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Clarify some borrowck errors

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Closes #17263.
This commit is contained in:
P1start 2014-09-15 21:20:05 +12:00
parent 36b85025df
commit 35ff2def5d
3 changed files with 125 additions and 27 deletions
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84
src/librustc/middle/borrowck/check_loans.rs
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@ -400,50 +400,82 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
for restr_path in loan1.restricted_paths.iter() {
if *restr_path != loan2_base_path { continue; }
let old_pronoun = if new_loan.loan_path == old_loan.loan_path {
// If new_loan is something like `x.a`, and old_loan is something like `x.b`, we would
// normally generate a rather confusing message (in this case, for multiple mutable
// borrows):
//
// error: cannot borrow `x.b` as mutable more than once at a time
// note: previous borrow of `x.a` occurs here; the mutable borrow prevents
// subsequent moves, borrows, or modification of `x.a` until the borrow ends
//
// What we want to do instead is get the 'common ancestor' of the two borrow paths and
// use that for most of the message instead, giving is something like this:
//
// error: cannot borrow `x` as mutable more than once at a time
// note: previous borrow of `x` occurs here (through borrowing `x.a`); the mutable
// borrow prevents subsequent moves, borrows, or modification of `x` until the
// borrow ends
let common = new_loan.loan_path.common(&*old_loan.loan_path);
let (nl, ol, new_loan_msg, old_loan_msg) =
if new_loan.loan_path.has_fork(&*old_loan.loan_path) && common.is_some() {
let nl = self.bccx.loan_path_to_string(&common.unwrap());
let ol = nl.clone();
let new_loan_msg = format!(" (here through borrowing `{}`)",
self.bccx.loan_path_to_string(
&*new_loan.loan_path));
let old_loan_msg = format!(" (through borrowing `{}`)",
self.bccx.loan_path_to_string(
&*old_loan.loan_path));
(nl, ol, new_loan_msg, old_loan_msg)
} else {
(self.bccx.loan_path_to_string(&*new_loan.loan_path),
self.bccx.loan_path_to_string(&*old_loan.loan_path),
String::new(), String::new())
};
let ol_pronoun = if new_loan.loan_path == old_loan.loan_path {
"it".to_string()
} else {
format!("`{}`",
self.bccx.loan_path_to_string(&*old_loan.loan_path))
format!("`{}`", ol)
};
match (new_loan.kind, old_loan.kind) {
(ty::MutBorrow, ty::MutBorrow) => {
self.bccx.span_err(
new_loan.span,
format!("cannot borrow `{}` as mutable \
format!("cannot borrow `{}`{} as mutable \
more than once at a time",
self.bccx.loan_path_to_string(
&*new_loan.loan_path)).as_slice());
nl, new_loan_msg).as_slice())
}
(ty::UniqueImmBorrow, _) => {
self.bccx.span_err(
new_loan.span,
format!("closure requires unique access to `{}` \
but {} is already borrowed",
self.bccx.loan_path_to_string(&*new_loan.loan_path),
old_pronoun).as_slice());
but {} is already borrowed{}",
nl, ol_pronoun, old_loan_msg).as_slice());
}
(_, ty::UniqueImmBorrow) => {
self.bccx.span_err(
new_loan.span,
format!("cannot borrow `{}` as {} because \
format!("cannot borrow `{}`{} as {} because \
previous closure requires unique access",
self.bccx.loan_path_to_string(&*new_loan.loan_path),
new_loan.kind.to_user_str()).as_slice());
nl, new_loan_msg, new_loan.kind.to_user_str()).as_slice());
}
(_, _) => {
self.bccx.span_err(
new_loan.span,
format!("cannot borrow `{}` as {} because \
{} is also borrowed as {}",
self.bccx.loan_path_to_string(&*new_loan.loan_path),
format!("cannot borrow `{}`{} as {} because \
{} is also borrowed as {}{}",
nl,
new_loan_msg,
new_loan.kind.to_user_str(),
old_pronoun,
old_loan.kind.to_user_str()).as_slice());
ol_pronoun,
old_loan.kind.to_user_str(),
old_loan_msg).as_slice());
}
}
@ -452,8 +484,7 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
self.bccx.span_note(
span,
format!("borrow occurs due to use of `{}` in closure",
self.bccx.loan_path_to_string(
&*new_loan.loan_path)).as_slice());
nl).as_slice());
}
_ => { }
}
@ -463,30 +494,29 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
format!("the mutable borrow prevents subsequent \
moves, borrows, or modification of `{0}` \
until the borrow ends",
self.bccx.loan_path_to_string(
&*old_loan.loan_path))
ol)
}
ty::ImmBorrow => {
format!("the immutable borrow prevents subsequent \
moves or mutable borrows of `{0}` \
until the borrow ends",
self.bccx.loan_path_to_string(&*old_loan.loan_path))
ol)
}
ty::UniqueImmBorrow => {
format!("the unique capture prevents subsequent \
moves or borrows of `{0}` \
until the borrow ends",
self.bccx.loan_path_to_string(&*old_loan.loan_path))
ol)
}
};
let borrow_summary = match old_loan.cause {
euv::ClosureCapture(_) => {
format!("previous borrow of `{}` occurs here due to \
format!("previous borrow of `{}` occurs here{} due to \
use in closure",
self.bccx.loan_path_to_string(&*old_loan.loan_path))
ol, old_loan_msg)
}
euv::OverloadedOperator(..) |
@ -496,8 +526,8 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
euv::ForLoop(..) |
euv::RefBinding(..) |
euv::MatchDiscriminant(..) => {
format!("previous borrow of `{}` occurs here",
self.bccx.loan_path_to_string(&*old_loan.loan_path))
format!("previous borrow of `{}` occurs here{}",
ol, old_loan_msg)
}
};

45
src/librustc/middle/borrowck/mod.rs
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@ -298,6 +298,51 @@ impl LoanPath {
LpExtend(ref base, _, _) => base.kill_scope(tcx),
}
}
fn has_fork(&self, other: &LoanPath) -> bool {
match (self, other) {
(&LpExtend(ref base, _, LpInterior(id)), &LpExtend(ref base2, _, LpInterior(id2))) =>
if id == id2 {
base.has_fork(&**base2)
} else {
true
},
(&LpExtend(ref base, _, LpDeref(_)), _) => base.has_fork(other),
(_, &LpExtend(ref base, _, LpDeref(_))) => self.has_fork(&**base),
_ => false,
}
}
fn depth(&self) -> uint {
match *self {
LpExtend(ref base, _, LpDeref(_)) => base.depth(),
LpExtend(ref base, _, LpInterior(_)) => base.depth() + 1,
_ => 0,
}
}
fn common(&self, other: &LoanPath) -> Option<LoanPath> {
match (self, other) {
(&LpExtend(ref base, a, LpInterior(id)), &LpExtend(ref base2, _, LpInterior(id2))) =>
if id == id2 {
base.common(&**base2).map(|x| {
let xd = x.depth();
if base.depth() == xd && base2.depth() == xd {
LpExtend(Rc::new(x), a, LpInterior(id))
} else {
x
}
})
} else {
base.common(&**base2)
},
(&LpExtend(ref base, _, LpDeref(_)), _) => base.common(other),
(_, &LpExtend(ref other, _, LpDeref(_))) => self.common(&**other),
(&LpVar(id), &LpVar(id2)) => if id == id2 { Some(LpVar(id)) } else { None },
(&LpUpvar(id), &LpUpvar(id2)) => if id == id2 { Some(LpUpvar(id)) } else { None },
_ => None,
}
}
}
pub fn opt_loan_path(cmt: &mc::cmt) -> Option<Rc<LoanPath>> {

23
src/test/compile-fail/issue-17263.rs Normal file
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@ -0,0 +1,23 @@
// Copyright 2014 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.
struct Foo { a: int, b: int }
fn main() {
let mut x = box Foo { a: 1, b: 2 };
let (a, b) = (&mut x.a, &mut x.b);
//~^ ERROR cannot borrow `x` (here through borrowing `x.b`) as mutable more than once at a time
//~^^ NOTE previous borrow of `x` occurs here (through borrowing `x.a`)
let mut foo = box Foo { a: 1, b: 2 };
let (c, d) = (&mut foo.a, &foo.b);
//~^ ERROR cannot borrow `foo` (here through borrowing `foo.b`) as immutable
//~^^ NOTE previous borrow of `foo` occurs here (through borrowing `foo.a`)
}