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Implement the occurs check 

In the writeback phase, the typechecker now checks that it isn't
replacing a type variable T with a type that contains T. It
also does an occurs check in do_autoderef in order to avoid
getting into an infinite chain of derefs.

I'm a bit worried that there are more places where the occurs
check needs to happen where I'm not doing it now, though.

Closes #768
This commit is contained in:
Tim Chevalier 2011-08-03 18:06:57 -07:00
parent d7d4b4fc38
commit 2baaeab784
4 changed files with 87 additions and 14 deletions
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72
src/comp/middle/ty.rs
View File

@ -184,6 +184,7 @@ export type_param;
export unify;
export variant_info;
export walk_ty;
export occurs_check_fails;
// Data types
tag mode { mo_val; mo_alias(bool); }
@ -655,6 +656,7 @@ fn walk_ty(cx: &ctxt, walker: ty_walk, ty: t) {
ty_str. {/* no-op */ }
ty_istr. {/* no-op */ }
ty_type. {/* no-op */ }
ty_task. {/* no-op */ }
ty_native(_) {/* no-op */ }
ty_box(tm) { walk_ty(cx, walker, tm.ty); }
ty_vec(tm) { walk_ty(cx, walker, tm.ty); }
@ -686,6 +688,9 @@ fn walk_ty(cx: &ctxt, walker: ty_walk, ty: t) {
walk_ty(cx, walker, sub);
for tp: t in tps { walk_ty(cx, walker, tp); }
}
ty_constr(sub, _) {
walk_ty(cx, walker, sub);
}
ty_var(_) {/* no-op */ }
ty_param(_,_) {/* no-op */ }
}
@ -1393,6 +1398,24 @@ fn type_param(cx: &ctxt, ty: &t) -> option::t[uint] {
ret none;
}
// Returns an ivec of all the type variables
// occurring in t. It may contain duplicates.
fn vars_in_type(cx:&ctxt, ty: &t) -> int[] {
fn collect_var(cx:&ctxt, vars: &@mutable int[], ty: t) {
alt struct(cx, ty) {
ty_var(v) {
*vars += ~[v];
}
_ {}
}
}
let rslt: @mutable int[] = @mutable (~[]);
walk_ty(cx, bind collect_var(cx, rslt, _), ty);
// Works because of a "convenient" bug that lets us
// return a mutable ivec as if it's immutable
ret *rslt;
}
fn type_autoderef(cx: &ctxt, t: &ty::t) -> ty::t {
let t1: ty::t = t;
while true {
@ -1990,6 +2013,28 @@ fn is_lval(expr: &@ast::expr) -> bool {
}
}
fn occurs_check_fails(tcx: &ctxt, sp: &option::t[span], vid: int, rt: &t)
-> bool {
// Occurs check!
if ivec::member(vid, vars_in_type(tcx, rt)) {
alt sp {
some (s) {
// Maybe this should be span_err -- however, there's an
// assertion later on that the type doesn't contain
// variables, so in this case we have to be sure to die.
tcx.sess.span_fatal(s,
"Type inference failed because I \
could not find a type\n that's both of the form " +
ty_to_str(tcx, ty::mk_var(tcx, (vid)))
+ " and of the form " + ty_to_str(tcx, rt)
+ ". Such a type would have to be infinitely \
large.");
}
_ { ret true; }
}
}
else { ret false; }
}
// Type unification via Robinson's algorithm (Robinson 1965). Implemented as
// described in Hoder and Voronkov:
@ -2318,9 +2363,6 @@ mod unify {
// TODO: rewrite this using tuple pattern matching when available, to
// avoid all this rightward drift and spikiness.
// TODO: occurs check, to make sure we don't loop forever when
// unifying e.g. 'a and option['a]
// Fast path.
if eq_ty(expected, actual) { ret ures_ok(expected); }
@ -2694,9 +2736,15 @@ mod unify {
}
// Fixups and substitutions
fn fixup_vars(tcx: ty_ctxt, vb: @var_bindings, typ: t) -> fixup_result {
fn subst_vars(tcx: ty_ctxt, vb: @var_bindings,
// Takes an optional span - complain about occurs check violations
// iff the span is present (so that if we already know we're going
// to error anyway, we don't complain)
fn fixup_vars(tcx: ty_ctxt, sp: &option::t[span],
vb: @var_bindings, typ: t) -> fixup_result {
fn subst_vars(tcx: ty_ctxt, sp: &option::t[span], vb: @var_bindings,
unresolved: @mutable option::t[int], vid: int) -> t {
// Should really return a fixup_result instead of a t, but fold_ty
// doesn't allow returning anything but a t.
if vid as uint >= ufindivec::set_count(vb.sets) {
*unresolved = some(vid);
ret ty::mk_var(tcx, vid);
@ -2705,15 +2753,18 @@ mod unify {
alt smallintmap::find[t](vb.types, root_id) {
none. { *unresolved = some(vid); ret ty::mk_var(tcx, vid); }
some(rt) {
if occurs_check_fails(tcx, sp, vid, rt) {
// Return the type unchanged, so we can error out downstream
ret rt;
}
ret fold_ty(tcx,
fm_var(bind subst_vars(tcx, vb, unresolved, _)),
rt);
fm_var(bind subst_vars(tcx, sp, vb, unresolved, _)), rt);
}
}
}
let unresolved = @mutable none[int];
let rty =
fold_ty(tcx, fm_var(bind subst_vars(tcx, vb, unresolved, _)),
fold_ty(tcx, fm_var(bind subst_vars(tcx, sp, vb, unresolved, _)),
typ);
let ur = *unresolved;
alt ur {
@ -2721,13 +2772,14 @@ mod unify {
some(var_id) { ret fix_err(var_id); }
}
}
fn resolve_type_var(tcx: &ty_ctxt, vb: &@var_bindings, vid: int) ->
fn resolve_type_var(tcx: &ty_ctxt, sp: &option::t[span],
vb: &@var_bindings, vid: int) ->
fixup_result {
if vid as uint >= ufindivec::set_count(vb.sets) { ret fix_err(vid); }
let root_id = ufindivec::find(vb.sets, vid as uint);
alt smallintmap::find[t](vb.types, root_id) {
none. { ret fix_err(vid); }
some(rt) { ret fixup_vars(tcx, vb, rt); }
some(rt) { ret fixup_vars(tcx, sp, vb, rt); }
}
}
}

20
src/comp/middle/typeck.rs
View File

@ -879,7 +879,18 @@ fn do_autoderef(fcx: &@fn_ctxt, sp: &span, t: &ty::t) -> ty::t {
let t1 = t;
while true {
alt structure_of(fcx, sp, t1) {
ty::ty_box(inner) { t1 = inner.ty; }
ty::ty_box(inner) {
alt ty::struct(fcx.ccx.tcx, t1) {
ty::ty_var(v1) {
if ty::occurs_check_fails(fcx.ccx.tcx, some(sp), v1,
ty::mk_box(fcx.ccx.tcx, inner)) {
break;
}
}
_ {}
}
t1 = inner.ty;
}
ty::ty_res(_, inner, tps) {
t1 = ty::substitute_type_params(fcx.ccx.tcx, tps, inner);
}
@ -942,7 +953,7 @@ fn do_fn_block_coerce(fcx: &@fn_ctxt, sp: &span, actual: &ty::t,
fn resolve_type_vars_if_possible(fcx: &@fn_ctxt, typ: ty::t) -> ty::t {
alt ty::unify::fixup_vars(fcx.ccx.tcx, fcx.var_bindings, typ) {
alt ty::unify::fixup_vars(fcx.ccx.tcx, none, fcx.var_bindings, typ) {
fix_ok(new_type) { ret new_type; }
fix_err(_) { ret typ; }
}
@ -1073,7 +1084,8 @@ mod writeback {
fn resolve_type_vars_in_type(fcx: &@fn_ctxt, sp: &span, typ: ty::t) ->
option::t[ty::t] {
if !ty::type_contains_vars(fcx.ccx.tcx, typ) { ret some(typ); }
alt ty::unify::fixup_vars(fcx.ccx.tcx, fcx.var_bindings, typ) {
alt ty::unify::fixup_vars(fcx.ccx.tcx, some(sp),
fcx.var_bindings, typ) {
fix_ok(new_type) { ret some(new_type); }
fix_err(vid) {
fcx.ccx.tcx.sess.span_err(sp,
@ -1139,7 +1151,7 @@ mod writeback {
if !wbcx.success { ret; }
let var_id = lookup_local(wbcx.fcx, l.span, l.node.id);
let fix_rslt =
ty::unify::resolve_type_var(wbcx.fcx.ccx.tcx,
ty::unify::resolve_type_var(wbcx.fcx.ccx.tcx, some(l.span),
wbcx.fcx.var_bindings, var_id);
alt fix_rslt {
fix_ok(lty) { write::ty_only(wbcx.fcx.ccx.tcx, l.node.id, lty); }

5
src/test/compile-fail/occurs-check-2.rs Normal file
View File

@ -0,0 +1,5 @@
// error-pattern: Type inference failed because I could not find
fn main() {
let f = @f;
f();
}

4
src/test/compile-fail/occurs-check.rs Normal file
View File

@ -0,0 +1,4 @@
// error-pattern: Type inference failed because I could not find
fn main() {
let f = @f;
}