trigger unsized coercions keyed on Sized bounds #56219

arielb1 · Nov 25, 2018

This PR causes unsized coercions to not be disabled by $0: Unsize<dyn Object> coercion obligations when we have an $0: Sized obligation somewhere.

This should be mostly backwards-compatible, because in these cases not doing the unsize coercion should have caused the $0: Sized obligation to fail.

Note that X: Unsize<dyn Object> obligations can't fail as obligations if X: Sized holds, so this still maintains some version of monotonicity (I think that an unsized coercion can't be converted to no coercion by unifying type variables).

Fixes #49593 (unblocking never_type).

r? @eddyb
cc @nikomatsakis

eddyb · Nov 25, 2018

eddyb reviewed Nov 25, 2018

View changes

eddyb · Nov 25, 2018

src/librustc_typeck/check/mod.rs

+    }
+}
+
+/// FIXME: impl Trait why u give me lifetime errors?


What did you try? You might've had to use + Captures<'tcx> etc.

eddyb · Nov 25, 2018

LGTM, r? @nikomatsakis

Centril · Nov 25, 2018

Centril reviewed Nov 25, 2018

View changes

src/librustc_typeck/check/mod.rs

src/test/ui/coercion/coerce-issue-49593-box-never.rs

Centril · Nov 25, 2018

This should be mostly backwards-compatible, because in these cases not doing the unsize coercion should have caused the $0: Sized obligation to fail.

We should test that with a crater run?

arielb1 · Nov 25, 2018

What did you try? You might've had to use + Captures<'tcx> etc.

I first tried to have the function return impl Iterator<Item=ty::PolyTraitRef<'tcx>> + 'b. That gave me the following error:

error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds  
    --> src/librustc_typeck/check/mod.rs:2788:12
     |
2788 |         -> impl Iterator<Item=ty::PolyTraitRef<'tcx>> + 'b
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
note: hidden type `std::iter::FilterMap<std::vec::IntoIter<rustc::traits::Obligation<'tcx, rustc::ty::Predicate<'tcx>>>, check::ObligationMapper<'b, 'gcx, 'tcx>>` captures the lifetime 'gcx as defined on the impl at 2786:10

I then tried to use the impl Iterator<Item=ty::PolyTraitRef<'tcx>> + 'a + 'b + 'gcx + 'tcx syntax, which gave me:

error: unsatisfied lifetime constraints==========================>   ] 106/111: rustc_ty...  
    --> src/librustc_typeck/check/mod.rs:2788:12
     |
2786 | impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     |          ----  ---- lifetime `'tcx` defined here
     |          |
     |          lifetime `'gcx` defined here
2787 |     fn obligations_for_self_ty<'b>(&'b self, self_ty: ty::TyVid)
2788 |         -> impl Iterator<Item=ty::PolyTraitRef<'tcx>> + 'a + 'b + 'gcx + 'tcx
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'tcx` must outlive `'gcx`

error: unsatisfied lifetime constraints
    --> src/librustc_typeck/check/mod.rs:2795:9
     |
2786 |   impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     |        --  ---- lifetime `'gcx` defined here
     |        |
     |        lifetime `'a` defined here
...
2795 | /         self.fulfillment_cx
2796 | |             .borrow()
2797 | |             .pending_obligations()
2798 | |             .into_iter()
2799 | |             .filter_map(ObligationMapper(self, ty_var_root))
     | |____________________________________________________________^ returning this value requires that `'a` must outlive `'gcx`

error: unsatisfied lifetime constraints
    --> src/librustc_typeck/check/mod.rs:2795:9
     |
2786 |   impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     |        -- lifetime `'a` defined here
2787 |       fn obligations_for_self_ty<'b>(&'b self, self_ty: ty::TyVid)
     |                                  -- lifetime `'b` defined here
...
2795 | /         self.fulfillment_cx
2796 | |             .borrow()
2797 | |             .pending_obligations()
2798 | |             .into_iter()
2799 | |             .filter_map(ObligationMapper(self, ty_var_root))
     | |____________________________________________________________^ returning this value requires that `'b` must outlive `'a`

Of course, my original design had closures instead of a struct implementing FnMut, but when I didn't have issues with move they gave a similar set of errors.

@eddyb

It would be nice if you could pinpoint a particular shortcoming with impl Trait that led to this problem.

eddyb · Nov 25, 2018

You need to write something like this (see #34511 (comment) for more details):

impl Iterator<Item=ty::PolyTraitRef<'tcx>> + Captures<'gcx> + 'b

In general, add Captures<'x> for every "hidden type ... captures the lifetime 'x".

rust-highfive · Nov 25, 2018

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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travis_time:end:058361b1:start=1543182799014667170,finish=1543182853764650520,duration=54749983350
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
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32,"is_primary":true,"text":[{"text":"    mem::replace(&mut x, &mut y);","highlight_start":26,"highlight_end":32}],"label":null,"suggested_replacement":"y","suggestion_applicability":"MachineApplicable","expansion":null}],"children":[],"rendered":null}],"rendered":"error[E0308]: mismatched types\n  --> /checkout/src/test/ui/coercion/coerce-issue-49593-box-never.rs:58:26\n   |\nLL |     mem::replace(&mut x, &mut y);\n   |                          ^^^^^^\n   |                          |\n   |                          expected enum `std::option::Option`, found mutable reference\n   |                          help: consider removing the borrow: `y`\n   |\n   = note: expected type `std::option::Option<S>`\n              found type `&mut std::option::Option<S>`\n\n"}
[00:49:15] {"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error\n\n"}
[00:49:15] {"message":"For more information about this error, try `rustc --explain E0308`.","code":null,"level":"","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0308`.\n"}
[00:49:15] ------------------------------------------
[00:49:15] 
[00:49:15] thread '[ui] ui/coercion/coerce-issue-49593-box-never.rs' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:3282:9
[00:49:15] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:49:15] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:49:15] 
[00:49:15] 
[00:49:15] failures:
[00:49:15]     [ui] ui/coercion/coerce-issue-49593-box-never.rs
[00:49:15] 
[00:49:15] test result: FAILED. 5033 passed; 1 failed; 24 ignoredor target 'check' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0e4296a0
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Sun Nov 25 22:43:37 UTC 2018
---
travis_time:end:0109a2fb:start=1543185818701633785,finish=1543185818707235483,duration=5601698
travis_fold:end:after_failure.3
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travis_time:start:0826cc3e
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [

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arielb1 · Nov 27, 2018

So does this require a lang-team FCP?

Centril · Nov 27, 2018

@arielb1 I think if not FCP we should at least discuss it (and so I've nominated it) since this has user observable type system changes.

nikomatsakis · Nov 29, 2018

We had a discussion in the @rust-lang/lang meeting and concluded that we should go ahead with this change, at least in some form. Although we are now taking more things into consideration for coercions (namely, pending coercions) and that's a bit risky, it's nice to fix a real problem, and this doesn't seem like a "fundamental complexity change" in the coercion rules. They've always taken into account "information gained from type-checking thus far" and now we are just expanding that to the "complete set of information" (i.e. not just equality constraints, but arbitrary obligations).

I'd still personally like to spend some time tomorrow to think over some examples and try to be careful about precisely which obligations we consider, but the lang team is 👍 on the general direction thus far. =)

nikomatsakis · Nov 30, 2018

nikomatsakis approved these changes Nov 30, 2018

View changes

OK, I read over the changes, which look good.

My only real hesitation is whether we want to try to be "intensional" about the set of obligations we consider for this role -- i.e., to trace the set of obligations that arise from the surrounding context in the same way we trace the 'expected type'. I believe that the existing closure checking code, which also uses obligations, was sort of aiming for this sort of limitation by tying itself to the case where (a) you have an expected type of an inference variable and (b) you have obligations directly referencing that variable. Pretty hacky, though.

I want to experiment a bit with this branch -- which for example checks for unifications of the expected type inference variable -- to see which things work and which things don't, and if there's much real change. I'm building a local copy for this purpose.

nikomatsakis · Nov 30, 2018

src/librustc_typeck/check/closure.rs

-                                proj_predicate
-                            )
-                        })
+                    Some(()).filter(|()| {


What is the purpose of this construct? I suspect there's a clearer one out there =)

arielb1 · Dec 1, 2018

My only real hesitation is whether we want to try to be "intensional" about the set of obligations we consider for this role -- i.e., to trace the set of obligations that arise from the surrounding context in the same way we trace the 'expected type'. I believe that the existing closure checking code, which also uses obligations, was sort of aiming for this sort of limitation by tying itself to the case where (a) you have an expected type of an inference variable and (b) you have obligations directly referencing that variable. Pretty hacky, though.

Note that the obligations here are on the "found type" (i.e., the result of Box::new, which will be Box<$0> where $0: Sized), not on the expected type.

Consider cases like

fn let_expansion() -> Box<Error> {
    let x = Box::new(!);
    x /* should this work? */
}

My personal expectation is that this code will work (BTW, it does not work ATM because of subtyping (the "is sized" relationship doesn't propagate through that) - should I change that?), with x getting the type Box<!> - the "found type" information is not destroyed when you go out of a let.

(b) you have obligations directly referencing that variable

I added the ty_var_root normalization to the closure code too. Which type variable is selected as a root when unifying is sufficiently erratic that I don't think relying on it will do any good. If there's an edge case where you do want to rely on it, maybe a test/comment would be better?

bors · Dec 8, 2018

☔️ The latest upstream changes (presumably #56578) made this pull request unmergeable. Please resolve the merge conflicts.

cramertj · Dec 13, 2018

@nikomatsakis

I want to experiment a bit with this branch -- which for example checks for unifications of the expected type inference variable -- to see which things work and which things don't, and if there's much real change. I'm building a local copy for this purpose.

Ping on this-- did you have a chance to experiment? I'd love to see this land, since it unblocks both never_type and TryFrom/TryInto traits, AFAICT.

bors · Dec 16, 2018

☔️ The latest upstream changes (presumably #56875) made this pull request unmergeable. Please resolve the merge conflicts.

nikomatsakis · Dec 18, 2018

@bors r+

OK. I did another read through the code and gave a bit of thought to alternative approaches -- I decided that I'm game to land this, but I would really like us to prioritize modeling the Rust type checker and its coercions (I expect that to be done in a chalk-like setting). It feels really important that we have a good handle on what the compiler knows in order to make its judgements at any time etc.

With regard to the subtyping rule, @arielb1, I'd rather take it slow. Let's open a FIXME perhaps and consider it as a follow-up?

I think in my ideal world we would try to model the type-checker in a chalk-like style, so that we can express these coercions in that world (and ideally try to address the problem of over-eager coercions as well, although I'm not sure how easy that will be). This would probably be best to do in the context of the Traits Working Group, I guess. Seems like maybe a good Rust All Hands discussion topic.

bors · Dec 18, 2018

📌 Commit eeb8c8d has been approved by nikomatsakis

arielb1 · Dec 19, 2018

@bors r=nikomatsakis

bors · Dec 19, 2018

📌 Commit 5b74438 has been approved by nikomatsakis

bors · Dec 19, 2018

⌛️ Testing commit 5b74438 with merge 3b1845c...

bors · Dec 19, 2018

💔 Test failed - status-travis

rust-highfive · Dec 19, 2018

The job dist-i686-freebsd of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

Building stage2 tool cargo (i686-unknown-freebsd)
[01:03:50]  Downloading crates ...
[01:04:10] warning: spurious network error (2 tries remaining): [6] Couldn't resolve host name (Could not resolve host: crates.io)
[01:04:30] warning: spurious network error (1 tries remaining): [6] Couldn't resolve host name (Could not resolve host: crates.io)
[01:04:50] error: failed to download from `https://crates.io/api/v1/crates/openssl-src/111.1.0+1.1.1a/download`
[01:04:50] Caused by:
[01:04:50]   [6] Couldn't resolve host name (Could not resolve host: crates.io)
[01:04:50] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "i686-unknown-freebsd" "-j" "4" "--release" "--locked" "--color" "always" "--manifest-path" "/checkout/src/tools/cargo/Cargo.toml" "--features" "rustc-workspace-hack/all-static" "--message-format" "json"
[01:04:50] expected success, got: exit code: 101
---
travis_time:end:0a564182:start=1545201916212762147,finish=1545201916220909275,duration=8147128
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0da8fb4e
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:0e53a404
travis_time:start:0e53a404
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:05b40c91
$ dmesg | grep -i kill

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kennytm · Dec 19, 2018

@bors retry travis-ci/travis-ci#9696

bors · Dec 19, 2018

⌛️ Testing commit 5b74438 with merge cb5e1d3...

bors · Dec 19, 2018

💔 Test failed - status-travis

rust-highfive · Dec 19, 2018

The job dist-x86_64-apple-alt of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:02:24]       Memory: 8 GB
[00:02:24]       Boot ROM Version: VMW71.00V.0.B64.1704110547
[00:02:24]       Apple ROM Info: [MS_VM_CERT/SHA1/27d66596a61c48dd3dc7216fd715126e33f59ae7]Welcome to the Virtual Machine
[00:02:24]       SMC Version (system): 2.8f0
[00:02:24]       Serial Number (system): VMFGEGuV3OlF
[00:02:24] 
[00:02:24] hw.ncpu: 4
[00:02:24] hw.byteorder: 1234
[00:02:24] hw.memsize: 8589934592
---
Building stage2 tool cargo (x86_64-apple-darwin)
[00:46:48]  Downloading crates ...
[00:46:48] warning: spurious network error (2 tries remaining): [6] Couldn't resolve host name (Could not resolve host: crates.io)
[00:46:48] warning: spurious network error (1 tries remaining): [6] Couldn't resolve host name (Could not resolve host: crates.io)
[00:46:48] error: failed to download from `https://crates.io/api/v1/crates/openssl-src/111.1.0+1.1.1a/download`
[00:46:48] Caused by:
[00:46:48]   [6] Couldn't resolve host name (Could not resolve host: crates.io)
[00:46:48] command did not execute successfully: "/Users/travis/build/rust-lang/rust/build/x86_64-apple-darwin/stage0/bin/cargo" "build" "--target" "x86_64-apple-darwin" "-j" "4" "--release" "--locked" "--color" "always" "--manifest-path" "/Users/travis/build/rust-lang/rust/src/tools/cargo/Cargo.toml" "--features" "rustc-workspace-hack/all-static" "--message-format" "json"
[00:46:48] expected success, got: exit code: 101
[00:46:48] expected success, got: exit code: 101
[00:46:48] failed to run: /Users/travis/build/rust-lang/rust/build/bootstrap/debug/bootstrap build
[00:46:48] Build completed unsuccessfully in 0:43:24
[00:46:48] make: *** [all] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:12ae13ad
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Dec 19 18:36:16 GMT 2018
---
travis_fold:start:after_failure.2
travis_time:start:0f182781
$ ls -lat $HOME/Library/Logs/DiagnosticReports/
total 0
drwx------+ 15 travis  staff  510 Jan 25  2018 ..
drwx------   2 travis  staff   68 Dec  6  2017 .
travis_fold:end:after_failure.2
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travis_time:start:16177b9b
$ find $HOME/Library/Logs/DiagnosticReports -type f -name '*.crash' -not -name '*.stage2-*.crash' -not -name 'com.apple.CoreSimulator.CoreSimulatorService-*.crash' -exec printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" {} \; -exec head -750 {} \; -exec echo travis_fold":"end:crashlog \; || true
$ find $HOME/Library/Logs/DiagnosticReports -type f -name '*.crash' -not -name '*.stage2-*.crash' -not -name 'com.apple.CoreSimulator.CoreSimulatorService-*.crash' -exec printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" {} \; -exec head -750 {} \; -exec echo travis_fold":"end:crashlog \; || true
travis_time:end:16177b9b:start=1545244582136754000,finish=1545244582150623000,duration=13869000
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:1e679175
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:05a04a88
travis_time:start:05a04a88
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:0a9c5e6e
$ dmesg | grep -i kill
$ dmesg | grep -i kill
Unable to obtain kernel buffer: Operation not permitted
usage: sudo dmesg
travis_fold:end:after_failure.6

Done. Your build exited with 1.

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arielb1 · Dec 19, 2018

[00:46:48] error: failed to download from `https://crates.io/api/v1/crates/openssl-src/111.1.0+1.1.1a/download`
[00:46:48] Caused by:
[00:46:48]   [6] Couldn't resolve host name (Could not resolve host: crates.io)

@bors retry

bors · Dec 20, 2018

⌛️ Testing commit 5b74438 with merge e42247f...

bors · Dec 20, 2018

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing e42247f to master...

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NLL: User type annotations refactor, associated constant patterns and ref bindings. #55937

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trigger unsized coercions keyed on Sized bounds #56219

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arielb1 commented Nov 25, 2018 •

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arielb1 force-pushed the arielb1:never-coerce-box branch 2 times, most recently from `116ecff` to `33db7c0` Nov 25, 2018

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arielb1 commented Dec 1, 2018 •

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arielb1 force-pushed the arielb1:never-coerce-box branch from `c0e0d82` to `98faeb5` Dec 15, 2018

arielb1 force-pushed the arielb1:never-coerce-box branch from `98faeb5` to `eeb8c8d` Dec 16, 2018

bors merged commit `5b74438` into rust-lang:master Dec 20, 2018
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