rust/src/librustc/middle/kind.rs

// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::prelude::*;

use middle::freevars::freevar_entry;
use middle::freevars;
use middle::liveness;
use middle::pat_util;
use middle::ty;
use middle::typeck;
use util::ppaux::{Repr, ty_to_str, tys_to_str};

use syntax::ast::*;
use syntax::attr::attrs_contains_name;
use syntax::codemap::span;
use syntax::print::pprust::expr_to_str;
use syntax::{visit, ast_util};

// Kind analysis pass.
//
// There are several kinds defined by various operations. The most restrictive
// kind is noncopyable. The noncopyable kind can be extended with any number
// of the following attributes.
//
//  send: Things that can be sent on channels or included in spawned closures.
//  copy: Things that can be copied.
//  const: Things thare are deeply immutable. They are guaranteed never to
//    change, and can be safely shared without copying between tasks.
//  owned: Things that do not contain borrowed pointers.
//
// Send includes scalar types as well as classes and unique types containing
// only sendable types.
//
// Copy includes boxes, closure and unique types containing copyable types.
//
// Const include scalar types, things without non-const fields, and pointers
// to const things.
//
// This pass ensures that type parameters are only instantiated with types
// whose kinds are equal or less general than the way the type parameter was
// annotated (with the `send`, `copy` or `const` keyword).
//
// It also verifies that noncopyable kinds are not copied. Sendability is not
// applied, since none of our language primitives send. Instead, the sending
// primitives in the stdlib are explicitly annotated to only take sendable
// types.

pub static try_adding: &'static str = "Try adding a move";

pub struct Context {
    tcx: ty::ctxt,
    method_map: typeck::method_map,
    last_use_map: liveness::last_use_map,
    current_item: node_id,
}

pub fn check_crate(tcx: ty::ctxt,
                   method_map: typeck::method_map,
                   last_use_map: liveness::last_use_map,
                   crate: @crate) {
    let ctx = Context {
        tcx: tcx,
        method_map: method_map,
        last_use_map: last_use_map,
        current_item: -1,
    };
    let visit = visit::mk_vt(@visit::Visitor {
        visit_arm: check_arm,
        visit_expr: check_expr,
        visit_fn: check_fn,
        visit_ty: check_ty,
        visit_item: check_item,
        visit_block: check_block,
        .. *visit::default_visitor()
    });
    visit::visit_crate(*crate, ctx, visit);
    tcx.sess.abort_if_errors();
}

type check_fn = @fn(Context, @freevar_entry);

fn check_struct_safe_for_destructor(cx: Context,
                                    span: span,
                                    struct_did: def_id) {
    let struct_tpt = ty::lookup_item_type(cx.tcx, struct_did);
    if !struct_tpt.generics.has_type_params() {
        let struct_ty = ty::mk_struct(cx.tcx, struct_did, ty::substs {
            self_r: None,
            self_ty: None,
            tps: ~[]
        });
        if !ty::type_is_owned(cx.tcx, struct_ty) {
            cx.tcx.sess.span_err(span,
                                 ~"cannot implement a destructor on a struct \
                                   that is not Owned");
            cx.tcx.sess.span_note(span,
                                  ~"use \"#[unsafe_destructor]\" on the \
                                    implementation to force the compiler to \
                                    allow this");
        }
    } else {
        cx.tcx.sess.span_err(span,
                             ~"cannot implement a destructor on a struct \
                               with type parameters");
        cx.tcx.sess.span_note(span,
                              ~"use \"#[unsafe_destructor]\" on the \
                                implementation to force the compiler to \
                                allow this");
    }
}

fn check_block(block: &blk, cx: Context, visitor: visit::vt<Context>) {
    visit::visit_block(block, cx, visitor);
}

fn check_item(item: @item, cx: Context, visitor: visit::vt<Context>) {
    // If this is a destructor, check kinds.
    if !attrs_contains_name(item.attrs, "unsafe_destructor") {
        match item.node {
            item_impl(_, Some(trait_ref), self_type, _) => {
                match cx.tcx.def_map.find(&trait_ref.ref_id) {
                    None => cx.tcx.sess.bug(~"trait ref not in def map!"),
                    Some(&trait_def) => {
                        let trait_def_id = ast_util::def_id_of_def(trait_def);
                        if cx.tcx.lang_items.drop_trait() == trait_def_id {
                            // Yes, it's a destructor.
                            match self_type.node {
                                ty_path(_, path_node_id) => {
                                    let struct_def = *cx.tcx.def_map.get(
                                        &path_node_id);
                                    let struct_did =
                                        ast_util::def_id_of_def(struct_def);
                                    check_struct_safe_for_destructor(
                                        cx,
                                        self_type.span,
                                        struct_did);
                                }
                                _ => {
                                    cx.tcx.sess.span_bug(self_type.span,
                                                         ~"the self type for \
                                                           the Drop trait \
                                                           impl is not a \
                                                           path");
                                }
                            }
                        }
                    }
                }
            }
            item_struct(struct_def, _) => {
                match struct_def.dtor {
                    None => {}
                    Some(ref dtor) => {
                        let struct_did = def_id { crate: 0, node: item.id };
                        check_struct_safe_for_destructor(cx,
                                                         dtor.span,
                                                         struct_did);
                    }
                }
            }
            _ => {}
        }
    }

    let cx = Context { current_item: item.id, ..cx };
    visit::visit_item(item, cx, visitor);
}

// Yields the appropriate function to check the kind of closed over
// variables. `id` is the node_id for some expression that creates the
// closure.
fn with_appropriate_checker(cx: Context, id: node_id, b: &fn(check_fn)) {
    fn check_for_uniq(cx: Context, fv: @freevar_entry) {
        // all captured data must be owned, regardless of whether it is
        // moved in or copied in.
        let id = ast_util::def_id_of_def(fv.def).node;
        let var_t = ty::node_id_to_type(cx.tcx, id);
        if !check_owned(cx, var_t, fv.span) { return; }

        // check that only immutable variables are implicitly copied in
        check_imm_free_var(cx, fv.def, fv.span);
    }

    fn check_for_box(cx: Context, fv: @freevar_entry) {
        // all captured data must be owned
        let id = ast_util::def_id_of_def(fv.def).node;
        let var_t = ty::node_id_to_type(cx.tcx, id);
        if !check_durable(cx.tcx, var_t, fv.span) { return; }

        // check that only immutable variables are implicitly copied in
        check_imm_free_var(cx, fv.def, fv.span);
    }

    fn check_for_block(_cx: Context, _fv: @freevar_entry) {
        // no restrictions
    }

    fn check_for_bare(cx: Context, fv: @freevar_entry) {
        cx.tcx.sess.span_err(
            fv.span,
            ~"attempted dynamic environment capture");
    }

    let fty = ty::node_id_to_type(cx.tcx, id);
    match ty::get(fty).sty {
        ty::ty_closure(ty::ClosureTy {sigil: OwnedSigil, _}) => {
            b(check_for_uniq)
        }
        ty::ty_closure(ty::ClosureTy {sigil: ManagedSigil, _}) => {
            b(check_for_box)
        }
        ty::ty_closure(ty::ClosureTy {sigil: BorrowedSigil, _}) => {
            b(check_for_block)
        }
        ty::ty_bare_fn(_) => {
            b(check_for_bare)
        }
        ref s => {
            cx.tcx.sess.bug(
                fmt!("expect fn type in kind checker, not %?", s));
        }
    }
}

// Check that the free variables used in a shared/sendable closure conform
// to the copy/move kind bounds. Then recursively check the function body.
fn check_fn(
    fk: &visit::fn_kind,
    decl: &fn_decl,
    body: &blk,
    sp: span,
    fn_id: node_id,
    cx: Context,
    v: visit::vt<Context>) {

    // Check kinds on free variables:
    do with_appropriate_checker(cx, fn_id) |chk| {
        for vec::each(*freevars::get_freevars(cx.tcx, fn_id)) |fv| {
            chk(cx, *fv);
        }
    }

    visit::visit_fn(fk, decl, body, sp, fn_id, cx, v);
}

fn check_arm(a: &arm, cx: Context, v: visit::vt<Context>) {
    for vec::each(a.pats) |p| {
        do pat_util::pat_bindings(cx.tcx.def_map, *p) |mode, id, span, _pth| {
            if mode == bind_by_copy {
                let t = ty::node_id_to_type(cx.tcx, id);
                let reason = "consider binding with `ref` or `move` instead";
                check_copy(cx, t, span, reason);
            }
        }
    }
    visit::visit_arm(a, cx, v);
}

pub fn check_expr(e: @expr, cx: Context, v: visit::vt<Context>) {
    debug!("kind::check_expr(%s)", expr_to_str(e, cx.tcx.sess.intr()));

    // Handle any kind bounds on type parameters
    let type_parameter_id = match e.node {
        expr_index(*)|expr_assign_op(*)|
        expr_unary(*)|expr_binary(*)|expr_method_call(*) => e.callee_id,
        _ => e.id
    };
    for cx.tcx.node_type_substs.find(&type_parameter_id).each |ts| {
        // FIXME(#5562): removing this copy causes a segfault before stage2
        let ts = /*bad*/ copy **ts;
        let type_param_defs = match e.node {
          expr_path(_) => {
            let did = ast_util::def_id_of_def(*cx.tcx.def_map.get(&e.id));
            ty::lookup_item_type(cx.tcx, did).generics.type_param_defs
          }
          _ => {
            // Type substitutions should only occur on paths and
            // method calls, so this needs to be a method call.

            // Even though the callee_id may have been the id with
            // node_type_substs, e.id is correct here.
            ty::method_call_type_param_defs(cx.tcx, cx.method_map, e.id).expect(
                ~"non path/method call expr has type substs??")
          }
        };
        if ts.len() != type_param_defs.len() {
            // Fail earlier to make debugging easier
            fail!(fmt!("internal error: in kind::check_expr, length \
                       mismatch between actual and declared bounds: actual = \
                        %s, declared = %s",
                       ts.repr(cx.tcx),
                       type_param_defs.repr(cx.tcx)));
        }
        for vec::each2(ts, *type_param_defs) |&ty, type_param_def| {
            check_bounds(cx, type_parameter_id, e.span, ty, type_param_def)
        }
    }

    match e.node {
        expr_cast(source, _) => {
            check_cast_for_escaping_regions(cx, source, e);
            check_kind_bounds_of_cast(cx, source, e);
        }
        expr_copy(expr) => {
            // Note: This is the only place where we must check whether the
            // argument is copyable.  This is not because this is the only
            // kind of expression that may copy things, but rather because all
            // other copies will have been converted to moves by by the
            // `moves` pass if the value is not copyable.
            check_copy(cx,
                       ty::expr_ty(cx.tcx, expr),
                       expr.span,
                       "explicit copy requires a copyable argument");
        }
        expr_repeat(element, count_expr, _) => {
            let count = ty::eval_repeat_count(cx.tcx, count_expr);
            if count > 1 {
                let element_ty = ty::expr_ty(cx.tcx, element);
                check_copy(cx, element_ty, element.span,
                           "repeated element will be copied");
            }
        }
        _ => {}
    }
    visit::visit_expr(e, cx, v);
}

fn check_ty(aty: @Ty, cx: Context, v: visit::vt<Context>) {
    match aty.node {
      ty_path(_, id) => {
        for cx.tcx.node_type_substs.find(&id).each |ts| {
            // FIXME(#5562): removing this copy causes a segfault before stage2
            let ts = /*bad*/ copy **ts;
            let did = ast_util::def_id_of_def(*cx.tcx.def_map.get(&id));
            let type_param_defs =
                ty::lookup_item_type(cx.tcx, did).generics.type_param_defs;
            for vec::each2(ts, *type_param_defs) |&ty, type_param_def| {
                check_bounds(cx, aty.id, aty.span, ty, type_param_def)
            }
        }
      }
      _ => {}
    }
    visit::visit_ty(aty, cx, v);
}

pub fn check_bounds(cx: Context,
                    _type_parameter_id: node_id,
                    sp: span,
                    ty: ty::t,
                    type_param_def: &ty::TypeParameterDef)
{
    let kind = ty::type_contents(cx.tcx, ty);
    let mut missing = ~[];
    for type_param_def.bounds.each |bound| {
        match *bound {
            ty::bound_trait(_) => {
                /* Not our job, checking in typeck */
            }

            ty::bound_copy => {
                if !kind.is_copy(cx.tcx) {
                    missing.push("Copy");
                }
            }

            ty::bound_durable => {
                if !kind.is_durable(cx.tcx) {
                    missing.push("'static");
                }
            }

            ty::bound_owned => {
                if !kind.is_owned(cx.tcx) {
                    missing.push("Owned");
                }
            }

            ty::bound_const => {
                if !kind.is_const(cx.tcx) {
                    missing.push("Const");
                }
            }
        }
    }

    if !missing.is_empty() {
        cx.tcx.sess.span_err(
            sp,
            fmt!("instantiating a type parameter with an incompatible type \
                  `%s`, which does not fulfill `%s`",
                 ty_to_str(cx.tcx, ty),
                 str::connect_slices(missing, " ")));
    }
}

fn is_nullary_variant(cx: Context, ex: @expr) -> bool {
    match ex.node {
      expr_path(_) => {
        match *cx.tcx.def_map.get(&ex.id) {
          def_variant(edid, vdid) => {
            vec::len(ty::enum_variant_with_id(cx.tcx, edid, vdid).args) == 0u
          }
          _ => false
        }
      }
      _ => false
    }
}

fn check_imm_free_var(cx: Context, def: def, sp: span) {
    match def {
        def_local(_, is_mutbl) => {
            if is_mutbl {
                cx.tcx.sess.span_err(
                    sp,
                    ~"mutable variables cannot be implicitly captured");
            }
        }
        def_arg(*) => { /* ok */ }
        def_upvar(_, def1, _, _) => { check_imm_free_var(cx, *def1, sp); }
        def_binding(*) | def_self(*) => { /*ok*/ }
        _ => {
            cx.tcx.sess.span_bug(
                sp,
                fmt!("unknown def for free variable: %?", def));
        }
    }
}

fn check_copy(cx: Context, ty: ty::t, sp: span, reason: &str) {
    debug!("type_contents(%s)=%s",
           ty_to_str(cx.tcx, ty),
           ty::type_contents(cx.tcx, ty).to_str());
    if !ty::type_is_copyable(cx.tcx, ty) {
        cx.tcx.sess.span_err(
            sp, fmt!("copying a value of non-copyable type `%s`",
                     ty_to_str(cx.tcx, ty)));
        cx.tcx.sess.span_note(sp, fmt!("%s", reason));
    }
}

pub fn check_owned(cx: Context, ty: ty::t, sp: span) -> bool {
    if !ty::type_is_owned(cx.tcx, ty) {
        cx.tcx.sess.span_err(
            sp, fmt!("value has non-owned type `%s`",
                     ty_to_str(cx.tcx, ty)));
        false
    } else {
        true
    }
}

// note: also used from middle::typeck::regionck!
pub fn check_durable(tcx: ty::ctxt, ty: ty::t, sp: span) -> bool {
    if !ty::type_is_durable(tcx, ty) {
        match ty::get(ty).sty {
          ty::ty_param(*) => {
            tcx.sess.span_err(sp, ~"value may contain borrowed \
                                    pointers; use `'static` bound");
          }
          _ => {
            tcx.sess.span_err(sp, ~"value may contain borrowed \
                                    pointers");
          }
        }
        false
    } else {
        true
    }
}

/// This is rather subtle.  When we are casting a value to a instantiated
/// trait like `a as trait<'r>`, regionck already ensures that any borrowed
/// pointers that appear in the type of `a` are bounded by `'r` (ed.: modulo
/// FIXME(#5723)).  However, it is possible that there are *type parameters*
/// in the type of `a`, and those *type parameters* may have borrowed pointers
/// within them.  We have to guarantee that the regions which appear in those
/// type parameters are not obscured.
///
/// Therefore, we ensure that one of three conditions holds:
///
/// (1) The trait instance cannot escape the current fn.  This is
/// guaranteed if the region bound `&r` is some scope within the fn
/// itself.  This case is safe because whatever borrowed pointers are
/// found within the type parameter, they must enclose the fn body
/// itself.
///
/// (2) The type parameter appears in the type of the trait.  For
/// example, if the type parameter is `T` and the trait type is
/// `deque<T>`, then whatever borrowed ptrs may appear in `T` also
/// appear in `deque<T>`.
///
/// (3) The type parameter is owned (and therefore does not contain
/// borrowed ptrs).
///
/// FIXME(#5723)---This code should probably move into regionck.
pub fn check_cast_for_escaping_regions(
    cx: Context,
    source: @expr,
    target: @expr)
{
    // Determine what type we are casting to; if it is not an trait, then no
    // worries.
    let target_ty = ty::expr_ty(cx.tcx, target);
    match ty::get(target_ty).sty {
        ty::ty_trait(*) => {}
        _ => { return; }
    }

    // Collect up the regions that appear in the target type.  We want to
    // ensure that these lifetimes are shorter than all lifetimes that are in
    // the source type.  See test `src/test/compile-fail/regions-trait-2.rs`
    let mut target_regions = ~[];
    ty::walk_regions_and_ty(
        cx.tcx,
        target_ty,
        |r| {
            if !r.is_bound() {
                target_regions.push(r);
            }
        },
        |_| true);

    // Check, based on the region associated with the trait, whether it can
    // possibly escape the enclosing fn item (note that all type parameters
    // must have been declared on the enclosing fn item).
    if target_regions.any(|r| is_re_scope(*r)) {
        return; /* case (1) */
    }

    // Assuming the trait instance can escape, then ensure that each parameter
    // either appears in the trait type or is owned.
    let target_params = ty::param_tys_in_type(target_ty);
    let source_ty = ty::expr_ty(cx.tcx, source);
    ty::walk_regions_and_ty(
        cx.tcx,
        source_ty,

        |_r| {
            // FIXME(#5723) --- turn this check on once &Objects are usable
            //
            // if !target_regions.any(|t_r| is_subregion_of(cx, *t_r, r)) {
            //     cx.tcx.sess.span_err(
            //         source.span,
            //         fmt!("source contains borrowed pointer with lifetime \
            //               not found in the target type `%s`",
            //              ty_to_str(cx.tcx, target_ty)));
            //     note_and_explain_region(
            //         cx.tcx, "source data is only valid for ", r, "");
            // }
        },

        |ty| {
            match ty::get(ty).sty {
                ty::ty_param(source_param) => {
                    if target_params.contains(&source_param) {
                        /* case (2) */
                    } else {
                        check_durable(cx.tcx, ty, source.span); /* case (3) */
                    }
                }
                _ => {}
            }
            true
        });

    fn is_re_scope(+r: ty::Region) -> bool {
        match r {
            ty::re_scope(*) => true,
            _ => false
        }
    }

    fn is_subregion_of(cx: Context, r_sub: ty::Region, r_sup: ty::Region) -> bool {
        cx.tcx.region_maps.is_subregion_of(r_sub, r_sup)
    }
}

/// Ensures that values placed into a ~Trait are copyable and sendable.
pub fn check_kind_bounds_of_cast(cx: Context, source: @expr, target: @expr) {
    let target_ty = ty::expr_ty(cx.tcx, target);
    match ty::get(target_ty).sty {
        ty::ty_trait(_, _, ty::UniqTraitStore) => {
            let source_ty = ty::expr_ty(cx.tcx, source);
            if !ty::type_is_owned(cx.tcx, source_ty) {
                cx.tcx.sess.span_err(
                    target.span,
                    ~"uniquely-owned trait objects must be sendable");
            }
        }
        _ => {} // Nothing to do.
    }
}

//
// Local Variables:
// mode: rust
// fill-column: 78;
// indent-tabs-mode: nil
// c-basic-offset: 4
// buffer-file-coding-system: utf-8-unix
// End:
//