servo/components/script_plugins/unrooted_must_root.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

use rustc::hir::{self, ExprKind};
use rustc::hir::intravisit as visit;
use rustc::hir::map as ast_map;
use rustc::lint::{LateContext, LintPass, LintArray, LateLintPass, LintContext};
use rustc::ty;
use syntax::{ast, codemap, symbol::Ident};
use utils::{match_def_path, in_derive_expn};

declare_lint!(UNROOTED_MUST_ROOT, Deny,
              "Warn and report usage of unrooted jsmanaged objects");

/// Lint for ensuring safe usage of unrooted pointers
///
/// This lint (disable with `-A unrooted-must-root`/`#[allow(unrooted_must_root)]`) ensures that `#[must_root]`
/// values are used correctly.
///
/// "Incorrect" usage includes:
///
///  - Not being used in a struct/enum field which is not `#[must_root]` itself
///  - Not being used as an argument to a function (Except onces named `new` and `new_inherited`)
///  - Not being bound locally in a `let` statement, assignment, `for` loop, or `match` statement.
///
/// This helps catch most situations where pointers like `JS<T>` are used in a way that they can be invalidated by a
/// GC pass.
///
/// Structs which have their own mechanism of rooting their unrooted contents (e.g. `ScriptThread`)
/// can be marked as `#[allow(unrooted_must_root)]`. Smart pointers which root their interior type
/// can be marked as `#[allow_unrooted_interior]`
pub struct UnrootedPass;

impl UnrootedPass {
    pub fn new() -> UnrootedPass {
        UnrootedPass
    }
}

/// Checks if a type is unrooted or contains any owned unrooted types
fn is_unrooted_ty(cx: &LateContext, ty: &ty::TyS, in_new_function: bool) -> bool {
    let mut ret = false;
    ty.maybe_walk(|t| {
        match t.sty {
            ty::TyAdt(did, _) => {
                if cx.tcx.has_attr(did.did, "must_root") {
                    ret = true;
                    false
                } else if cx.tcx.has_attr(did.did, "allow_unrooted_interior") {
                    false
                } else if match_def_path(cx, did.did, &["core", "cell", "Ref"])
                        || match_def_path(cx, did.did, &["core", "cell", "RefMut"])
                        || match_def_path(cx, did.did, &["core", "slice", "Iter"])
                        || match_def_path(cx, did.did, &["core", "slice", "IterMut"])
                        || match_def_path(cx, did.did, &["std", "collections", "hash", "map", "Entry"])
                        || match_def_path(cx, did.did, &["std", "collections", "hash", "map", "OccupiedEntry"])
                        || match_def_path(cx, did.did, &["std", "collections", "hash", "map", "VacantEntry"])
                        || match_def_path(cx, did.did, &["std", "collections", "hash", "map", "Iter"])
                        || match_def_path(cx, did.did, &["std", "collections", "hash", "set", "Iter"]) {
                    // Structures which are semantically similar to an &ptr.
                    false
                } else if did.is_box() && in_new_function {
                    // box in new() is okay
                    false
                } else {
                    true
                }
            },
            ty::TyRef(..) => false, // don't recurse down &ptrs
            ty::TyRawPtr(..) => false, // don't recurse down *ptrs
            ty::TyFnDef(..) | ty::TyFnPtr(_) => false,
            _ => true
        }
    });
    ret
}

impl LintPass for UnrootedPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(UNROOTED_MUST_ROOT)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnrootedPass {
    /// All structs containing #[must_root] types must be #[must_root] themselves
    fn check_struct_def(&mut self,
                        cx: &LateContext,
                        def: &hir::VariantData,
                        _n: ast::Name,
                        _gen: &hir::Generics,
                        id: ast::NodeId) {
        let item = match cx.tcx.hir.get(id) {
            ast_map::Node::NodeItem(item) => item,
            _ => cx.tcx.hir.expect_item(cx.tcx.hir.get_parent(id)),
        };
        if item.attrs.iter().all(|a| !a.check_name("must_root")) {
            for ref field in def.fields() {
                let def_id = cx.tcx.hir.local_def_id(field.id);
                if is_unrooted_ty(cx, cx.tcx.type_of(def_id), false) {
                    cx.span_lint(UNROOTED_MUST_ROOT, field.span,
                                 "Type must be rooted, use #[must_root] on the struct definition to propagate")
                }
            }
        }
    }

    /// All enums containing #[must_root] types must be #[must_root] themselves
    fn check_variant(&mut self, cx: &LateContext, var: &hir::Variant, _gen: &hir::Generics) {
        let ref map = cx.tcx.hir;
        if map.expect_item(map.get_parent(var.node.data.id())).attrs.iter().all(|a| !a.check_name("must_root")) {
            match var.node.data {
                hir::VariantData::Tuple(ref fields, _) => {
                    for ref field in fields {
                        let def_id = cx.tcx.hir.local_def_id(field.id);
                        if is_unrooted_ty(cx, cx.tcx.type_of(def_id), false) {
                            cx.span_lint(UNROOTED_MUST_ROOT, field.ty.span,
                                         "Type must be rooted, use #[must_root] on \
                                          the enum definition to propagate")
                        }
                    }
                }
                _ => () // Struct variants already caught by check_struct_def
            }
        }
    }
    /// Function arguments that are #[must_root] types are not allowed
    fn check_fn(&mut self,
                cx: &LateContext<'a, 'tcx>,
                kind: visit::FnKind,
                decl: &'tcx hir::FnDecl,
                body: &'tcx hir::Body,
                span: codemap::Span,
                id: ast::NodeId) {
        let in_new_function = match kind {
            visit::FnKind::ItemFn(n, _, _, _, _) |
            visit::FnKind::Method(Ident { name: n, .. }, _, _, _) => {
                &*n.as_str() == "new" || n.as_str().starts_with("new_")
            }
            visit::FnKind::Closure(_) => return,
        };

        if !in_derive_expn(span) {
            let def_id = cx.tcx.hir.local_def_id(id);
            let sig = cx.tcx.type_of(def_id).fn_sig(cx.tcx);

            for (arg, ty) in decl.inputs.iter().zip(sig.inputs().skip_binder().iter()) {
                if is_unrooted_ty(cx, ty, false) {
                    cx.span_lint(UNROOTED_MUST_ROOT, arg.span, "Type must be rooted")
                }
            }

            if !in_new_function {
                if is_unrooted_ty(cx, sig.output().skip_binder(), false) {
                    cx.span_lint(UNROOTED_MUST_ROOT, decl.output.span(), "Type must be rooted")
                }
            }
        }

        let mut visitor = FnDefVisitor {
            cx: cx,
            in_new_function: in_new_function,
        };
        visit::walk_expr(&mut visitor, &body.value);
    }
}

struct FnDefVisitor<'a, 'b: 'a, 'tcx: 'a + 'b> {
    cx: &'a LateContext<'b, 'tcx>,
    in_new_function: bool,
}

impl<'a, 'b, 'tcx> visit::Visitor<'tcx> for FnDefVisitor<'a, 'b, 'tcx> {
    fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
        let cx = self.cx;

        fn require_rooted(cx: &LateContext, in_new_function: bool, subexpr: &hir::Expr) {
            let ty = cx.tables.expr_ty(&subexpr);
            if is_unrooted_ty(cx, ty, in_new_function) {
                cx.span_lint(UNROOTED_MUST_ROOT,
                             subexpr.span,
                             &format!("Expression of type {:?} must be rooted", ty))
            }
        }

        match expr.node {
            // Trait casts from #[must_root] types are not allowed
            ExprKind::Cast(ref subexpr, _) => require_rooted(cx, self.in_new_function, &*subexpr),
            // This catches assignments... the main point of this would be to catch mutable
            // references to `JS<T>`.
            // FIXME: Enable this? Triggers on certain kinds of uses of DomRefCell.
            // hir::ExprAssign(_, ref rhs) => require_rooted(cx, self.in_new_function, &*rhs),
            // This catches calls; basically, this enforces the constraint that only constructors
            // can call other constructors.
            // FIXME: Enable this? Currently triggers with constructs involving DomRefCell, and
            // constructs like Vec<JS<T>> and RootedVec<JS<T>>.
            // hir::ExprCall(..) if !self.in_new_function => {
            //     require_rooted(cx, self.in_new_function, expr);
            // }
            _ => {
                // TODO(pcwalton): Check generics with a whitelist of allowed generics.
            }
        }

        visit::walk_expr(self, expr);
    }

    fn visit_pat(&mut self, pat: &'tcx hir::Pat) {
        let cx = self.cx;

        // We want to detect pattern bindings that move a value onto the stack.
        // When "default binding modes" https://github.com/rust-lang/rust/issues/42640
        // are implemented, the `Unannotated` case could cause false-positives.
        // These should be fixable by adding an explicit `ref`.
        match pat.node {
            hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, _, _) |
            hir::PatKind::Binding(hir::BindingAnnotation::Mutable, _, _, _) => {
                let ty = cx.tables.pat_ty(pat);
                if is_unrooted_ty(cx, ty, self.in_new_function) {
                    cx.span_lint(UNROOTED_MUST_ROOT,
                                pat.span,
                                &format!("Expression of type {:?} must be rooted", ty))
                }
            }
            _ => {}
        }

        visit::walk_pat(self, pat);
    }

    fn visit_ty(&mut self, _: &'tcx hir::Ty) {}

    fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
        hir::intravisit::NestedVisitorMap::OnlyBodies(&self.cx.tcx.hir)
    }
}