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Can cast an unusable value to a raw pointer #20791

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wthrowe opened this issue Jan 9, 2015 · 6 comments · Fixed by #24191
Closed

Can cast an unusable value to a raw pointer #20791

wthrowe opened this issue Jan 9, 2015 · 6 comments · Fixed by #24191
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@nikomatsakis
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A-type-system Area: Type system P-medium Medium priority
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1.0

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@wthrowe
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wthrowe commented Jan 9, 2015 

fn main() {
    let x: &i32;
    //x; // error: use of possibly uninitialized variable: `x`
    println!("{:?}", x as *const _);
}

This is accepted and prints 0x0 everywhere I've tested (rustc 0.13.0-dev (496dc4e) and the playpen). A similar access is allowed to moved values and mutably borrowed values.

I assume I should not be able to access uninitialized values without some unsafe code, although I am new to the language and might be missing something.
@Thiez
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Thiez commented Jan 9, 2015

You assumption is correct, the reference explicitly defines this as unsafe, see the 4th rule here.

@kmcallister
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Nice catch! Nominating.

@nikomatsakis
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Weird. Perhaps an oversight in the EUV?

@brson brson removed the I-nominated label Jan 15, 2015
@nikomatsakis
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cc me

@brson brson added this to the 1.0 milestone Jan 15, 2015
@brson brson added the P-medium Medium priority label Jan 15, 2015
@kmcallister
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Still compiles successfully (I made a minor rustup edit). Closed by mistake?

rustc 1.0.0-dev (8160fc4 2015-01-22 16:50:17 +0000)

@kmcallister kmcallister reopened this Jan 22, 2015
@nikomatsakis
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Um, it appears I got confused -- I added a run-pass test ;)

@nikomatsakis nikomatsakis self-assigned this Apr 2, 2015
@nikomatsakis nikomatsakis mentioned this issue Apr 8, 2015
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Manishearth added a commit to Manishearth/rust that referenced this issue Apr 9, 2015
@Manishearth
Rollup merge of rust-lang#24191 - nikomatsakis:issue-20791, r=pnkfelix
c065873 
 Modify the ExprUseVisitor to walk each part of an AutoRef, and in
particular to treat an AutoUnsize as as kind of \"instantaneous\" borrow
of the value being unsized. This prevents us from feeding uninitialized
data.

This caused a problem for the eager reborrow of comparison traits,
because that wound up introducing a \"double AutoRef\", which was not
being thoroughly checked before but turned out not to type check.
Fortunately, we can just remove that \"eager reborrow\" as it is no longer
needed now that `PartialEq` doesn't force both LHS and RHS to have the
same type (and even if we did have this problem, the better way would be
to lean on introducing a common supertype).

Fixes rust-lang#20791.

r? @nrc
Manishearth added a commit to Manishearth/rust that referenced this issue Apr 9, 2015
@Manishearth
Rollup merge of rust-lang#24191 - nikomatsakis:issue-20791, r=pnkfelix
3b87140 
 Modify the ExprUseVisitor to walk each part of an AutoRef, and in
particular to treat an AutoUnsize as as kind of \"instantaneous\" borrow
of the value being unsized. This prevents us from feeding uninitialized
data.

This caused a problem for the eager reborrow of comparison traits,
because that wound up introducing a \"double AutoRef\", which was not
being thoroughly checked before but turned out not to type check.
Fortunately, we can just remove that \"eager reborrow\" as it is no longer
needed now that `PartialEq` doesn't force both LHS and RHS to have the
same type (and even if we did have this problem, the better way would be
to lean on introducing a common supertype).

Fixes rust-lang#20791.

r? @nrc
@snargleplax snargleplax mentioned this issue Sep 24, 2016
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Modify the ExprUseVisitor to walk each part of an AutoRef nikomatsakis/rust
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@brson @nikomatsakis @Thiez @kmcallister @wthrowe