/
coercion.rs
478 lines (429 loc) · 18.2 KB
/
coercion.rs
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// 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.
//! # Type Coercion
//!
//! Under certain circumstances we will coerce from one type to another,
//! for example by auto-borrowing. This occurs in situations where the
//! compiler has a firm 'expected type' that was supplied from the user,
//! and where the actual type is similar to that expected type in purpose
//! but not in representation (so actual subtyping is inappropriate).
//!
//! ## Reborrowing
//!
//! Note that if we are expecting a reference, we will *reborrow*
//! even if the argument provided was already a reference. This is
//! useful for freezing mut/const things (that is, when the expected is &T
//! but you have &const T or &mut T) and also for avoiding the linearity
//! of mut things (when the expected is &mut T and you have &mut T). See
//! the various `src/test/run-pass/coerce-reborrow-*.rs` tests for
//! examples of where this is useful.
//!
//! ## Subtle note
//!
//! When deciding what type coercions to consider, we do not attempt to
//! resolve any type variables we may encounter. This is because `b`
//! represents the expected type "as the user wrote it", meaning that if
//! the user defined a generic function like
//!
//! fn foo<A>(a: A, b: A) { ... }
//!
//! and then we wrote `foo(&1, @2)`, we will not auto-borrow
//! either argument. In older code we went to some lengths to
//! resolve the `b` variable, which could mean that we'd
//! auto-borrow later arguments but not earlier ones, which
//! seems very confusing.
//!
//! ## Subtler note
//!
//! However, right now, if the user manually specifies the
//! values for the type variables, as so:
//!
//! foo::<&int>(@1, @2)
//!
//! then we *will* auto-borrow, because we can't distinguish this from a
//! function that declared `&int`. This is inconsistent but it's easiest
//! at the moment. The right thing to do, I think, is to consider the
//! *unsubstituted* type when deciding whether to auto-borrow, but the
//! *substituted* type when considering the bounds and so forth. But most
//! of our methods don't give access to the unsubstituted type, and
//! rightly so because they'd be error-prone. So maybe the thing to do is
//! to actually determine the kind of coercions that should occur
//! separately and pass them in. Or maybe it's ok as is. Anyway, it's
//! sort of a minor point so I've opted to leave it for later---after all
//! we may want to adjust precisely when coercions occur.
use check::{autoderef, FnCtxt, LvaluePreference, UnresolvedTypeAction};
use middle::infer::{self, Coercion};
use middle::traits::{self, ObligationCause};
use middle::traits::{predicate_for_trait_def, report_selection_error};
use middle::ty::{AutoDerefRef, AdjustDerefRef};
use middle::ty::{self, mt, Ty};
use middle::ty_relate::RelateResult;
use util::common::indent;
use util::ppaux::Repr;
use std::cell::RefCell;
use std::collections::VecDeque;
use syntax::ast;
struct Coerce<'a, 'tcx: 'a> {
fcx: &'a FnCtxt<'a, 'tcx>,
origin: infer::TypeOrigin,
unsizing_obligations: RefCell<Vec<traits::PredicateObligation<'tcx>>>,
}
type CoerceResult<'tcx> = RelateResult<'tcx, Option<ty::AutoAdjustment<'tcx>>>;
impl<'f, 'tcx> Coerce<'f, 'tcx> {
fn tcx(&self) -> &ty::ctxt<'tcx> {
self.fcx.tcx()
}
fn subtype(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
try!(self.fcx.infcx().sub_types(false, self.origin.clone(), a, b));
Ok(None) // No coercion required.
}
fn unpack_actual_value<T, F>(&self, a: Ty<'tcx>, f: F) -> T where
F: FnOnce(Ty<'tcx>) -> T,
{
f(self.fcx.infcx().shallow_resolve(a))
}
fn coerce(&self,
expr_a: &ast::Expr,
a: Ty<'tcx>,
b: Ty<'tcx>)
-> CoerceResult<'tcx> {
debug!("Coerce.tys({} => {})",
a.repr(self.tcx()),
b.repr(self.tcx()));
// Consider coercing the subtype to a DST
let unsize = self.unpack_actual_value(a, |a| {
self.coerce_unsized(a, b)
});
if unsize.is_ok() {
return unsize;
}
// Examine the supertype and consider auto-borrowing.
//
// Note: does not attempt to resolve type variables we encounter.
// See above for details.
match b.sty {
ty::TyRawPtr(mt_b) => {
return self.unpack_actual_value(a, |a| {
self.coerce_unsafe_ptr(a, b, mt_b.mutbl)
});
}
ty::TyRef(_, mt_b) => {
return self.unpack_actual_value(a, |a| {
self.coerce_borrowed_pointer(expr_a, a, b, mt_b.mutbl)
});
}
_ => {}
}
self.unpack_actual_value(a, |a| {
match a.sty {
ty::TyBareFn(Some(_), a_f) => {
// Function items are coercible to any closure
// type; function pointers are not (that would
// require double indirection).
self.coerce_from_fn_item(a, a_f, b)
}
ty::TyBareFn(None, a_f) => {
// We permit coercion of fn pointers to drop the
// unsafe qualifier.
self.coerce_from_fn_pointer(a, a_f, b)
}
_ => {
// Otherwise, just use subtyping rules.
self.subtype(a, b)
}
}
})
}
/// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
/// To match `A` with `B`, autoderef will be performed,
/// calling `deref`/`deref_mut` where necessary.
fn coerce_borrowed_pointer(&self,
expr_a: &ast::Expr,
a: Ty<'tcx>,
b: Ty<'tcx>,
mutbl_b: ast::Mutability)
-> CoerceResult<'tcx> {
debug!("coerce_borrowed_pointer(a={}, b={})",
a.repr(self.tcx()),
b.repr(self.tcx()));
// If we have a parameter of type `&M T_a` and the value
// provided is `expr`, we will be adding an implicit borrow,
// meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
// to type check, we will construct the type that `&M*expr` would
// yield.
match a.sty {
ty::TyRef(_, mt_a) => {
try!(coerce_mutbls(mt_a.mutbl, mutbl_b));
}
_ => return self.subtype(a, b)
}
let coercion = Coercion(self.origin.span());
let r_borrow = self.fcx.infcx().next_region_var(coercion);
let r_borrow = self.tcx().mk_region(r_borrow);
let autoref = Some(ty::AutoPtr(r_borrow, mutbl_b));
let lvalue_pref = LvaluePreference::from_mutbl(mutbl_b);
let mut first_error = None;
let (_, autoderefs, success) = autoderef(self.fcx,
expr_a.span,
a,
Some(expr_a),
UnresolvedTypeAction::Ignore,
lvalue_pref,
|inner_ty, autoderef| {
if autoderef == 0 {
// Don't let this pass, otherwise it would cause
// &T to autoref to &&T.
return None;
}
let ty = ty::mk_rptr(self.tcx(), r_borrow,
mt {ty: inner_ty, mutbl: mutbl_b});
if let Err(err) = self.subtype(ty, b) {
if first_error.is_none() {
first_error = Some(err);
}
None
} else {
Some(())
}
});
match success {
Some(_) => {
Ok(Some(AdjustDerefRef(AutoDerefRef {
autoderefs: autoderefs,
autoref: autoref,
unsize: None
})))
}
None => {
// Return original error as if overloaded deref was never
// attempted, to avoid irrelevant/confusing error messages.
Err(first_error.expect("coerce_borrowed_pointer failed with no error?"))
}
}
}
// &[T; n] or &mut [T; n] -> &[T]
// or &mut [T; n] -> &mut [T]
// or &Concrete -> &Trait, etc.
fn coerce_unsized(&self,
source: Ty<'tcx>,
target: Ty<'tcx>)
-> CoerceResult<'tcx> {
debug!("coerce_unsized(source={}, target={})",
source.repr(self.tcx()),
target.repr(self.tcx()));
let traits = (self.tcx().lang_items.unsize_trait(),
self.tcx().lang_items.coerce_unsized_trait());
let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
(u, cu)
} else {
debug!("Missing Unsize or CoerceUnsized traits");
return Err(ty::terr_mismatch);
};
// Note, we want to avoid unnecessary unsizing. We don't want to coerce to
// a DST unless we have to. This currently comes out in the wash since
// we can't unify [T] with U. But to properly support DST, we need to allow
// that, at which point we will need extra checks on the target here.
// Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
let (source, reborrow) = match (&source.sty, &target.sty) {
(&ty::TyRef(_, mt_a), &ty::TyRef(_, mt_b)) => {
try!(coerce_mutbls(mt_a.mutbl, mt_b.mutbl));
let coercion = Coercion(self.origin.span());
let r_borrow = self.fcx.infcx().next_region_var(coercion);
let region = self.tcx().mk_region(r_borrow);
(mt_a.ty, Some(ty::AutoPtr(region, mt_b.mutbl)))
}
(&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) => {
try!(coerce_mutbls(mt_a.mutbl, mt_b.mutbl));
(mt_a.ty, Some(ty::AutoUnsafe(mt_b.mutbl)))
}
_ => (source, None)
};
let source = ty::adjust_ty_for_autoref(self.tcx(), source, reborrow);
let mut selcx = traits::SelectionContext::new(self.fcx.infcx(), self.fcx);
// Use a FIFO queue for this custom fulfillment procedure.
let mut queue = VecDeque::new();
let mut leftover_predicates = vec![];
// Create an obligation for `Source: CoerceUnsized<Target>`.
let cause = ObligationCause::misc(self.origin.span(), self.fcx.body_id);
queue.push_back(predicate_for_trait_def(self.tcx(),
cause,
coerce_unsized_did,
0,
source,
vec![target]));
// Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
// emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
// inference might unify those two inner type variables later.
let traits = [coerce_unsized_did, unsize_did];
while let Some(obligation) = queue.pop_front() {
debug!("coerce_unsized resolve step: {}", obligation.repr(self.tcx()));
let trait_ref = match obligation.predicate {
ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
tr.clone()
}
_ => {
leftover_predicates.push(obligation);
continue;
}
};
match selcx.select(&obligation.with(trait_ref)) {
// Uncertain or unimplemented.
Ok(None) | Err(traits::Unimplemented) => {
debug!("coerce_unsized: early return - can't prove obligation");
return Err(ty::terr_mismatch);
}
// Object safety violations or miscellaneous.
Err(err) => {
report_selection_error(self.fcx.infcx(), &obligation, &err);
// Treat this like an obligation and follow through
// with the unsizing - the lack of a coercion should
// be silent, as it causes a type mismatch later.
}
Ok(Some(vtable)) => {
for obligation in vtable.nested_obligations() {
queue.push_back(obligation);
}
}
}
}
let mut obligations = self.unsizing_obligations.borrow_mut();
assert!(obligations.is_empty());
*obligations = leftover_predicates;
let adjustment = AutoDerefRef {
autoderefs: if reborrow.is_some() { 1 } else { 0 },
autoref: reborrow,
unsize: Some(target)
};
debug!("Success, coerced with {}", adjustment.repr(self.tcx()));
Ok(Some(AdjustDerefRef(adjustment)))
}
fn coerce_from_fn_pointer(&self,
a: Ty<'tcx>,
fn_ty_a: &'tcx ty::BareFnTy<'tcx>,
b: Ty<'tcx>)
-> CoerceResult<'tcx>
{
/*!
* Attempts to coerce from the type of a Rust function item
* into a closure or a `proc`.
*/
self.unpack_actual_value(b, |b| {
debug!("coerce_from_fn_pointer(a={}, b={})",
a.repr(self.tcx()), b.repr(self.tcx()));
if let ty::TyBareFn(None, fn_ty_b) = b.sty {
match (fn_ty_a.unsafety, fn_ty_b.unsafety) {
(ast::Unsafety::Normal, ast::Unsafety::Unsafe) => {
let unsafe_a = self.tcx().safe_to_unsafe_fn_ty(fn_ty_a);
try!(self.subtype(unsafe_a, b));
return Ok(Some(ty::AdjustUnsafeFnPointer));
}
_ => {}
}
}
self.subtype(a, b)
})
}
fn coerce_from_fn_item(&self,
a: Ty<'tcx>,
fn_ty_a: &'tcx ty::BareFnTy<'tcx>,
b: Ty<'tcx>)
-> CoerceResult<'tcx> {
/*!
* Attempts to coerce from the type of a Rust function item
* into a closure or a `proc`.
*/
self.unpack_actual_value(b, |b| {
debug!("coerce_from_fn_item(a={}, b={})",
a.repr(self.tcx()), b.repr(self.tcx()));
match b.sty {
ty::TyBareFn(None, _) => {
let a_fn_pointer = ty::mk_bare_fn(self.tcx(), None, fn_ty_a);
try!(self.subtype(a_fn_pointer, b));
Ok(Some(ty::AdjustReifyFnPointer))
}
_ => self.subtype(a, b)
}
})
}
fn coerce_unsafe_ptr(&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
mutbl_b: ast::Mutability)
-> CoerceResult<'tcx> {
debug!("coerce_unsafe_ptr(a={}, b={})",
a.repr(self.tcx()),
b.repr(self.tcx()));
let (is_ref, mt_a) = match a.sty {
ty::TyRef(_, mt) => (true, mt),
ty::TyRawPtr(mt) => (false, mt),
_ => {
return self.subtype(a, b);
}
};
// Check that the types which they point at are compatible.
let a_unsafe = ty::mk_ptr(self.tcx(), ty::mt{ mutbl: mutbl_b, ty: mt_a.ty });
try!(self.subtype(a_unsafe, b));
try!(coerce_mutbls(mt_a.mutbl, mutbl_b));
// Although references and unsafe ptrs have the same
// representation, we still register an AutoDerefRef so that
// regionck knows that the region for `a` must be valid here.
if is_ref {
Ok(Some(AdjustDerefRef(AutoDerefRef {
autoderefs: 1,
autoref: Some(ty::AutoUnsafe(mutbl_b)),
unsize: None
})))
} else {
Ok(None)
}
}
}
pub fn mk_assignty<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
expr: &ast::Expr,
a: Ty<'tcx>,
b: Ty<'tcx>)
-> RelateResult<'tcx, ()> {
debug!("mk_assignty({} -> {})", a.repr(fcx.tcx()), b.repr(fcx.tcx()));
let mut unsizing_obligations = vec![];
let adjustment = try!(indent(|| {
fcx.infcx().commit_if_ok(|_| {
let coerce = Coerce {
fcx: fcx,
origin: infer::ExprAssignable(expr.span),
unsizing_obligations: RefCell::new(vec![])
};
let adjustment = try!(coerce.coerce(expr, a, b));
unsizing_obligations = coerce.unsizing_obligations.into_inner();
Ok(adjustment)
})
}));
if let Some(AdjustDerefRef(auto)) = adjustment {
if auto.unsize.is_some() {
for obligation in unsizing_obligations {
fcx.register_predicate(obligation);
}
}
}
if let Some(adjustment) = adjustment {
debug!("Success, coerced with {}", adjustment.repr(fcx.tcx()));
fcx.write_adjustment(expr.id, adjustment);
}
Ok(())
}
fn coerce_mutbls<'tcx>(from_mutbl: ast::Mutability,
to_mutbl: ast::Mutability)
-> CoerceResult<'tcx> {
match (from_mutbl, to_mutbl) {
(ast::MutMutable, ast::MutMutable) |
(ast::MutImmutable, ast::MutImmutable) |
(ast::MutMutable, ast::MutImmutable) => Ok(None),
(ast::MutImmutable, ast::MutMutable) => Err(ty::terr_mutability)
}
}