servo/components/plugins/lints/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 syntax::{ast, codemap, visit, ast_map};
use syntax::attr::AttrMetaMethods;
use rustc::lint::{Context, LintPass, LintArray};
use rustc::middle::ty::expr_ty;
use rustc::middle::{ty, def};
use rustc::util::ppaux::Repr;
use utils::unsafe_context;

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.
pub struct UnrootedPass;

// Checks if a type has the #[must_root] annotation.
// Unwraps pointers as well
// TODO (#3874, sort of): unwrap other types like Vec/Option/HashMap/etc
fn lint_unrooted_ty(cx: &Context, ty: &ast::Ty, warning: &str) {
    match ty.node {
        ast::TyVec(ref t) | ast::TyFixedLengthVec(ref t, _) |
        ast::TyPtr(ast::MutTy { ty: ref t, ..}) | ast::TyRptr(_, ast::MutTy { ty: ref t, ..}) =>
            lint_unrooted_ty(cx, &**t, warning),
        ast::TyPath(..) => {
                match cx.tcx.def_map.borrow()[&ty.id] {
                    def::PathResolution{ base_def: def::DefTy(def_id, _), .. } => {
                        if ty::has_attr(cx.tcx, def_id, "must_root") {
                            cx.span_lint(UNROOTED_MUST_ROOT, ty.span, warning);
                        }
                    }
                    _ => (),
                }
            }
            _ => (),
    };
}

impl LintPass for UnrootedPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(UNROOTED_MUST_ROOT)
    }
    /// All structs containing #[must_root] types must be #[must_root] themselves
    fn check_struct_def(&mut self,
                        cx: &Context,
                        def: &ast::StructDef,
                        _i: ast::Ident,
                        _gen: &ast::Generics,
                        id: ast::NodeId) {
        let item = match cx.tcx.map.get(id) {
            ast_map::Node::NodeItem(item) => item,
            _ => cx.tcx.map.expect_item(cx.tcx.map.get_parent(id)),
        };
        if item.attrs.iter().all(|a| !a.check_name("must_root")) {
            for ref field in def.fields.iter() {
                lint_unrooted_ty(cx, &*field.node.ty,
                                 "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: &Context, var: &ast::Variant, _gen: &ast::Generics) {
        let ref map = cx.tcx.map;
        if map.expect_item(map.get_parent(var.node.id)).attrs.iter().all(|a| !a.check_name("must_root")) {
            match var.node.kind {
                ast::TupleVariantKind(ref vec) => {
                    for ty in vec.iter() {
                        lint_unrooted_ty(cx, &*ty.ty,
                                         "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: &Context, kind: visit::FnKind, decl: &ast::FnDecl,
                block: &ast::Block, _span: codemap::Span, id: ast::NodeId) {
        match kind {
            visit::FkItemFn(i, _, _, _, _, _) |
            visit::FkMethod(i, _, _) if i.as_str() == "new" || i.as_str() == "new_inherited" => {
                return;
            },
            visit::FkItemFn(_, _, style, _, _, _) => match style {
                ast::Unsafety::Unsafe => return,
                _ => ()
            },
            _ => ()
        }

        if unsafe_context(&cx.tcx.map, id) {
            return;
        }

        match block.rules {
            ast::DefaultBlock => {
                for arg in decl.inputs.iter() {
                    lint_unrooted_ty(cx, &*arg.ty,
                                     "Type must be rooted")
                }
            }
            _ => () // fn is `unsafe`
        }
    }

    // Partially copied from rustc::middle::lint::builtin
    // Catches `let` statements and assignments which store a #[must_root] value
    // Expressions which return out of blocks eventually end up in a `let` or assignment
    // statement or a function return (which will be caught when it is used elsewhere)
    fn check_stmt(&mut self, cx: &Context, s: &ast::Stmt) {

        match s.node {
            ast::StmtDecl(_, id) |
            ast::StmtExpr(_, id) |
            ast::StmtSemi(_, id) if unsafe_context(&cx.tcx.map, id) => {
                return
            },
            _ => ()
        };

        let expr = match s.node {
            // Catch a `let` binding
            ast::StmtDecl(ref decl, _) => match decl.node {
                ast::DeclLocal(ref loc) => match loc.init {
                    Some(ref e) => &**e,
                    _ => return
                },
                _ => return
            },
            ast::StmtExpr(ref expr, _) => match expr.node {
                // This catches deferred `let` statements
                ast::ExprAssign(_, ref e) |
                // Match statements allow you to bind onto the variable later in an arm
                // We need not check arms individually since enum/struct fields are already
                // linted in `check_struct_def` and `check_variant`
                // (so there is no way of destructuring out a `#[must_root]` field)
                ast::ExprMatch(ref e, _, _) |
                // For loops allow you to bind a return value locally
                ast::ExprForLoop(_, ref e, _, _) => &**e,
                // XXXManishearth look into `if let` once it lands in our rustc
                _ => return
            },
            _ => return
        };

        let t = expr_ty(cx.tcx, &*expr);
        match t.sty {
            ty::ty_struct(did, _) |
            ty::ty_enum(did, _) => {
                if ty::has_attr(cx.tcx, did, "must_root") {
                    cx.span_lint(UNROOTED_MUST_ROOT, expr.span,
                                 &format!("Expression of type {} must be rooted", t.repr(cx.tcx)));
                }
            }
            _ => {}
        }
    }
}