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//! The region check is a final pass that runs over the AST after we have
//! inferred the type constraints but before we have actually finalized
//! the types. Its purpose is to embed a variety of region constraints.
//! Inserting these constraints as a separate pass is good because (1) it
//! localizes the code that has to do with region inference and (2) often
//! we cannot know what constraints are needed until the basic types have
//! been inferred.
//!
//! ### Interaction with the borrow checker
//!
//! In general, the job of the borrowck module (which runs later) is to
//! check that all soundness criteria are met, given a particular set of
//! regions. The job of *this* module is to anticipate the needs of the
//! borrow checker and infer regions that will satisfy its requirements.
//! It is generally true that the inference doesn't need to be sound,
//! meaning that if there is a bug and we inferred bad regions, the borrow
//! checker should catch it. This is not entirely true though; for
//! example, the borrow checker doesn't check subtyping, and it doesn't
//! check that region pointers are always live when they are used. It
//! might be worthwhile to fix this so that borrowck serves as a kind of
//! verification step -- that would add confidence in the overall
//! correctness of the compiler, at the cost of duplicating some type
//! checks and effort.
//!
//! ### Inferring the duration of borrows, automatic and otherwise
//!
//! Whenever we introduce a borrowed pointer, for example as the result of
//! a borrow expression `let x = &data`, the lifetime of the pointer `x`
//! is always specified as a region inference variable. `regionck` has the
//! job of adding constraints such that this inference variable is as
//! narrow as possible while still accommodating all uses (that is, every
//! dereference of the resulting pointer must be within the lifetime).
//!
//! #### Reborrows
//!
//! Generally speaking, `regionck` does NOT try to ensure that the data
//! `data` will outlive the pointer `x`. That is the job of borrowck. The
//! one exception is when "re-borrowing" the contents of another borrowed
//! pointer. For example, imagine you have a borrowed pointer `b` with
//! lifetime `L1` and you have an expression `&*b`. The result of this
//! expression will be another borrowed pointer with lifetime `L2` (which is
//! an inference variable). The borrow checker is going to enforce the
//! constraint that `L2 < L1`, because otherwise you are re-borrowing data
//! for a lifetime larger than the original loan. However, without the
//! routines in this module, the region inferencer would not know of this
//! dependency and thus it might infer the lifetime of `L2` to be greater
//! than `L1` (issue #3148).
//!
//! There are a number of troublesome scenarios in the tests
//! `region-dependent-*.rs`, but here is one example:
//!
//! struct Foo { i: i32 }
//! struct Bar { foo: Foo }
//! fn get_i<'a>(x: &'a Bar) -> &'a i32 {
//! let foo = &x.foo; // Lifetime L1
//! &foo.i // Lifetime L2
//! }
//!
//! Note that this comes up either with `&` expressions, `ref`
//! bindings, and `autorefs`, which are the three ways to introduce
//! a borrow.
//!
//! The key point here is that when you are borrowing a value that
//! is "guaranteed" by a borrowed pointer, you must link the
//! lifetime of that borrowed pointer (`L1`, here) to the lifetime of
//! the borrow itself (`L2`). What do I mean by "guaranteed" by a
//! borrowed pointer? I mean any data that is reached by first
//! dereferencing a borrowed pointer and then either traversing
//! interior offsets or boxes. We say that the guarantor
//! of such data is the region of the borrowed pointer that was
//! traversed. This is essentially the same as the ownership
//! relation, except that a borrowed pointer never owns its
//! contents.
use crate::check::dropck;
use crate::check::FnCtxt;
use crate::middle::mem_categorization as mc;
use crate::middle::mem_categorization::Categorization;
use crate::middle::region;
use rustc::hir::def_id::DefId;
use rustc::infer::outlives::env::OutlivesEnvironment;
use rustc::infer::{self, RegionObligation, SuppressRegionErrors};
use rustc::ty::adjustment;
use rustc::ty::subst::{SubstsRef, UnpackedKind};
use rustc::ty::{self, Ty};
use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
use rustc::hir::{self, PatKind};
use std::mem;
use std::ops::Deref;
use std::rc::Rc;
use syntax_pos::Span;
// a variation on try that just returns unit
macro_rules! ignore_err {
($e:expr) => {
match $e {
Ok(e) => e,
Err(_) => {
debug!("ignoring mem-categorization error!");
return ();
}
}
};
}
///////////////////////////////////////////////////////////////////////////
// PUBLIC ENTRY POINTS
impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
pub fn regionck_expr(&self, body: &'tcx hir::Body) {
let subject = self.tcx.hir().body_owner_def_id(body.id());
let id = body.value.hir_id;
let mut rcx = RegionCtxt::new(
self,
RepeatingScope(id),
id,
Subject(subject),
self.param_env,
);
// There are no add'l implied bounds when checking a
// standalone expr (e.g., the `E` in a type like `[u32; E]`).
rcx.outlives_environment.save_implied_bounds(id);
if !self.errors_reported_since_creation() {
// regionck assumes typeck succeeded
rcx.visit_body(body);
rcx.visit_region_obligations(id);
}
rcx.resolve_regions_and_report_errors(SuppressRegionErrors::when_nll_is_enabled(self.tcx));
assert!(self.tables.borrow().free_region_map.is_empty());
self.tables.borrow_mut().free_region_map = rcx.outlives_environment.into_free_region_map();
}
/// Region checking during the WF phase for items. `wf_tys` are the
/// types from which we should derive implied bounds, if any.
pub fn regionck_item(&self, item_id: hir::HirId, span: Span, wf_tys: &[Ty<'tcx>]) {
debug!("regionck_item(item.id={:?}, wf_tys={:?})", item_id, wf_tys);
let subject = self.tcx.hir().local_def_id(item_id);
let mut rcx = RegionCtxt::new(
self,
RepeatingScope(item_id),
item_id,
Subject(subject),
self.param_env,
);
rcx.outlives_environment
.add_implied_bounds(self, wf_tys, item_id, span);
rcx.outlives_environment.save_implied_bounds(item_id);
rcx.visit_region_obligations(item_id);
rcx.resolve_regions_and_report_errors(SuppressRegionErrors::default());
}
/// Region check a function body. Not invoked on closures, but
/// only on the "root" fn item (in which closures may be
/// embedded). Walks the function body and adds various add'l
/// constraints that are needed for region inference. This is
/// separated both to isolate "pure" region constraints from the
/// rest of type check and because sometimes we need type
/// inference to have completed before we can determine which
/// constraints to add.
pub fn regionck_fn(&self, fn_id: hir::HirId, body: &'tcx hir::Body) {
debug!("regionck_fn(id={})", fn_id);
let subject = self.tcx.hir().body_owner_def_id(body.id());
let hir_id = body.value.hir_id;
let mut rcx = RegionCtxt::new(
self,
RepeatingScope(hir_id),
hir_id,
Subject(subject),
self.param_env,
);
if !self.errors_reported_since_creation() {
// regionck assumes typeck succeeded
rcx.visit_fn_body(fn_id, body, self.tcx.hir().span(fn_id));
}
rcx.resolve_regions_and_report_errors(SuppressRegionErrors::when_nll_is_enabled(self.tcx));
// In this mode, we also copy the free-region-map into the
// tables of the enclosing fcx. In the other regionck modes
// (e.g., `regionck_item`), we don't have an enclosing tables.
assert!(self.tables.borrow().free_region_map.is_empty());
self.tables.borrow_mut().free_region_map = rcx.outlives_environment.into_free_region_map();
}
}
///////////////////////////////////////////////////////////////////////////
// INTERNALS
pub struct RegionCtxt<'a, 'tcx> {
pub fcx: &'a FnCtxt<'a, 'tcx>,
pub region_scope_tree: &'tcx region::ScopeTree,
outlives_environment: OutlivesEnvironment<'tcx>,
// id of innermost fn body id
body_id: hir::HirId,
body_owner: DefId,
// call_site scope of innermost fn
call_site_scope: Option<region::Scope>,
// id of innermost fn or loop
repeating_scope: hir::HirId,
// id of AST node being analyzed (the subject of the analysis).
subject_def_id: DefId,
}
impl<'a, 'tcx> Deref for RegionCtxt<'a, 'tcx> {
type Target = FnCtxt<'a, 'tcx>;
fn deref(&self) -> &Self::Target {
&self.fcx
}
}
pub struct RepeatingScope(hir::HirId);
pub struct Subject(DefId);
impl<'a, 'tcx> RegionCtxt<'a, 'tcx> {
pub fn new(
fcx: &'a FnCtxt<'a, 'tcx>,
RepeatingScope(initial_repeating_scope): RepeatingScope,
initial_body_id: hir::HirId,
Subject(subject): Subject,
param_env: ty::ParamEnv<'tcx>,
) -> RegionCtxt<'a, 'tcx> {
let region_scope_tree = fcx.tcx.region_scope_tree(subject);
let outlives_environment = OutlivesEnvironment::new(param_env);
RegionCtxt {
fcx,
region_scope_tree,
repeating_scope: initial_repeating_scope,
body_id: initial_body_id,
body_owner: subject,
call_site_scope: None,
subject_def_id: subject,
outlives_environment,
}
}
fn set_repeating_scope(&mut self, scope: hir::HirId) -> hir::HirId {
mem::replace(&mut self.repeating_scope, scope)
}
/// Try to resolve the type for the given node, returning `t_err` if an error results. Note that
/// we never care about the details of the error, the same error will be detected and reported
/// in the writeback phase.
///
/// Note one important point: we do not attempt to resolve *region variables* here. This is
/// because regionck is essentially adding constraints to those region variables and so may yet
/// influence how they are resolved.
///
/// Consider this silly example:
///
/// ```
/// fn borrow(x: &i32) -> &i32 {x}
/// fn foo(x: @i32) -> i32 { // block: B
/// let b = borrow(x); // region: <R0>
/// *b
/// }
/// ```
///
/// Here, the region of `b` will be `<R0>`. `<R0>` is constrained to be some subregion of the
/// block B and some superregion of the call. If we forced it now, we'd choose the smaller
/// region (the call). But that would make the *b illegal. Since we don't resolve, the type
/// of b will be `&<R0>.i32` and then `*b` will require that `<R0>` be bigger than the let and
/// the `*b` expression, so we will effectively resolve `<R0>` to be the block B.
pub fn resolve_type(&self, unresolved_ty: Ty<'tcx>) -> Ty<'tcx> {
self.resolve_vars_if_possible(&unresolved_ty)
}
/// Try to resolve the type for the given node.
fn resolve_node_type(&self, id: hir::HirId) -> Ty<'tcx> {
let t = self.node_ty(id);
self.resolve_type(t)
}
/// Try to resolve the type for the given node.
pub fn resolve_expr_type_adjusted(&mut self, expr: &hir::Expr) -> Ty<'tcx> {
let ty = self.tables.borrow().expr_ty_adjusted(expr);
self.resolve_type(ty)
}
/// This is the "main" function when region-checking a function item or a closure
/// within a function item. It begins by updating various fields (e.g., `call_site_scope`
/// and `outlives_environment`) to be appropriate to the function and then adds constraints
/// derived from the function body.
///
/// Note that it does **not** restore the state of the fields that
/// it updates! This is intentional, since -- for the main
/// function -- we wish to be able to read the final
/// `outlives_environment` and other fields from the caller. For
/// closures, however, we save and restore any "scoped state"
/// before we invoke this function. (See `visit_fn` in the
/// `intravisit::Visitor` impl below.)
fn visit_fn_body(
&mut self,
id: hir::HirId, // the id of the fn itself
body: &'tcx hir::Body,
span: Span,
) {
// When we enter a function, we can derive
debug!("visit_fn_body(id={:?})", id);
let body_id = body.id();
self.body_id = body_id.hir_id;
self.body_owner = self.tcx.hir().body_owner_def_id(body_id);
let call_site = region::Scope {
id: body.value.hir_id.local_id,
data: region::ScopeData::CallSite,
};
self.call_site_scope = Some(call_site);
let fn_sig = {
match self.tables.borrow().liberated_fn_sigs().get(id) {
Some(f) => f.clone(),
None => {
bug!("No fn-sig entry for id={:?}", id);
}
}
};
// Collect the types from which we create inferred bounds.
// For the return type, if diverging, substitute `bool` just
// because it will have no effect.
//
// FIXME(#27579) return types should not be implied bounds
let fn_sig_tys: Vec<_> = fn_sig
.inputs()
.iter()
.cloned()
.chain(Some(fn_sig.output()))
.collect();
self.outlives_environment.add_implied_bounds(
self.fcx,
&fn_sig_tys[..],
body_id.hir_id,
span,
);
self.outlives_environment
.save_implied_bounds(body_id.hir_id);
self.link_fn_args(
region::Scope {
id: body.value.hir_id.local_id,
data: region::ScopeData::Node,
},
&body.arguments,
);
self.visit_body(body);
self.visit_region_obligations(body_id.hir_id);
let call_site_scope = self.call_site_scope.unwrap();
debug!(
"visit_fn_body body.id {:?} call_site_scope: {:?}",
body.id(),
call_site_scope
);
let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope));
self.type_of_node_must_outlive(infer::CallReturn(span), body_id.hir_id, call_site_region);
self.constrain_opaque_types(
&self.fcx.opaque_types.borrow(),
self.outlives_environment.free_region_map(),
);
}
fn visit_region_obligations(&mut self, hir_id: hir::HirId) {
debug!("visit_region_obligations: hir_id={:?}", hir_id);
// region checking can introduce new pending obligations
// which, when processed, might generate new region
// obligations. So make sure we process those.
self.select_all_obligations_or_error();
}
fn resolve_regions_and_report_errors(&self, suppress: SuppressRegionErrors) {
self.infcx.process_registered_region_obligations(
self.outlives_environment.region_bound_pairs_map(),
self.implicit_region_bound,
self.param_env,
);
self.fcx.resolve_regions_and_report_errors(
self.subject_def_id,
&self.region_scope_tree,
&self.outlives_environment,
suppress,
);
}
fn constrain_bindings_in_pat(&mut self, pat: &hir::Pat) {
debug!("regionck::visit_pat(pat={:?})", pat);
pat.each_binding(|_, hir_id, span, _| {
// If we have a variable that contains region'd data, that
// data will be accessible from anywhere that the variable is
// accessed. We must be wary of loops like this:
//
// // from src/test/compile-fail/borrowck-lend-flow.rs
// let mut v = box 3, w = box 4;
// let mut x = &mut w;
// loop {
// **x += 1; // (2)
// borrow(v); //~ ERROR cannot borrow
// x = &mut v; // (1)
// }
//
// Typically, we try to determine the region of a borrow from
// those points where it is dereferenced. In this case, one
// might imagine that the lifetime of `x` need only be the
// body of the loop. But of course this is incorrect because
// the pointer that is created at point (1) is consumed at
// point (2), meaning that it must be live across the loop
// iteration. The easiest way to guarantee this is to require
// that the lifetime of any regions that appear in a
// variable's type enclose at least the variable's scope.
let var_scope = self.region_scope_tree.var_scope(hir_id.local_id);
let var_region = self.tcx.mk_region(ty::ReScope(var_scope));
let origin = infer::BindingTypeIsNotValidAtDecl(span);
self.type_of_node_must_outlive(origin, hir_id, var_region);
let typ = self.resolve_node_type(hir_id);
let body_id = self.body_id;
let _ = dropck::check_safety_of_destructor_if_necessary(
self, typ, span, body_id, var_scope,
);
})
}
}
impl<'a, 'tcx> Visitor<'tcx> for RegionCtxt<'a, 'tcx> {
// (..) FIXME(#3238) should use visit_pat, not visit_arm/visit_local,
// However, right now we run into an issue whereby some free
// regions are not properly related if they appear within the
// types of arguments that must be inferred. This could be
// addressed by deferring the construction of the region
// hierarchy, and in particular the relationships between free
// regions, until regionck, as described in #3238.
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
fn visit_fn(
&mut self,
fk: intravisit::FnKind<'tcx>,
_: &'tcx hir::FnDecl,
body_id: hir::BodyId,
span: Span,
hir_id: hir::HirId,
) {
assert!(
match fk {
intravisit::FnKind::Closure(..) => true,
_ => false,
},
"visit_fn invoked for something other than a closure"
);
// Save state of current function before invoking
// `visit_fn_body`. We will restore afterwards.
let old_body_id = self.body_id;
let old_body_owner = self.body_owner;
let old_call_site_scope = self.call_site_scope;
let env_snapshot = self.outlives_environment.push_snapshot_pre_closure();
let body = self.tcx.hir().body(body_id);
self.visit_fn_body(hir_id, body, span);
// Restore state from previous function.
self.outlives_environment
.pop_snapshot_post_closure(env_snapshot);
self.call_site_scope = old_call_site_scope;
self.body_id = old_body_id;
self.body_owner = old_body_owner;
}
//visit_pat: visit_pat, // (..) see above
fn visit_arm(&mut self, arm: &'tcx hir::Arm) {
// see above
for p in &arm.pats {
self.constrain_bindings_in_pat(p);
}
intravisit::walk_arm(self, arm);
}
fn visit_local(&mut self, l: &'tcx hir::Local) {
// see above
self.constrain_bindings_in_pat(&l.pat);
self.link_local(l);
intravisit::walk_local(self, l);
}
fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
debug!(
"regionck::visit_expr(e={:?}, repeating_scope={:?})",
expr, self.repeating_scope
);
// No matter what, the type of each expression must outlive the
// scope of that expression. This also guarantees basic WF.
let expr_ty = self.resolve_node_type(expr.hir_id);
// the region corresponding to this expression
let expr_region = self.tcx.mk_region(ty::ReScope(region::Scope {
id: expr.hir_id.local_id,
data: region::ScopeData::Node,
}));
self.type_must_outlive(
infer::ExprTypeIsNotInScope(expr_ty, expr.span),
expr_ty,
expr_region,
);
let is_method_call = self.tables.borrow().is_method_call(expr);
// If we are calling a method (either explicitly or via an
// overloaded operator), check that all of the types provided as
// arguments for its type parameters are well-formed, and all the regions
// provided as arguments outlive the call.
if is_method_call {
let origin = match expr.node {
hir::ExprKind::MethodCall(..) => infer::ParameterOrigin::MethodCall,
hir::ExprKind::Unary(op, _) if op == hir::UnDeref => {
infer::ParameterOrigin::OverloadedDeref
}
_ => infer::ParameterOrigin::OverloadedOperator,
};
let substs = self.tables.borrow().node_substs(expr.hir_id);
self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
// Arguments (sub-expressions) are checked via `constrain_call`, below.
}
// Check any autoderefs or autorefs that appear.
let cmt_result = self.constrain_adjustments(expr);
// If necessary, constrain destructors in this expression. This will be
// the adjusted form if there is an adjustment.
match cmt_result {
Ok(head_cmt) => {
self.check_safety_of_rvalue_destructor_if_necessary(&head_cmt, expr.span);
}
Err(..) => {
self.tcx.sess.delay_span_bug(expr.span, "cat_expr Errd");
}
}
debug!(
"regionck::visit_expr(e={:?}, repeating_scope={:?}) - visiting subexprs",
expr, self.repeating_scope
);
match expr.node {
hir::ExprKind::Path(_) => {
let substs = self.tables.borrow().node_substs(expr.hir_id);
let origin = infer::ParameterOrigin::Path;
self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
}
hir::ExprKind::Call(ref callee, ref args) => {
if is_method_call {
self.constrain_call(expr, Some(&callee), args.iter().map(|e| &*e));
} else {
self.constrain_callee(&callee);
self.constrain_call(expr, None, args.iter().map(|e| &*e));
}
intravisit::walk_expr(self, expr);
}
hir::ExprKind::MethodCall(.., ref args) => {
self.constrain_call(expr, Some(&args[0]), args[1..].iter().map(|e| &*e));
intravisit::walk_expr(self, expr);
}
hir::ExprKind::AssignOp(_, ref lhs, ref rhs) => {
if is_method_call {
self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
}
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Index(ref lhs, ref rhs) if is_method_call => {
self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Binary(_, ref lhs, ref rhs) if is_method_call => {
// As `ExprKind::MethodCall`, but the call is via an overloaded op.
self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
// If you do `x OP y`, then the types of `x` and `y` must
// outlive the operation you are performing.
let lhs_ty = self.resolve_expr_type_adjusted(&lhs);
let rhs_ty = self.resolve_expr_type_adjusted(&rhs);
for &ty in &[lhs_ty, rhs_ty] {
self.type_must_outlive(infer::Operand(expr.span), ty, expr_region);
}
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Unary(hir::UnDeref, ref base) => {
// For *a, the lifetime of a must enclose the deref
if is_method_call {
self.constrain_call(expr, Some(base), None::<hir::Expr>.iter());
}
// For overloaded derefs, base_ty is the input to `Deref::deref`,
// but it's a reference type uing the same region as the output.
let base_ty = self.resolve_expr_type_adjusted(base);
if let ty::Ref(r_ptr, _, _) = base_ty.sty {
self.mk_subregion_due_to_dereference(expr.span, expr_region, r_ptr);
}
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Unary(_, ref lhs) if is_method_call => {
// As above.
self.constrain_call(expr, Some(&lhs), None::<hir::Expr>.iter());
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Index(ref vec_expr, _) => {
// For a[b], the lifetime of a must enclose the deref
let vec_type = self.resolve_expr_type_adjusted(&vec_expr);
self.constrain_index(expr, vec_type);
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Cast(ref source, _) => {
// Determine if we are casting `source` to a trait
// instance. If so, we have to be sure that the type of
// the source obeys the trait's region bound.
self.constrain_cast(expr, &source);
intravisit::walk_expr(self, expr);
}
hir::ExprKind::AddrOf(m, ref base) => {
self.link_addr_of(expr, m, &base);
// Require that when you write a `&expr` expression, the
// resulting pointer has a lifetime that encompasses the
// `&expr` expression itself. Note that we constraining
// the type of the node expr.id here *before applying
// adjustments*.
//
// FIXME(https://github.com/rust-lang/rfcs/issues/811)
// nested method calls requires that this rule change
let ty0 = self.resolve_node_type(expr.hir_id);
self.type_must_outlive(infer::AddrOf(expr.span), ty0, expr_region);
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Match(ref discr, ref arms, _) => {
self.link_match(&discr, &arms[..]);
intravisit::walk_expr(self, expr);
}
hir::ExprKind::Closure(.., body_id, _, _) => {
self.check_expr_fn_block(expr, body_id);
}
hir::ExprKind::Loop(ref body, _, _) => {
let repeating_scope = self.set_repeating_scope(body.hir_id);
intravisit::walk_expr(self, expr);
self.set_repeating_scope(repeating_scope);
}
hir::ExprKind::Ret(Some(ref ret_expr)) => {
let call_site_scope = self.call_site_scope;
debug!(
"visit_expr ExprKind::Ret ret_expr.hir_id {} call_site_scope: {:?}",
ret_expr.hir_id, call_site_scope
);
let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope.unwrap()));
self.type_of_node_must_outlive(
infer::CallReturn(ret_expr.span),
ret_expr.hir_id,
call_site_region,
);
intravisit::walk_expr(self, expr);
}
_ => {
intravisit::walk_expr(self, expr);
}
}
}
}
impl<'a, 'tcx> RegionCtxt<'a, 'tcx> {
fn constrain_cast(&mut self, cast_expr: &hir::Expr, source_expr: &hir::Expr) {
debug!(
"constrain_cast(cast_expr={:?}, source_expr={:?})",
cast_expr, source_expr
);
let source_ty = self.resolve_node_type(source_expr.hir_id);
let target_ty = self.resolve_node_type(cast_expr.hir_id);
self.walk_cast(cast_expr, source_ty, target_ty);
}
fn walk_cast(&mut self, cast_expr: &hir::Expr, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) {
debug!("walk_cast(from_ty={:?}, to_ty={:?})", from_ty, to_ty);
match (&from_ty.sty, &to_ty.sty) {
/*From:*/
(&ty::Ref(from_r, from_ty, _), /*To: */ &ty::Ref(to_r, to_ty, _)) => {
// Target cannot outlive source, naturally.
self.sub_regions(infer::Reborrow(cast_expr.span), to_r, from_r);
self.walk_cast(cast_expr, from_ty, to_ty);
}
/*From:*/
(_, /*To: */ &ty::Dynamic(.., r)) => {
// When T is existentially quantified as a trait
// `Foo+'to`, it must outlive the region bound `'to`.
self.type_must_outlive(infer::RelateObjectBound(cast_expr.span), from_ty, r);
}
/*From:*/
(&ty::Adt(from_def, _), /*To: */ &ty::Adt(to_def, _))
if from_def.is_box() && to_def.is_box() =>
{
self.walk_cast(cast_expr, from_ty.boxed_ty(), to_ty.boxed_ty());
}
_ => {}
}
}
fn check_expr_fn_block(&mut self, expr: &'tcx hir::Expr, body_id: hir::BodyId) {
let repeating_scope = self.set_repeating_scope(body_id.hir_id);
intravisit::walk_expr(self, expr);
self.set_repeating_scope(repeating_scope);
}
fn constrain_callee(&mut self, callee_expr: &hir::Expr) {
let callee_ty = self.resolve_node_type(callee_expr.hir_id);
match callee_ty.sty {
ty::FnDef(..) | ty::FnPtr(_) => {}
_ => {
// this should not happen, but it does if the program is
// erroneous
//
// bug!(
// callee_expr.span,
// "Calling non-function: {}",
// callee_ty);
}
}
}
fn constrain_call<'b, I: Iterator<Item = &'b hir::Expr>>(
&mut self,
call_expr: &hir::Expr,
receiver: Option<&hir::Expr>,
arg_exprs: I,
) {
//! Invoked on every call site (i.e., normal calls, method calls,
//! and overloaded operators). Constrains the regions which appear
//! in the type of the function. Also constrains the regions that
//! appear in the arguments appropriately.
debug!(
"constrain_call(call_expr={:?}, receiver={:?})",
call_expr, receiver
);
// `callee_region` is the scope representing the time in which the
// call occurs.
//
// FIXME(#6268) to support nested method calls, should be callee_id
let callee_scope = region::Scope {
id: call_expr.hir_id.local_id,
data: region::ScopeData::Node,
};
let callee_region = self.tcx.mk_region(ty::ReScope(callee_scope));
debug!("callee_region={:?}", callee_region);
for arg_expr in arg_exprs {
debug!("argument: {:?}", arg_expr);
// ensure that any regions appearing in the argument type are
// valid for at least the lifetime of the function:
self.type_of_node_must_outlive(
infer::CallArg(arg_expr.span),
arg_expr.hir_id,
callee_region,
);
}
// as loop above, but for receiver
if let Some(r) = receiver {
debug!("receiver: {:?}", r);
self.type_of_node_must_outlive(infer::CallRcvr(r.span), r.hir_id, callee_region);
}
}
/// Creates a temporary `MemCategorizationContext` and pass it to the closure.
fn with_mc<F, R>(&self, f: F) -> R
where
F: for<'b> FnOnce(mc::MemCategorizationContext<'b, 'tcx>) -> R,
{
f(mc::MemCategorizationContext::with_infer(
&self.infcx,
self.body_owner,
&self.region_scope_tree,
&self.tables.borrow(),
))
}
/// Invoked on any adjustments that occur. Checks that if this is a region pointer being
/// dereferenced, the lifetime of the pointer includes the deref expr.
fn constrain_adjustments(&mut self, expr: &hir::Expr) -> mc::McResult<mc::cmt_<'tcx>> {
debug!("constrain_adjustments(expr={:?})", expr);
let mut cmt = self.with_mc(|mc| mc.cat_expr_unadjusted(expr))?;
let tables = self.tables.borrow();
let adjustments = tables.expr_adjustments(&expr);
if adjustments.is_empty() {
return Ok(cmt);
}
debug!("constrain_adjustments: adjustments={:?}", adjustments);
// If necessary, constrain destructors in the unadjusted form of this
// expression.
self.check_safety_of_rvalue_destructor_if_necessary(&cmt, expr.span);
let expr_region = self.tcx.mk_region(ty::ReScope(region::Scope {
id: expr.hir_id.local_id,
data: region::ScopeData::Node,
}));
for adjustment in adjustments {
debug!(
"constrain_adjustments: adjustment={:?}, cmt={:?}",
adjustment, cmt
);
if let adjustment::Adjust::Deref(Some(deref)) = adjustment.kind {
debug!("constrain_adjustments: overloaded deref: {:?}", deref);
// Treat overloaded autoderefs as if an AutoBorrow adjustment
// was applied on the base type, as that is always the case.
let input = self.tcx.mk_ref(
deref.region,
ty::TypeAndMut {
ty: cmt.ty,
mutbl: deref.mutbl,
},
);
let output = self.tcx.mk_ref(
deref.region,
ty::TypeAndMut {
ty: adjustment.target,
mutbl: deref.mutbl,
},
);
self.link_region(
expr.span,
deref.region,
ty::BorrowKind::from_mutbl(deref.mutbl),
&cmt,
);
// Specialized version of constrain_call.
self.type_must_outlive(infer::CallRcvr(expr.span), input, expr_region);
self.type_must_outlive(infer::CallReturn(expr.span), output, expr_region);
}
if let adjustment::Adjust::Borrow(ref autoref) = adjustment.kind {
self.link_autoref(expr, &cmt, autoref);
// Require that the resulting region encompasses
// the current node.
//
// FIXME(#6268) remove to support nested method calls
self.type_of_node_must_outlive(
infer::AutoBorrow(expr.span),
expr.hir_id,
expr_region,
);
}
cmt = self.with_mc(|mc| mc.cat_expr_adjusted(expr, cmt, &adjustment))?;
if let Categorization::Deref(_, mc::BorrowedPtr(_, r_ptr)) = cmt.cat {
self.mk_subregion_due_to_dereference(expr.span, expr_region, r_ptr);
}
}
Ok(cmt)
}
pub fn mk_subregion_due_to_dereference(
&mut self,
deref_span: Span,
minimum_lifetime: ty::Region<'tcx>,
maximum_lifetime: ty::Region<'tcx>,
) {
self.sub_regions(
infer::DerefPointer(deref_span),
minimum_lifetime,
maximum_lifetime,
)
}
fn check_safety_of_rvalue_destructor_if_necessary(&mut self, cmt: &mc::cmt_<'tcx>, span: Span) {
if let Categorization::Rvalue(region) = cmt.cat {
match *region {
ty::ReScope(rvalue_scope) => {
let typ = self.resolve_type(cmt.ty);
let body_id = self.body_id;
let _ = dropck::check_safety_of_destructor_if_necessary(
self,
typ,
span,
body_id,
rvalue_scope,
);
}
ty::ReStatic => {}
_ => {
span_bug!(
span,
"unexpected rvalue region in rvalue \
destructor safety checking: `{:?}`",
region
);
}
}
}
}
/// Invoked on any index expression that occurs. Checks that if this is a slice
/// being indexed, the lifetime of the pointer includes the deref expr.
fn constrain_index(&mut self, index_expr: &hir::Expr, indexed_ty: Ty<'tcx>) {
debug!(
"constrain_index(index_expr=?, indexed_ty={}",
self.ty_to_string(indexed_ty)
);
let r_index_expr = ty::ReScope(region::Scope {
id: index_expr.hir_id.local_id,
data: region::ScopeData::Node,
});
if let ty::Ref(r_ptr, r_ty, _) = indexed_ty.sty {
match r_ty.sty {
ty::Slice(_) | ty::Str => {
self.sub_regions(
infer::IndexSlice(index_expr.span),
self.tcx.mk_region(r_index_expr),
r_ptr,
);
}
_ => {}
}
}
}
/// Guarantees that any lifetimes that appear in the type of the node `id` (after applying
/// adjustments) are valid for at least `minimum_lifetime`.
fn type_of_node_must_outlive(
&mut self,
origin: infer::SubregionOrigin<'tcx>,
hir_id: hir::HirId,
minimum_lifetime: ty::Region<'tcx>,
) {
// Try to resolve the type. If we encounter an error, then typeck
// is going to fail anyway, so just stop here and let typeck
// report errors later on in the writeback phase.
let ty0 = self.resolve_node_type(hir_id);
let ty = self.tables
.borrow()
.adjustments()
.get(hir_id)
.and_then(|adj| adj.last())
.map_or(ty0, |adj| adj.target);
let ty = self.resolve_type(ty);
debug!(
"constrain_regions_in_type_of_node(\
ty={}, ty0={}, id={:?}, minimum_lifetime={:?})",
ty, ty0, hir_id, minimum_lifetime
);
self.type_must_outlive(origin, ty, minimum_lifetime);
}
/// Adds constraints to inference such that `T: 'a` holds (or
/// reports an error if it cannot).
///
/// # Parameters
///
/// - `origin`, the reason we need this constraint
/// - `ty`, the type `T`
/// - `region`, the region `'a`
pub fn type_must_outlive(
&self,
origin: infer::SubregionOrigin<'tcx>,
ty: Ty<'tcx>,
region: ty::Region<'tcx>,
) {
self.infcx.register_region_obligation(
self.body_id,
RegionObligation {
sub_region: region,
sup_type: ty,
origin,
},
);
}
/// Computes the guarantor for an expression `&base` and then ensures that the lifetime of the
/// resulting pointer is linked to the lifetime of its guarantor (if any).
fn link_addr_of(&mut self, expr: &hir::Expr, mutability: hir::Mutability, base: &hir::Expr) {
debug!("link_addr_of(expr={:?}, base={:?})", expr, base);
let cmt = ignore_err!(self.with_mc(|mc| mc.cat_expr(base)));
debug!("link_addr_of: cmt={:?}", cmt);
self.link_region_from_node_type(expr.span, expr.hir_id, mutability, &cmt);
}
/// Computes the guarantors for any ref bindings in a `let` and
/// then ensures that the lifetime of the resulting pointer is
/// linked to the lifetime of the initialization expression.
fn link_local(&self, local: &hir::Local) {
debug!("regionck::for_local()");
let init_expr = match local.init {
None => {
return;
}
Some(ref expr) => &**expr,
};
let discr_cmt = Rc::new(ignore_err!(self.with_mc(|mc| mc.cat_expr(init_expr))));
self.link_pattern(discr_cmt, &local.pat);
}
/// Computes the guarantors for any ref bindings in a match and
/// then ensures that the lifetime of the resulting pointer is
/// linked to the lifetime of its guarantor (if any).
fn link_match(&self, discr: &hir::Expr, arms: &[hir::Arm]) {
debug!("regionck::for_match()");
let discr_cmt = Rc::new(ignore_err!(self.with_mc(|mc| mc.cat_expr(discr))));
debug!("discr_cmt={:?}", discr_cmt);
for arm in arms {
for root_pat in &arm.pats {
self.link_pattern(discr_cmt.clone(), &root_pat);
}
}
}
/// Computes the guarantors for any ref bindings in a match and
/// then ensures that the lifetime of the resulting pointer is
/// linked to the lifetime of its guarantor (if any).
fn link_fn_args(&self, body_scope: region::Scope, args: &[hir::Arg]) {
debug!("regionck::link_fn_args(body_scope={:?})", body_scope);
for arg in args {
let arg_ty = self.node_ty(arg.hir_id);
let re_scope = self.tcx.mk_region(ty::ReScope(body_scope));
let arg_cmt = self.with_mc(|mc| {
Rc::new(mc.cat_rvalue(arg.hir_id, arg.pat.span, re_scope, arg_ty))
});
debug!("arg_ty={:?} arg_cmt={:?} arg={:?}", arg_ty, arg_cmt, arg);
self.link_pattern(arg_cmt, &arg.pat);
}
}
/// Link lifetimes of any ref bindings in `root_pat` to the pointers found
/// in the discriminant, if needed.
fn link_pattern(&self, discr_cmt: mc::cmt<'tcx>, root_pat: &hir::Pat) {
debug!(
"link_pattern(discr_cmt={:?}, root_pat={:?})",
discr_cmt, root_pat
);
ignore_err!(self.with_mc(|mc| {
mc.cat_pattern(discr_cmt, root_pat, |sub_cmt, sub_pat| {
// `ref x` pattern
if let PatKind::Binding(..) = sub_pat.node {
if let Some(&bm) = mc.tables.pat_binding_modes().get(sub_pat.hir_id) {
if let ty::BindByReference(mutbl) = bm {
self.link_region_from_node_type(
sub_pat.span,
sub_pat.hir_id,
mutbl,
&sub_cmt,
);
}
} else {
self.tcx
.sess
.delay_span_bug(sub_pat.span, "missing binding mode");
}
}
})
}));
}
/// Link lifetime of borrowed pointer resulting from autoref to lifetimes in the value being
/// autoref'd.
fn link_autoref(
&self,
expr: &hir::Expr,
expr_cmt: &mc::cmt_<'tcx>,
autoref: &adjustment::AutoBorrow<'tcx>,
) {
debug!(
"link_autoref(autoref={:?}, expr_cmt={:?})",
autoref, expr_cmt
);
match *autoref {
adjustment::AutoBorrow::Ref(r, m) => {
self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m.into()), expr_cmt);
}
adjustment::AutoBorrow::RawPtr(m) => {
let r = self.tcx.mk_region(ty::ReScope(region::Scope {
id: expr.hir_id.local_id,
data: region::ScopeData::Node,
}));
self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m), expr_cmt);
}
}
}
/// Like `link_region()`, except that the region is extracted from the type of `id`,
/// which must be some reference (`&T`, `&str`, etc).
fn link_region_from_node_type(
&self,
span: Span,
id: hir::HirId,
mutbl: hir::Mutability,
cmt_borrowed: &mc::cmt_<'tcx>,
) {
debug!(
"link_region_from_node_type(id={:?}, mutbl={:?}, cmt_borrowed={:?})",
id, mutbl, cmt_borrowed
);
let rptr_ty = self.resolve_node_type(id);
if let ty::Ref(r, _, _) = rptr_ty.sty {
debug!("rptr_ty={}", rptr_ty);
self.link_region(span, r, ty::BorrowKind::from_mutbl(mutbl), cmt_borrowed);
}
}
/// Informs the inference engine that `borrow_cmt` is being borrowed with
/// kind `borrow_kind` and lifetime `borrow_region`.
/// In order to ensure borrowck is satisfied, this may create constraints
/// between regions, as explained in `link_reborrowed_region()`.
fn link_region(
&self,
span: Span,
borrow_region: ty::Region<'tcx>,
borrow_kind: ty::BorrowKind,
borrow_cmt: &mc::cmt_<'tcx>,
) {
let origin = infer::DataBorrowed(borrow_cmt.ty, span);
self.type_must_outlive(origin, borrow_cmt.ty, borrow_region);
let mut borrow_kind = borrow_kind;
let mut borrow_cmt_cat = borrow_cmt.cat.clone();
loop {
debug!(
"link_region(borrow_region={:?}, borrow_kind={:?}, borrow_cmt={:?})",
borrow_region, borrow_kind, borrow_cmt
);
match borrow_cmt_cat {
Categorization::Deref(ref_cmt, mc::BorrowedPtr(ref_kind, ref_region)) => {
match self.link_reborrowed_region(
span,
borrow_region,
borrow_kind,
ref_cmt,
ref_region,
ref_kind,
borrow_cmt.note,
) {
Some((c, k)) => {
borrow_cmt_cat = c.cat.clone();
borrow_kind = k;
}
None => {
return;
}
}
}
Categorization::Downcast(cmt_base, _)
| Categorization::Deref(cmt_base, mc::Unique)
| Categorization::Interior(cmt_base, _) => {
// Borrowing interior or owned data requires the base
// to be valid and borrowable in the same fashion.
borrow_cmt_cat = cmt_base.cat.clone();
borrow_kind = borrow_kind;
}
Categorization::Deref(_, mc::UnsafePtr(..))
| Categorization::StaticItem
| Categorization::Upvar(..)
| Categorization::Local(..)
| Categorization::ThreadLocal(..)
| Categorization::Rvalue(..) => {
// These are all "base cases" with independent lifetimes
// that are not subject to inference
return;
}
}
}
}
/// This is the most complicated case: the path being borrowed is
/// itself the referent of a borrowed pointer. Let me give an
/// example fragment of code to make clear(er) the situation:
///
/// let r: &'a mut T = ...; // the original reference "r" has lifetime 'a
/// ...
/// &'z *r // the reborrow has lifetime 'z
///
/// Now, in this case, our primary job is to add the inference
/// constraint that `'z <= 'a`. Given this setup, let's clarify the
/// parameters in (roughly) terms of the example:
///
/// ```plain,ignore (pseudo-Rust)
/// A borrow of: `& 'z bk * r` where `r` has type `& 'a bk T`
/// borrow_region ^~ ref_region ^~
/// borrow_kind ^~ ref_kind ^~
/// ref_cmt ^
/// ```
///
/// Here `bk` stands for some borrow-kind (e.g., `mut`, `uniq`, etc).
///
/// Unfortunately, there are some complications beyond the simple
/// scenario I just painted:
///
/// 1. The reference `r` might in fact be a "by-ref" upvar. In that
/// case, we have two jobs. First, we are inferring whether this reference
/// should be an `&T`, `&mut T`, or `&uniq T` reference, and we must
/// adjust that based on this borrow (e.g., if this is an `&mut` borrow,
/// then `r` must be an `&mut` reference). Second, whenever we link
/// two regions (here, `'z <= 'a`), we supply a *cause*, and in this
/// case we adjust the cause to indicate that the reference being
/// "reborrowed" is itself an upvar. This provides a nicer error message
/// should something go wrong.
///
/// 2. There may in fact be more levels of reborrowing. In the
/// example, I said the borrow was like `&'z *r`, but it might
/// in fact be a borrow like `&'z **q` where `q` has type `&'a
/// &'b mut T`. In that case, we want to ensure that `'z <= 'a`
/// and `'z <= 'b`. This is explained more below.
///
/// The return value of this function indicates whether we need to
/// recurse and process `ref_cmt` (see case 2 above).
fn link_reborrowed_region(
&self,
span: Span,
borrow_region: ty::Region<'tcx>,
borrow_kind: ty::BorrowKind,
ref_cmt: mc::cmt<'tcx>,
ref_region: ty::Region<'tcx>,
mut ref_kind: ty::BorrowKind,
note: mc::Note,
) -> Option<(mc::cmt<'tcx>, ty::BorrowKind)> {
// Possible upvar ID we may need later to create an entry in the
// maybe link map.
// Detect by-ref upvar `x`:
let cause = match note {
mc::NoteUpvarRef(ref upvar_id) => {
match self.tables.borrow().upvar_capture_map.get(upvar_id) {
Some(&ty::UpvarCapture::ByRef(ref upvar_borrow)) => {
// The mutability of the upvar may have been modified
// by the above adjustment, so update our local variable.
ref_kind = upvar_borrow.kind;
infer::ReborrowUpvar(span, *upvar_id)
}
_ => {
span_bug!(span, "Illegal upvar id: {:?}", upvar_id);
}
}
}
mc::NoteClosureEnv(ref upvar_id) => {
// We don't have any mutability changes to propagate, but
// we do want to note that an upvar reborrow caused this
// link
infer::ReborrowUpvar(span, *upvar_id)
}
_ => infer::Reborrow(span),
};
debug!(
"link_reborrowed_region: {:?} <= {:?}",
borrow_region, ref_region
);
self.sub_regions(cause, borrow_region, ref_region);
// If we end up needing to recurse and establish a region link
// with `ref_cmt`, calculate what borrow kind we will end up
// needing. This will be used below.
//
// One interesting twist is that we can weaken the borrow kind
// when we recurse: to reborrow an `&mut` referent as mutable,
// borrowck requires a unique path to the `&mut` reference but not
// necessarily a *mutable* path.
let new_borrow_kind = match borrow_kind {
ty::ImmBorrow => ty::ImmBorrow,
ty::MutBorrow | ty::UniqueImmBorrow => ty::UniqueImmBorrow,
};
// Decide whether we need to recurse and link any regions within
// the `ref_cmt`. This is concerned for the case where the value
// being reborrowed is in fact a borrowed pointer found within
// another borrowed pointer. For example:
//
// let p: &'b &'a mut T = ...;
// ...
// &'z **p
//
// What makes this case particularly tricky is that, if the data
// being borrowed is a `&mut` or `&uniq` borrow, borrowck requires
// not only that `'z <= 'a`, (as before) but also `'z <= 'b`
// (otherwise the user might mutate through the `&mut T` reference
// after `'b` expires and invalidate the borrow we are looking at
// now).
//
// So let's re-examine our parameters in light of this more
// complicated (possible) scenario:
//
// A borrow of: `& 'z bk * * p` where `p` has type `&'b bk & 'a bk T`
// borrow_region ^~ ref_region ^~
// borrow_kind ^~ ref_kind ^~
// ref_cmt ^~~
//
// (Note that since we have not examined `ref_cmt.cat`, we don't
// know whether this scenario has occurred; but I wanted to show
// how all the types get adjusted.)
match ref_kind {
ty::ImmBorrow => {
// The reference being reborrowed is a shareable ref of
// type `&'a T`. In this case, it doesn't matter where we
// *found* the `&T` pointer, the memory it references will
// be valid and immutable for `'a`. So we can stop here.
//
// (Note that the `borrow_kind` must also be ImmBorrow or
// else the user is borrowed imm memory as mut memory,
// which means they'll get an error downstream in borrowck
// anyhow.)
return None;
}
ty::MutBorrow | ty::UniqueImmBorrow => {
// The reference being reborrowed is either an `&mut T` or
// `&uniq T`. This is the case where recursion is needed.
return Some((ref_cmt, new_borrow_kind));
}
}
}
/// Checks that the values provided for type/region arguments in a given
/// expression are well-formed and in-scope.
fn substs_wf_in_scope(
&mut self,
origin: infer::ParameterOrigin,
substs: SubstsRef<'tcx>,
expr_span: Span,
expr_region: ty::Region<'tcx>,
) {
debug!(
"substs_wf_in_scope(substs={:?}, \
expr_region={:?}, \
origin={:?}, \
expr_span={:?})",
substs, expr_region, origin, expr_span
);
let origin = infer::ParameterInScope(origin, expr_span);
for kind in substs {
match kind.unpack() {
UnpackedKind::Lifetime(lt) => {
self.sub_regions(origin.clone(), expr_region, lt);
}
UnpackedKind::Type(ty) => {
let ty = self.resolve_type(ty);
self.type_must_outlive(origin.clone(), ty, expr_region);
}
UnpackedKind::Const(_) => {
// Const parameters don't impose constraints.
}
}
}
}
}
You can’t perform that action at this time.