Allow trait methods to be called on concrete types in a const context #68847

ecstatic-morse · 2020-02-05T05:10:43Z

This partially implements RFC 2632 by const-checking methods inside an impl const block and allowing those methods to be called on concrete types. Calling trait methods on type parameters in a const context is not yet allowed. Implementing this will require much more work. Since we are only concerned with methods on concrete types, we are able to take advantage of the machinery in Instance::resolve, which is doing most of the work.

This also propagates #[rustc_const_unstable] from parent items to child items, making that attribute behave like #[stable] and #[unstable] do. This allows trait methods to be marked as unstably const.

cc #67792 #57563
cc @rust-lang/wg-const-eval
r? @oli-obk

oli-obk · 2020-02-05T09:09:42Z

src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method.rs

+    fn plus(self, rhs: Self) -> Self;
+}
+
+impl const Plus for i32 {


Would this test work the same if the Add trait were used instead of Plus?

Yes. I'll add a type with a const impl of Add to this test. Obviously I can't do this directly on i32 outside the standard library without conflicting impls. Note that, with the current implementation, it's possible to have const unstable trait impls even though it's not possible to have unstable ones. This means we could have const unstable implementations of arithmetic traits for the primitive types. I will try to find an approach to trait solving that preserves this property.

Note that, with the current implementation, it's possible to have const unstable trait impls even though it's not possible to have unstable ones. This means we could have const unstable implementations of arithmetic traits for the primitive types. I will try to find an approach to trait solving that preserves this property.

Oh wow, that would be a neat feature

ecstatic-morse · 2020-02-08T19:43:05Z

@oli-obk, I'm conflicted about this PR. On the one hand, it allows a lot of useful operations within const initializers on nightly. Notably, you can use the various arithmetic operators on custom types. On the other, it's a hack that can't reason about generic types at all. For now, associated type bounds within const impls are not required to be const, violating the RFC, although actually trying to call methods on associated types will always fail in a const context. Perhaps this is okay since this feature gate is marked experimental.

The next step is to teach the trait solver and method resolution about the constness of impls. This will require a lot of small changes to performance-sensitive code, so it will take a while.

oli-obk · 2020-02-10T15:17:33Z

For now, associated type bounds within const impls are not required to be const, violating the RFC, although actually trying to call methods on associated types will always fail in a const context. Perhaps this is okay since this feature gate is marked experimental.

Yea that's ok. You can mark the feature as partially implemented to make sure users get a warning.

I'm conflicted about this PR. On the one hand, it allows a lot of useful operations within const initializers on nightly. Notably, you can use the various arithmetic operators on custom types. On the other, it's a hack that can't reason about generic types at all.

Can you elaborate on the

can't reason about generic types at all.

part? Do you mean const fn foo<T: Add>(arg: SomeStruct<T>) { arg + arg; } won't work even if there's an Add impl for SomeStruct<T: Arg>?

ecstatic-morse · 2020-02-10T19:28:58Z

Do you mean const fn foo<T: Add>(arg: SomeStruct) { arg + arg; } won't work even if there's an Add impl for SomeStruct<T: Arg>?

I thought that Instance::resolve only worked when the types were fully monomorphized, but apparently that's not quite right. Clearly I need more tests. I added a passing one for calling <Struct<T> as ops::Add>::add. Calling a trait method directly on T doesn't work, however.

src/test/ui/rfc-2632-const-trait-impl/stability.rs

oli-obk · 2020-02-11T14:39:28Z

r=me with a stability test for the above worry

bors · 2020-02-13T12:56:53Z

☔ The latest upstream changes (presumably #69023) made this pull request unmergeable. Please resolve the merge conflicts.

ecstatic-morse · 2020-02-15T22:12:59Z

#69036 will change the effects of this, so I'll hold off until that is merged. cc @eddyb who will probably not be happy with Instance::resolve being used in this way.

eddyb · 2020-02-16T01:12:58Z

This seems straight-forward and the use of Instance::resolve looks correct.
However, I think you should make sure that the ParamEnv is UserFacing, not RevealAll, because you want maximally conservative behavior around specialization - similar to associated types in typeck.

cc @nikomatsakis

src/librustc_mir/transform/check_consts/validation.rs

eddyb · 2020-02-16T01:14:49Z

src/librustc_mir/transform/check_consts/validation.rs

+                // See if this is a trait method for a concrete type whose impl of that trait is
+                // `const`.
+                if self.tcx.features().const_trait_impl {
+                    let instance = Instance::resolve(self.tcx, self.param_env, def_id, substs);


As long as self.param_env is tcx.param_env(some_def_id) without calling .with_reveal_all() or w/e, it should be fine.

rust-highfive · 2020-02-18T23:53:04Z

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2020-02-18T23:48:37.4540883Z ========================== Starting Command Output ===========================
2020-02-18T23:48:37.4542718Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/e4b4bba3-5650-4953-b8aa-89b99984d0f5.sh
2020-02-18T23:48:37.4542760Z 
2020-02-18T23:48:37.4546093Z ##[section]Finishing: Disable git automatic line ending conversion
2020-02-18T23:48:37.4555315Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68847/merge to s
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2020-02-18T23:48:37.4557450Z Version      : 1.0.0
2020-02-18T23:48:37.4557546Z Author       : Microsoft
---
2020-02-18T23:48:38.4294324Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-02-18T23:48:38.4305883Z ##[command]git config gc.auto 0
2020-02-18T23:48:38.4308826Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-02-18T23:48:38.4311006Z ##[command]git config --get-all http.proxy
2020-02-18T23:48:38.4317879Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/68847/merge:refs/remotes/pull/68847/merge
---
2020-02-18T23:52:51.3785679Z extracting /checkout/obj/build/cache/2020-01-31/rustfmt-nightly-x86_64-unknown-linux-gnu.tar.gz
2020-02-18T23:52:51.4510270Z error: failed to resolve patches for `https://github.com/rust-lang/crates.io-index`
2020-02-18T23:52:51.4510424Z 
2020-02-18T23:52:51.4510518Z Caused by:
2020-02-18T23:52:51.4510977Z   patch for `rustfmt-nightly` in `https://github.com/rust-lang/crates.io-index` did not resolve to any crates. If this is unexpected, you may wish to consult: https://github.com/rust-lang/cargo/issues/4678
2020-02-18T23:52:51.4521301Z Build completed unsuccessfully in 0:00:23
2020-02-18T23:52:51.4563651Z == clock drift check ==
2020-02-18T23:52:51.4570851Z   local time: Tue Feb 18 23:52:51 UTC 2020
2020-02-18T23:52:51.7443716Z   network time: Tue, 18 Feb 2020 23:52:51 GMT
2020-02-18T23:52:51.7443716Z   network time: Tue, 18 Feb 2020 23:52:51 GMT
2020-02-18T23:52:51.7448434Z == end clock drift check ==
2020-02-18T23:52:59.2789069Z 
2020-02-18T23:52:59.2904004Z ##[error]Bash exited with code '1'.
2020-02-18T23:52:59.2919995Z ##[section]Finishing: Run build
2020-02-18T23:52:59.2937350Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68847/merge to s
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2020-02-18T23:52:59.2939801Z Version      : 1.0.0
2020-02-18T23:52:59.2939847Z Author       : Microsoft
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The job x86_64-gnu-llvm-7 of your PR failed (pretty log, 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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2020-02-19T05:04:24.5104204Z ========================== Starting Command Output ===========================
2020-02-19T05:04:24.5106760Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/ff622616-d5c4-4e72-ba7a-ca54359cbd47.sh
2020-02-19T05:04:24.5106795Z 
2020-02-19T05:04:24.5109436Z ##[section]Finishing: Disable git automatic line ending conversion
2020-02-19T05:04:24.5114526Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68847/merge to s
2020-02-19T05:04:24.5116166Z Task         : Get sources
2020-02-19T05:04:24.5116195Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-02-19T05:04:24.5116239Z Version      : 1.0.0
2020-02-19T05:04:24.5116267Z Author       : Microsoft
---
2020-02-19T05:04:25.5540912Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-02-19T05:04:25.5637773Z ##[command]git config gc.auto 0
2020-02-19T05:04:25.5718557Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-02-19T05:04:25.5774648Z ##[command]git config --get-all http.proxy
2020-02-19T05:04:25.5930823Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/68847/merge:refs/remotes/pull/68847/merge
---
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---
2020-02-19T06:05:28.2027549Z 4 LL |     fn foo() { non_const() }
2020-02-19T06:05:28.2027903Z -    |     ^^^^^^^^^^^^^^^^^^^^^^^^
2020-02-19T06:05:28.2028063Z +    |                ^^^^^^^^^^^
2020-02-19T06:05:28.2028190Z 6    |
2020-02-19T06:05:28.2028708Z 7    = note: see issue #57563 <***/issues/57563> for more information
2020-02-19T06:05:28.2028897Z 8    = help: add `#![feature(const_fn)]` to the crate attributes to enable
2020-02-19T06:05:28.2029144Z 
2020-02-19T06:05:28.2029271Z The actual stderr differed from the expected stderr.
2020-02-19T06:05:28.2029744Z Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl/const-check-fns-in-const-impl.stderr
2020-02-19T06:05:28.2029744Z Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl/const-check-fns-in-const-impl.stderr
2020-02-19T06:05:28.2030118Z To update references, rerun the tests and pass the `--bless` flag
2020-02-19T06:05:28.2030548Z To only update this specific test, also pass `--test-args rfc-2632-const-trait-impl/const-check-fns-in-const-impl.rs`
2020-02-19T06:05:28.2030831Z error: 1 errors occurred comparing output.
2020-02-19T06:05:28.2031183Z status: exit code: 1
2020-02-19T06:05:28.2031183Z status: exit code: 1
2020-02-19T06:05:28.2032291Z command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-A" "unused" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl/auxiliary"
2020-02-19T06:05:28.2033223Z ------------------------------------------
2020-02-19T06:05:28.2033392Z 
2020-02-19T06:05:28.2033712Z ------------------------------------------
2020-02-19T06:05:28.2033884Z stderr:
2020-02-19T06:05:28.2033884Z stderr:
2020-02-19T06:05:28.2034215Z ------------------------------------------
2020-02-19T06:05:28.2034395Z error[E0723]: can only call other `const fn` within a `const fn`, but `const non_const` is not stable as `const fn`
2020-02-19T06:05:28.2034772Z   --> /checkout/src/test/ui/rfc-2632-const-trait-impl/const-check-fns-in-const-impl.rs:12:16
2020-02-19T06:05:28.2034963Z    |
2020-02-19T06:05:28.2035092Z LL |     fn foo() { non_const() }
2020-02-19T06:05:28.2035233Z    |                ^^^^^^^^^^^
2020-02-19T06:05:28.2035360Z    |
2020-02-19T06:05:28.2035941Z    = note: see issue #57563 <***/issues/57563> for more information
2020-02-19T06:05:28.2036139Z    = help: add `#![feature(const_fn)]` to the crate attributes to enable
2020-02-19T06:05:28.2036390Z error: aborting due to previous error
2020-02-19T06:05:28.2036677Z 
2020-02-19T06:05:28.2037024Z For more information about this error, try `rustc --explain E0723`.
2020-02-19T06:05:28.2037170Z 
---
2020-02-19T06:05:28.2042168Z thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:348:22
2020-02-19T06:05:28.2042418Z note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2020-02-19T06:05:28.2052519Z 
2020-02-19T06:05:28.2052742Z 
2020-02-19T06:05:28.2057951Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-7/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "7.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
2020-02-19T06:05:28.2059280Z 
2020-02-19T06:05:28.2059309Z 
2020-02-19T06:05:28.2090894Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
2020-02-19T06:05:28.2091004Z Build completed unsuccessfully in 0:55:29
2020-02-19T06:05:28.2091004Z Build completed unsuccessfully in 0:55:29
2020-02-19T06:05:28.2124900Z == clock drift check ==
2020-02-19T06:05:28.2152430Z   local time: Wed Feb 19 06:05:28 UTC 2020
2020-02-19T06:05:28.5111936Z   network time: Wed, 19 Feb 2020 06:05:28 GMT
2020-02-19T06:05:28.5112035Z == end clock drift check ==
2020-02-19T06:05:28.9678273Z 
2020-02-19T06:05:28.9768522Z ##[error]Bash exited with code '1'.
2020-02-19T06:05:28.9780643Z ##[section]Finishing: Run build
2020-02-19T06:05:28.9800359Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68847/merge to s
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2020-02-19T06:05:29.3806818Z Terminate orphan process: pid (3582) (python)
2020-02-19T06:05:29.3993593Z ##[section]Finishing: Finalize Job

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oli-obk · 2020-02-19T13:26:30Z

r=me with tests blessed

ecstatic-morse · 2020-02-19T22:53:37Z

@bors r=oli-obk

bors · 2020-02-19T22:53:38Z

📌 Commit 19801b1 has been approved by oli-obk

bors · 2020-02-20T08:41:24Z

⌛ Testing commit 19801b1 with merge 6af388b...

bors · 2020-02-20T12:06:04Z

☀️ Test successful - checks-azure
Approved by: oli-obk
Pushing 6af388b to master...

ecstatic-morse added A-const-eval Area: Constant evaluation (MIR interpretation) F-const_trait_impl `#![feature(const_trait_impl)]` labels Feb 5, 2020

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ecstatic-morse changed the title ~~[WIP] Allow trait methods to be called on concrete types in a const context~~ Allow trait methods to be called on concrete types in a const context Feb 7, 2020

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ecstatic-morse force-pushed the const-impl branch from f0cc77e to cede204 Compare February 18, 2020 23:48

ecstatic-morse added 7 commits February 18, 2020 19:27

Add is_const_impl_raw query

62ff11f

Const-check functions in a const impl

3e0b060

Remove "not yet implemented" warning

5e422ef

Check for trait methods on concrete types in const checking

7a019b1

Add tests for calling trait methods on concrete types

323ff19

Split const trait method test and impl ops::Add

d6d6d25

Ensure const impl cannot coexist with non-const impl

70f7879

ecstatic-morse added 8 commits February 18, 2020 21:03

Use early return when forbidding unstable attrs

0a5abca

Propagate rustc_const_unstable to children

8e7609b

Test rustc_const_unstable on trait fns

4992eb2

Add ignored test for associated types in const impl

c8f0abb

Remove outdated FIXME

9a36824

Add passing test for Add on generic struct

160e630

Make fn_queries helpers module-private

cb81712

Prevent const trait methods from being marked stable

5f06ce2

ecstatic-morse force-pushed the const-impl branch from cede204 to a06734d Compare February 19, 2020 05:03

Update tests

19801b1

ecstatic-morse force-pushed the const-impl branch from a06734d to 19801b1 Compare February 19, 2020 17:40

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 19, 2020

bors added the merged-by-bors This PR was explicitly merged by bors. label Feb 20, 2020

bors merged commit 6af388b into rust-lang:master Feb 20, 2020

calebcartwright mentioned this pull request Mar 14, 2020

rustfmt silently deletes the word const when it appears before impl while using the experimental const_trait_impl feature rust-lang/rustfmt#4084

Closed

This was referenced May 7, 2020

const fn in Trait implementation. #71971

Closed

Constant operator overloading for bitflags bitflags/bitflags#217

Closed

ecstatic-morse deleted the const-impl branch October 6, 2020 01:42

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Allow trait methods to be called on concrete types in a const context #68847

Allow trait methods to be called on concrete types in a const context #68847

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Allow trait methods to be called on concrete types in a const context #68847

Allow trait methods to be called on concrete types in a const context #68847

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