rust/src/rustc/middle/region.rs

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/*!
This file actually contains two passes related to regions. The first
pass builds up the `region_map`, which describes the parent links in
the region hierarchy. The second pass infers which types must be
region parameterized.
*/
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import driver::session::session;
import middle::ty;
import syntax::{ast, visit};
import syntax::codemap::span;
import syntax::print::pprust;
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import syntax::ast_util::new_def_hash;
import syntax::ast_map;
import dvec::{dvec, extensions};
import metadata::csearch;
import std::list;
import std::list::list;
import std::map::{hashmap, int_hash};
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type parent = option<ast::node_id>;
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/* Records the parameter ID of a region name. */
type binding = {node_id: ast::node_id,
name: ~str,
br: ty::bound_region};
/// Mapping from a block/expr/binding to the innermost scope that
/// bounds its lifetime. For a block/expression, this is the lifetime
/// in which it will be evaluated. For a binding, this is the lifetime
/// in which is in scope.
type region_map = hashmap<ast::node_id, ast::node_id>;
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type ctxt = {
sess: session,
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def_map: resolve3::DefMap,
region_map: region_map,
// Generally speaking, expressions are parented to their innermost
// enclosing block. But some kinds of expressions serve as
// parents: calls, methods, etc. In addition, some expressions
// serve as parents by virtue of where they appear. For example,
// the condition in a while loop is always a parent. In those
// cases, we add the node id of such an expression to this set so
// that when we visit it we can view it as a parent.
root_exprs: hashmap<ast::node_id, ()>,
// The parent scope is the innermost block, call, or alt
// expression during the execution of which the current expression
// will be evaluated. Generally speaking, the innermost parent
// scope is also the closest suitable ancestor in the AST tree.
//
// There is a subtle point concerning call arguments. Imagine
// you have a call:
//
// { // block a
// foo( // call b
// x,
// y);
// }
//
// In what lifetime are the expressions `x` and `y` evaluated? At
// first, I imagine the answer was the block `a`, as the arguments
// are evaluated before the call takes place. But this turns out
// to be wrong. The lifetime of the call must encompass the
// argument evaluation as well.
//
// The reason is that evaluation of an earlier argument could
// create a borrow which exists during the evaluation of later
// arguments. Consider this torture test, for example,
//
// fn test1(x: @mut ~int) {
// foo(&**x, *x = ~5);
// }
//
// Here, the first argument `&**x` will be a borrow of the `~int`,
// but the second argument overwrites that very value! Bad.
// (This test is borrowck-pure-scope-in-call.rs, btw)
parent: parent
};
/// Returns true if `subscope` is equal to or is lexically nested inside
/// `superscope` and false otherwise.
fn scope_contains(region_map: region_map, superscope: ast::node_id,
subscope: ast::node_id) -> bool {
let mut subscope = subscope;
while superscope != subscope {
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match region_map.find(subscope) {
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none => return false,
some(scope) => subscope = scope
}
}
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return true;
}
/// Determines whether one region is a subregion of another. This is
/// intended to run *after inference* and sadly the logic is somewhat
/// duplicated with the code in infer.rs.
fn is_subregion_of(region_map: region_map,
sub_region: ty::region,
super_region: ty::region) -> bool {
sub_region == super_region ||
match (sub_region, super_region) {
(_, ty::re_static) => {
true
}
(ty::re_scope(sub_scope), ty::re_scope(super_scope)) |
(ty::re_scope(sub_scope), ty::re_free(super_scope, _)) => {
scope_contains(region_map, super_scope, sub_scope)
}
_ => {
false
}
}
}
/// Finds the nearest common ancestor (if any) of two scopes. That
/// is, finds the smallest scope which is greater than or equal to
/// both `scope_a` and `scope_b`.
fn nearest_common_ancestor(region_map: region_map, scope_a: ast::node_id,
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scope_b: ast::node_id) -> option<ast::node_id> {
fn ancestors_of(region_map: region_map, scope: ast::node_id)
-> ~[ast::node_id] {
let mut result = ~[scope];
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let mut scope = scope;
loop {
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match region_map.find(scope) {
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none => return result,
some(superscope) => {
vec::push(result, superscope);
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scope = superscope;
}
}
}
}
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if scope_a == scope_b { return some(scope_a); }
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let a_ancestors = ancestors_of(region_map, scope_a);
let b_ancestors = ancestors_of(region_map, scope_b);
let mut a_index = vec::len(a_ancestors) - 1u;
let mut b_index = vec::len(b_ancestors) - 1u;
// Here, ~[ab]_ancestors is a vector going from narrow to broad.
// The end of each vector will be the item where the scope is
// defined; if there are any common ancestors, then the tails of
// the vector will be the same. So basically we want to walk
// backwards from the tail of each vector and find the first point
// where they diverge. If one vector is a suffix of the other,
// then the corresponding scope is a superscope of the other.
if a_ancestors[a_index] != b_ancestors[b_index] {
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return none;
}
loop {
// Loop invariant: a_ancestors[a_index] == b_ancestors[b_index]
// for all indices between a_index and the end of the array
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if a_index == 0u { return some(scope_a); }
if b_index == 0u { return some(scope_b); }
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a_index -= 1u;
b_index -= 1u;
if a_ancestors[a_index] != b_ancestors[b_index] {
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return some(a_ancestors[a_index + 1u]);
}
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}
}
/// Extracts that current parent from cx, failing if there is none.
fn parent_id(cx: ctxt, span: span) -> ast::node_id {
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match cx.parent {
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none => {
cx.sess.span_bug(span, ~"crate should not be parent here");
}
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some(parent_id) => {
parent_id
}
}
}
/// Records the current parent (if any) as the parent of `child_id`.
fn record_parent(cx: ctxt, child_id: ast::node_id) {
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match cx.parent {
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none => { /* no-op */ }
some(parent_id) => {
debug!{"parent of node %d is node %d", child_id, parent_id};
cx.region_map.insert(child_id, parent_id);
}
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}
}
fn resolve_block(blk: ast::blk, cx: ctxt, visitor: visit::vt<ctxt>) {
// Record the parent of this block.
record_parent(cx, blk.node.id);
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// Descend.
let new_cx: ctxt = {parent: some(blk.node.id) with cx};
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visit::visit_block(blk, new_cx, visitor);
}
fn resolve_arm(arm: ast::arm, cx: ctxt, visitor: visit::vt<ctxt>) {
visit::visit_arm(arm, cx, visitor);
}
fn resolve_pat(pat: @ast::pat, cx: ctxt, visitor: visit::vt<ctxt>) {
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match pat.node {
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ast::pat_ident(_, path, _) => {
let defn_opt = cx.def_map.find(pat.id);
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match defn_opt {
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some(ast::def_variant(_,_)) => {
/* Nothing to do; this names a variant. */
}
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_ => {
/* This names a local. Bind it to the containing scope. */
record_parent(cx, pat.id);
}
}
}
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_ => { /* no-op */ }
}
visit::visit_pat(pat, cx, visitor);
}
fn resolve_expr(expr: @ast::expr, cx: ctxt, visitor: visit::vt<ctxt>) {
record_parent(cx, expr.id);
let mut new_cx = cx;
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match expr.node {
ast::expr_call(*) => {
debug!{"node %d: %s", expr.id, pprust::expr_to_str(expr)};
new_cx.parent = some(expr.id);
}
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ast::expr_match(subexpr, _, _) => {
debug!{"node %d: %s", expr.id, pprust::expr_to_str(expr)};
new_cx.parent = some(expr.id);
}
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ast::expr_fn(_, _, _, cap_clause) |
ast::expr_fn_block(_, _, cap_clause) => {
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// although the capture items are not expressions per se, they
// do get "evaluated" in some sense as copies or moves of the
// relevant variables so we parent them like an expression
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for (*cap_clause).each |cap_item| {
record_parent(new_cx, cap_item.id);
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}
}
ast::expr_while(cond, _) => {
new_cx.root_exprs.insert(cond.id, ());
}
_ => {}
};
if new_cx.root_exprs.contains_key(expr.id) {
new_cx.parent = some(expr.id);
}
visit::visit_expr(expr, new_cx, visitor);
}
fn resolve_local(local: @ast::local, cx: ctxt, visitor: visit::vt<ctxt>) {
record_parent(cx, local.node.id);
visit::visit_local(local, cx, visitor);
}
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fn resolve_item(item: @ast::item, cx: ctxt, visitor: visit::vt<ctxt>) {
// Items create a new outer block scope as far as we're concerned.
let new_cx: ctxt = {parent: none with cx};
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visit::visit_item(item, new_cx, visitor);
}
fn resolve_fn(fk: visit::fn_kind, decl: ast::fn_decl, body: ast::blk,
sp: span, id: ast::node_id, cx: ctxt,
visitor: visit::vt<ctxt>) {
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let fn_cx = match fk {
visit::fk_item_fn(*) | visit::fk_method(*) |
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visit::fk_ctor(*) | visit::fk_dtor(*) => {
// Top-level functions are a root scope.
{parent: some(id) with cx}
}
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visit::fk_anon(*) | visit::fk_fn_block(*) => {
// Closures continue with the inherited scope.
cx
}
};
debug!{"visiting fn with body %d. cx.parent: %? \
fn_cx.parent: %?",
body.node.id, cx.parent, fn_cx.parent};
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for decl.inputs.each |input| {
cx.region_map.insert(input.id, body.node.id);
}
visit::visit_fn(fk, decl, body, sp, id, fn_cx, visitor);
}
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fn resolve_crate(sess: session, def_map: resolve3::DefMap,
crate: @ast::crate) -> region_map {
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let cx: ctxt = {sess: sess,
def_map: def_map,
region_map: int_hash(),
root_exprs: int_hash(),
parent: none};
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let visitor = visit::mk_vt(@{
visit_block: resolve_block,
visit_item: resolve_item,
visit_fn: resolve_fn,
visit_arm: resolve_arm,
visit_pat: resolve_pat,
visit_expr: resolve_expr,
visit_local: resolve_local
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with *visit::default_visitor()
});
visit::visit_crate(*crate, cx, visitor);
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return cx.region_map;
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}
// ___________________________________________________________________________
// Determining region parameterization
//
// Infers which type defns must be region parameterized---this is done
// by scanning their contents to see whether they reference a region
// type, directly or indirectly. This is a fixed-point computation.
//
// We do it in two passes. First we walk the AST and construct a map
// from each type defn T1 to other defns which make use of it. For example,
// if we have a type like:
//
// type S = *int;
// type T = S;
//
// Then there would be a map entry from S to T. During the same walk,
// we also construct add any types that reference regions to a set and
// a worklist. We can then process the worklist, propagating indirect
// dependencies until a fixed point is reached.
type region_paramd_items = hashmap<ast::node_id, ()>;
type dep_map = hashmap<ast::node_id, @dvec<ast::node_id>>;
type determine_rp_ctxt_ = {
sess: session,
ast_map: ast_map::map,
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def_map: resolve3::DefMap,
region_paramd_items: region_paramd_items,
dep_map: dep_map,
worklist: dvec<ast::node_id>,
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// the innermost enclosing item id
mut item_id: ast::node_id,
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// true when we are within an item but not within a method.
// see long discussion on region_is_relevant()
mut anon_implies_rp: bool
};
enum determine_rp_ctxt {
determine_rp_ctxt_(@determine_rp_ctxt_)
}
impl methods for determine_rp_ctxt {
fn add_rp(id: ast::node_id) {
assert id != 0;
if self.region_paramd_items.insert(id, ()) {
debug!{"add region-parameterized item: %d (%s)",
id, ast_map::node_id_to_str(self.ast_map, id)};
self.worklist.push(id);
} else {
debug!{"item %d already region-parameterized", id};
}
}
fn add_dep(from: ast::node_id, to: ast::node_id) {
debug!{"add dependency from %d -> %d (%s -> %s)",
from, to,
ast_map::node_id_to_str(self.ast_map, from),
ast_map::node_id_to_str(self.ast_map, to)};
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let vec = match self.dep_map.find(from) {
some(vec) => {vec}
none => {
let vec = @dvec();
self.dep_map.insert(from, vec);
vec
}
};
if !vec.contains(to) { vec.push(to); }
}
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// Determines whether a reference to a region that appears in the
// AST implies that the enclosing type is region-parameterized.
//
// This point is subtle. Here are four examples to make it more
// concrete.
//
// 1. impl foo for &int { ... }
// 2. impl foo for &self/int { ... }
// 3. impl foo for bar { fn m() -> &self/int { ... } }
// 4. impl foo for bar { fn m() -> &int { ... } }
//
// In case 1, the anonymous region is being referenced,
// but it appears in a context where the anonymous region
// resolves to self, so the impl foo is region-parameterized.
//
// In case 2, the self parameter is written explicitly.
//
// In case 3, the method refers to self, so that implies that the
// impl must be region parameterized. (If the type bar is not
// region parameterized, that is an error, because the self region
// is effectively unconstrained, but that is detected elsewhere).
//
// In case 4, the anonymous region is referenced, but it
// bound by the method, so it does not refer to self. This impl
// need not be region parameterized.
//
// So the rules basically are: the `self` region always implies
// that the enclosing type is region parameterized. The anonymous
// region also does, unless it appears within a method, in which
// case it is bound. We handle this by setting a flag
// (anon_implies_rp) to true when we enter an item and setting
// that flag to false when we enter a method.
fn region_is_relevant(r: @ast::region) -> bool {
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match r.node {
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ast::re_anon => self.anon_implies_rp,
ast::re_named(@~"self") => true,
ast::re_named(_) => false
}
}
fn with(item_id: ast::node_id, anon_implies_rp: bool, f: fn()) {
let old_item_id = self.item_id;
let old_anon_implies_rp = self.anon_implies_rp;
self.item_id = item_id;
self.anon_implies_rp = anon_implies_rp;
debug!{"with_item_id(%d, %b)", item_id, anon_implies_rp};
let _i = util::common::indenter();
f();
self.item_id = old_item_id;
self.anon_implies_rp = old_anon_implies_rp;
}
}
fn determine_rp_in_item(item: @ast::item,
&&cx: determine_rp_ctxt,
visitor: visit::vt<determine_rp_ctxt>) {
do cx.with(item.id, true) {
visit::visit_item(item, cx, visitor);
}
}
fn determine_rp_in_fn(fk: visit::fn_kind,
decl: ast::fn_decl,
body: ast::blk,
sp: span,
id: ast::node_id,
&&cx: determine_rp_ctxt,
visitor: visit::vt<determine_rp_ctxt>) {
do cx.with(cx.item_id, false) {
visit::visit_fn(fk, decl, body, sp, id, cx, visitor);
}
}
fn determine_rp_in_ty_method(ty_m: ast::ty_method,
&&cx: determine_rp_ctxt,
visitor: visit::vt<determine_rp_ctxt>) {
do cx.with(cx.item_id, false) {
visit::visit_ty_method(ty_m, cx, visitor);
}
}
fn determine_rp_in_ty(ty: @ast::ty,
&&cx: determine_rp_ctxt,
visitor: visit::vt<determine_rp_ctxt>) {
// we are only interesting in types that will require an item to
// be region-parameterized. if cx.item_id is zero, then this type
// is not a member of a type defn nor is it a constitutent of an
// impl etc. So we can ignore it and its components.
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if cx.item_id == 0 { return; }
// if this type directly references a region, either via a
// region pointer like &r.ty or a region-parameterized path
// like path/r, add to the worklist/set
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match ty.node {
ast::ty_rptr(r, _) |
ast::ty_path(@{rp: some(r), _}, _) => {
debug!{"referenced type with regions %s", pprust::ty_to_str(ty)};
if cx.region_is_relevant(r) {
cx.add_rp(cx.item_id);
}
}
_ => {}
}
// if this references another named type, add the dependency
// to the dep_map. If the type is not defined in this crate,
// then check whether it is region-parameterized and consider
// that as a direct dependency.
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match ty.node {
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ast::ty_path(_, id) => {
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match cx.def_map.get(id) {
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ast::def_ty(did) | ast::def_class(did, _) => {
if did.crate == ast::local_crate {
cx.add_dep(did.node, cx.item_id);
} else {
let cstore = cx.sess.cstore;
if csearch::get_region_param(cstore, did) {
debug!{"reference to external, rp'd type %s",
pprust::ty_to_str(ty)};
cx.add_rp(cx.item_id);
}
}
}
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_ => {}
}
}
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_ => {}
}
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match ty.node {
ast::ty_fn(*) => {
do cx.with(cx.item_id, false) {
visit::visit_ty(ty, cx, visitor);
}
}
_ => {
visit::visit_ty(ty, cx, visitor);
}
}
}
fn determine_rp_in_crate(sess: session,
ast_map: ast_map::map,
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def_map: resolve3::DefMap,
crate: @ast::crate) -> region_paramd_items {
let cx = determine_rp_ctxt_(@{sess: sess,
ast_map: ast_map,
def_map: def_map,
region_paramd_items: int_hash(),
dep_map: int_hash(),
worklist: dvec(),
mut item_id: 0,
mut anon_implies_rp: false});
// gather up the base set, worklist and dep_map:
let visitor = visit::mk_vt(@{
visit_fn: determine_rp_in_fn,
visit_item: determine_rp_in_item,
visit_ty: determine_rp_in_ty,
visit_ty_method: determine_rp_in_ty_method,
with *visit::default_visitor()
});
visit::visit_crate(*crate, cx, visitor);
// propagate indirect dependencies
while cx.worklist.len() != 0 {
let id = cx.worklist.pop();
debug!{"popped %d from worklist", id};
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match cx.dep_map.find(id) {
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none => {}
some(vec) => {
for vec.each |to_id| {
cx.add_rp(to_id);
}
}
}
}
// return final set
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return cx.region_paramd_items;
}