypeFlags::HAS_FREE_LOCAL_NAMES) {
                    db.span_help(ty_span, &format!("consider requiring `{}` to be `Copy`", ty));
                }
            },
            Source::Expr { .. } => {
                db.help("assign this const to a local or static variable, and use the variable here");
            },
        }
    });
}

pub struct NonCopyConst;

impl LintPass for NonCopyConst {
    fn get_lints(&self) -> LintArray {
        lint_array!(DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST)
    }

    fn name(&self) -> &'static str {
        "NonCopyConst"
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonCopyConst {
    fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, it: &'tcx Item) {
        if let ItemKind::Const(hir_ty, ..) = &it.node {
            let ty = hir_ty_to_ty(cx.tcx, hir_ty);
            verify_ty_bound(cx, ty, Source::Item { item: it.span });
        }
    }

    fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, trait_item: &'tcx TraitItem) {
        if let TraitItemKind::Const(hir_ty, ..) = &trait_item.node {
            let ty = hir_ty_to_ty(cx.tcx, hir_ty);
            verify_ty_bound(
                cx,
                ty,
                Source::Assoc {
                    ty: hir_ty.span,
                    item: trait_item.span,
                },
            );
        }
    }

    fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, impl_item: &'tcx ImplItem) {
        if let ImplItemKind::Const(hir_ty, ..) = &impl_item.node {
            let item_hir_id = cx.tcx.hir().get_parent_node_by_hir_id(impl_item.hir_id);
            let item = cx.tcx.hir().expect_item_by_hir_id(item_hir_id);
            // ensure the impl is an inherent impl.
            if let ItemKind::Impl(_, _, _, _, None, _, _) = item.node {
                let ty = hir_ty_to_ty(cx.tcx, hir_ty);
                verify_ty_bound(
                    cx,
                    ty,
                    Source::Assoc {
                        ty: hir_ty.span,
                        item: impl_item.span,
                    },
                );
            }
        }
    }

    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        if let ExprKind::Path(qpath) = &expr.node {
            // Only lint if we use the const item inside a function.
            if in_constant(cx, expr.hir_id) {
                return;
            }

            // make sure it is a const item.
            match cx.tables.qpath_def(qpath, expr.hir_id) {
                Def::Const(_) | Def::AssociatedConst(_) => {},
                _ => return,
            };

            // climb up to resolve any field access and explicit referencing.
            let mut cur_expr = expr;
            let mut dereferenced_expr = expr;
            let mut needs_check_adjustment = true;
            loop {
                let parent_id = cx.tcx.hir().get_parent_node_by_hir_id(cur_expr.hir_id);
                if parent_id == cur_expr.hir_id {
                    break;
                }
                if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find_by_hir_id(parent_id) {
                    match &parent_expr.node {
                        ExprKind::AddrOf(..) => {
                            // `&e` => `e` must be referenced
                            needs_check_adjustment = false;
                        },
                        ExprKind::Field(..) => {
                            dereferenced_expr = parent_expr;
                            needs_check_adjustment = true;
                        },
                        ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
                            // `e[i]` => desugared to `*Index::index(&e, i)`,
                            // meaning `e` must be referenced.
                            // no need to go further up since a method call is involved now.
                            needs_check_adjustment = false;
                            break;
                        },
                        ExprKind::Unary(UnDeref, _) => {
                            // `*e` => desugared to `*Deref::deref(&e)`,
                            // meaning `e` must be referenced.
                            // no need to go further up since a method call is involved now.
                            needs_check_adjustment = false;
                            break;
                        },
                        _ => break,
                    }
                    cur_expr = parent_expr;
                } else {
                    break;
                }
            }

            let ty = if needs_check_adjustment {
                let adjustments = cx.tables.expr_adjustments(dereferenced_expr);
                if let Some(i) = adjustments.iter().position(|adj| match adj.kind {
                    Adjust::Borrow(_) | Adjust::Deref(_) => true,
                    _ => false,
                }) {
                    if i == 0 {
                        cx.tables.expr_ty(dereferenced_expr)
                    } else {
                        adjustments[i - 1].target
                    }
                } else {
                    // No borrow adjustments = the entire const is moved.
                    return;
                }
            } else {
                cx.tables.expr_ty(dereferenced_expr)
            };

            verify_ty_bound(cx, ty, Source::Expr { expr: expr.span });
        }
    }
}