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Unconditionally breaking loops not allowed in constants. #62272

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eddyb opened this issue Jul 1, 2019 · 2 comments · Fixed by #62274
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Unconditionally breaking loops not allowed in constants. #62272

eddyb opened this issue Jul 1, 2019 · 2 comments · Fixed by #62274
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A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html regression-from-stable-to-stable Performance or correctness regression from one stable version to another.

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@eddyb
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eddyb commented Jul 1, 2019

Technically a regression from 1.24 to 1.25, but I don't think anyone used this.
Example code which used to be allowed:

pub const FOO: () = loop { break; };

I'm pretty sure this changed with #47802, which added the FalseUnwind terminator to MIR, but didn't handle it in const-checking (which otherwise ignores cleanup blocks).

cc @matthewjasper @nikomatsakis
@eddyb eddyb added A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html regression-from-stable-to-stable Performance or correctness regression from one stable version to another. A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) labels Jul 1, 2019
This was referenced Jul 1, 2019
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Centril commented Jul 1, 2019

I think this is a technicality and that we can continue to let this be broken since no one said a peep about it since 1.25. The upside is that the syntactic surface of const eval is clearer.

@eddyb eddyb mentioned this issue Jul 1, 2019
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@eddyb eddyb self-assigned this Jul 1, 2019
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eddyb commented Jul 5, 2019

On the PR I explained why I think we should fix this even if we want to ban loops: #62274 (comment)

@Centril Centril mentioned this issue Jul 5, 2019
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Centril added a commit to Centril/rust that referenced this issue Jul 12, 2019
@Centril
Rollup merge of rust-lang#62274 - eddyb:const-false-unwind, r=pnkfelix
9ffeb26 
rustc_mir: follow FalseUnwind's real_target edge in qualify_consts.

As far as I can tell, this was accidentally omitted from rust-lang#47802.
Fixes rust-lang#62272.

r? @matthewjasper or @nikomatsakis
@ecstatic-morse ecstatic-morse mentioned this issue Nov 6, 2019
Merged
ecstatic-morse added a commit to ecstatic-morse/rust that referenced this issue Nov 9, 2019
@ecstatic-morse
Bless back-compat breakages
bc5ba5d 
This PR BREAKS CODE THAT WAS ACCEPTED ON STABLE. It's arguably a bug
that this was accepted in the first place, but here we are. See rust-lang#62272
for more info.
bors added a commit that referenced this issue Nov 11, 2019
@bors
Auto merge of #66170 - ecstatic-morse:hir-const-check, r=Centril,oli-obk
47904d2 
Add a HIR pass to check consts for `if`, `loop`, etc.

Resolves #66125.

This PR adds a HIR pass to check for high-level control flow constructs that are forbidden in a const-context. The MIR const-checker is unable to provide good spans for these since they are lowered to control flow primitives (e.g., `Goto` and `SwitchInt`), and these often don't map back to the underlying statement as a whole. This PR is intended only to improve diagnostics once `if` and `match` become commonplace in constants (behind a feature flag). The MIR const-checker will continue to operate unchanged, and will catch anything this check might miss.

In this implementation, the HIR const-checking pass is run much earlier than the MIR one, so it will supersede any errors from the latter. I will need some mentoring if we wish to change this, since I'm not familiar with the diagnostics system. Moving this pass into the same phase as the MIR const-checker could also help keep backwards compatibility for items like `const _: () = loop { break; };`, which are currently (erroneously?) accepted by the MIR const-checker (see #62272).

r? @Centril
cc @eddyb (since they filed #62272)
ecstatic-morse added a commit to ecstatic-morse/rust that referenced this issue Nov 13, 2019
@ecstatic-morse
Bless back-compat breakages
281e898 
This PR BREAKS CODE THAT WAS ACCEPTED ON STABLE. It's arguably a bug
that this was accepted in the first place, but here we are. See rust-lang#62272
for more info.
bors added a commit that referenced this issue Nov 13, 2019
@bors
Auto merge of #66170 - ecstatic-morse:hir-const-check, r=Centril,oli-obk
ded5ee0 
Add a HIR pass to check consts for `if`, `loop`, etc.

Resolves #66125.

This PR adds a HIR pass to check for high-level control flow constructs that are forbidden in a const-context. The MIR const-checker is unable to provide good spans for these since they are lowered to control flow primitives (e.g., `Goto` and `SwitchInt`), and these often don't map back to the underlying statement as a whole. This PR is intended only to improve diagnostics once `if` and `match` become commonplace in constants (behind a feature flag). The MIR const-checker will continue to operate unchanged, and will catch anything this check might miss.

In this implementation, the HIR const-checking pass is run much earlier than the MIR one, so it will supersede any errors from the latter. I will need some mentoring if we wish to change this, since I'm not familiar with the diagnostics system. Moving this pass into the same phase as the MIR const-checker could also help keep backwards compatibility for items like `const _: () = loop { break; };`, which are currently (erroneously?) accepted by the MIR const-checker (see #62272).

r? @Centril
cc @eddyb (since they filed #62272)
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A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html regression-from-stable-to-stable Performance or correctness regression from one stable version to another.
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rustc_mir: follow FalseUnwind's real_target edge in qualify_consts. eddyb/rust
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@eddyb @Centril