[WIP] Never type experiments #84573

Mark-Simulacrum · 2021-04-26T02:39:11Z

This currently just rebases #79470 (hopefully all correctly), but will "soon" follow up with an implementation as per rust-lang/lang-team#60 (comment).

r? @ghost

Mark-Simulacrum · 2021-04-26T23:02:34Z

As an update - I just pushed the new v2 fallback algorithm we had originally discussed; this currently doesn't quite pass tests in-tree but does address some of the known cases the v1 fallback algorithm failed at.

Niko and I spent a bit of time discussing possible solutions to the problems v2 brings up but didn't arrive at anything concrete yet.

bors · 2021-05-14T01:20:36Z

☔ The latest upstream changes (presumably #84107) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2021-05-18T23:26:31Z

☔ The latest upstream changes (presumably #84767) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2021-05-22T20:24:56Z

☔ The latest upstream changes (presumably #85078) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2021-05-25T08:51:18Z

☔ The latest upstream changes (presumably #84985) made this pull request unmergeable. Please resolve the merge conflicts.

Instead, we now rely on the code that looks for a NeverToAny adjustment.

This could cause us to visit the start node twice.

Extend the `DepthFirstSearch` iterator so that it can be re-used and extended with add'l start nodes. Then replace the FxHashSets of nodes we were using in the fallback analysis with a single iterator. This way we won't re-walk portions of the graph that are reached more than once, and we also do less allocation etc.

This avoids select_obligations_where_possible having an effect, which future proofs the algorithm.

This doesn't yet fully work -- running coercions seems to be prone to causing errors in the environment, which creates a little noise in some cases. It should be enough to let us re-run crater, though, I think.

rust-log-analyzer · 2021-08-30T22:36:31Z

The job x86_64-gnu-llvm-10 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Some tests failed in compiletest suite=mir-opt mode=mir-opt host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
..............................i.............F...................
failures:

---- [mir-opt] mir-opt/separate_const_switch.rs stdout ----
1 - // MIR for `identity` before SeparateConstSwitch
2 + // MIR for `identity` after SeparateConstSwitch
+   
+   
4   fn identity(_1: Result<i32, i32>) -> Result<i32, i32> {
5       debug x => _1;                       // in scope 0 at $DIR/separate_const_switch.rs:28:13: 28:14
6       let mut _0: std::result::Result<i32, i32>; // return place in scope 0 at $DIR/separate_const_switch.rs:28:37: 28:53
15       scope 1 {
15       scope 1 {
16           debug residual => _6;            // in scope 1 at $DIR/separate_const_switch.rs:29:9: 29:10
17           scope 2 {
-               scope 8 (inlined <Result<i32, i32> as FromResidual<Result<Infallible, i32>>>::from_residual) { // at $DIR/separate_const_switch.rs:29:8: 29:10
+               scope 8 (inlined <Result<i32, i32> as FromResidual<Result<!, i32>>>::from_residual) { // at $DIR/separate_const_switch.rs:29:8: 29:10
19                   debug residual => _8;    // in scope 8 at $DIR/separate_const_switch.rs:29:8: 29:10
20                   let _16: i32;            // in scope 8 at $DIR/separate_const_switch.rs:29:8: 29:10
21                   let mut _17: i32;        // in scope 8 at $DIR/separate_const_switch.rs:29:8: 29:10

49               debug e => _13;              // in scope 7 at $DIR/separate_const_switch.rs:29:8: 29:10
51       }
- 
+   
53       bb0: {
53       bb0: {
54           StorageLive(_2);                 // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
55           StorageLive(_3);                 // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10

60 -         switchInt(move _10) -> [0_isize: bb6, 1_isize: bb4, otherwise: bb5]; // scope 5 at $DIR/separate_const_switch.rs:29:8: 29:10
61 +         switchInt(move _10) -> [0_isize: bb5, 1_isize: bb3, otherwise: bb4]; // scope 5 at $DIR/separate_const_switch.rs:29:8: 29:10
- 
+   
64       bb1: {
64       bb1: {
65 -         StorageDead(_10);                // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
66 -         StorageDead(_4);                 // scope 0 at $DIR/separate_const_switch.rs:29:9: 29:10

67 -         _5 = discriminant(_3);           // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
68 -         switchInt(move _5) -> [0_isize: bb2, otherwise: bb3]; // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
69 -     }
+ - 
71 -     bb2: {
71 -     bb2: {
72           StorageLive(_9);                 // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
73           _9 = ((_3 as Continue).0: i32);  // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10

79           StorageDead(_3);                 // scope 0 at $DIR/separate_const_switch.rs:30:1: 30:2
80           return;                          // scope 0 at $DIR/separate_const_switch.rs:30:2: 30:2
- 
+   
83 -     bb3: {
84 +     bb2: {
84 +     bb2: {
85           StorageLive(_6);                 // scope 0 at $DIR/separate_const_switch.rs:29:9: 29:10

103           StorageDead(_3);                 // scope 0 at $DIR/separate_const_switch.rs:30:1: 30:2
104           return;                          // scope 0 at $DIR/separate_const_switch.rs:30:2: 30:2
- 
+   
107 -     bb4: {
108 +     bb3: {
108 +     bb3: {
109           StorageLive(_13);                // scope 5 at $DIR/separate_const_switch.rs:29:8: 29:10

124 +         _5 = discriminant(_3);           // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
125 +         switchInt(move _5) -> [0_isize: bb1, otherwise: bb2]; // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
- 
+   
128 -     bb5: {
129 +     bb4: {
129 +     bb4: {
130           unreachable;                     // scope 5 at $DIR/separate_const_switch.rs:29:8: 29:10
131       }
- 
+   
133 -     bb6: {
133 -     bb6: {
134 +     bb5: {
135           StorageLive(_11);                // scope 5 at $DIR/separate_const_switch.rs:29:8: 29:10

147 +         switchInt(move _5) -> [0_isize: bb1, otherwise: bb2]; // scope 0 at $DIR/separate_const_switch.rs:29:8: 29:10
149   }
- 
+   
151 
151 

thread '[mir-opt] mir-opt/separate_const_switch.rs' panicked at 'Actual MIR output differs from expected MIR output /checkout/src/test/mir-opt/separate_const_switch.identity.SeparateConstSwitch.diff', src/tools/compiletest/src/runtest.rs:3588:25


failures:
    [mir-opt] mir-opt/separate_const_switch.rs
    [mir-opt] mir-opt/separate_const_switch.rs

test result: FAILED. 160 passed; 1 failed; 3 ignored; 0 measured; 0 filtered out; finished in 5.24s



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/mir-opt" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/mir-opt" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--suite" "mir-opt" "--mode" "mir-opt" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-10/bin/FileCheck" "--nodejs" "/usr/bin/node" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python3" "--lldb-python" "/usr/bin/python3" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "10.0.0" "--llvm-components" "aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cfguard codegen core coroutines coverage debuginfocodeview debuginfodwarf debuginfogsym debuginfomsf debuginfopdb demangle dlltooldriver dwarflinker engine executionengine frontendopenmp fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo instcombine instrumentation interpreter ipo irreader jitlink lanai lanaiasmparser lanaicodegen lanaidesc lanaidisassembler lanaiinfo libdriver lineeditor linker lto mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts object objectyaml option orcerror orcjit passes perfjitevents powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvutils runtimedyld scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target textapi transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86utils xcore xcorecodegen xcoredesc xcoredisassembler xcoreinfo xray" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--channel" "nightly" "--color" "always"


Build completed unsuccessfully in 0:13:25

jackh726 · 2021-09-02T23:37:39Z

@Mark-Simulacrum what's the plan here? Are you planning to split out some of this into more PRs? I can definitely try to review some, especially code that was mostly written by Niko :)

Mark-Simulacrum · 2021-09-03T13:26:54Z

Yeah, the next steps are:

Split some more work out, probably, into smaller PRs -- likely will ask for your review on most of these
Document the algorithm in this PR (Niko's + some further adjustments since then)
T-lang discussion + potential stabilization report

Mark-Simulacrum · 2021-09-17T16:58:47Z

Going to go ahead and close this -- work is proceeding in #88804 and this isn't useful to keep open for now.

…matsakis,jackh726 Revise never type fallback algorithm This is a rebase of rust-lang#84573, but dropping the stabilization of never type (and the accompanying large test diff). Each commit builds & has tests updated alongside it, and could be reviewed in a more or less standalone fashion. But it may make more sense to review the PR as a whole, I'm not sure. It should be noted that tests being updated isn't really a good indicator of final behavior -- never_type_fallback is not enabled by default in this PR, so we can't really see the full effects of the commits here. This combines the work by Niko, which is [documented in this gist](https://gist.github.com/nikomatsakis/7a07b265dc12f5c3b3bd0422018fa660), with some additional rules largely derived to target specific known patterns that regress with the algorithm solely derived by Niko. We build these from an intuition that: * In general, fallback to `()` is *sound* in all cases * But, in general, we *prefer* fallback to `!` as it accepts more code, particularly that written to intentionally use `!` (e.g., Result's with a Infallible/! variant). When evaluating Niko's proposed algorithm, we find that there are certain cases where fallback to `!` leads to compilation failures in real-world code, and fallback to `()` fixes those errors. In order to allow for stabilization, we need to fix a good portion of these patterns. The final rule set this PR proposes is that, by default, we fallback from `?T` to `!`, with the following exceptions: 1. `?T: Foo` and `Bar::Baz = ?T` and `(): Foo`, then fallback to `()` 2. Per [Niko's algorithm](https://gist.github.com/nikomatsakis/7a07b265dc12f5c3b3bd0422018fa660#proposal-fallback-chooses-between--and--based-on-the-coercion-graph), the "live" `?T` also fallback to `()`. The first rule is necessary to address a fairly common pattern which boils down to something like the snippet below. Without rule 1, we do not see the closure's return type as needing a () fallback, which leads to compilation failure. ```rust #![feature(never_type_fallback)] trait Bar { } impl Bar for () { } impl Bar for u32 { } fn foo<R: Bar>(_: impl Fn() -> R) {} fn main() { foo(|| panic!()); } ``` r? `@jackh726`

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never type stabilization rust-lang/lang-team#60

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nikomatsakis and others added 8 commits August 30, 2021 17:55

stop categorizing inference variables as diverging when created

543b241

Instead, we now rely on the code that looks for a NeverToAny adjustment.

fix bug in DepthFirstSearch where start node was not visited

141246c

This could cause us to visit the start node twice.

stabilize never type

9a332c8

WIP: make Infallible an alias for !

be24f7b

v2 of fallback algo

7531f59

v2 algorithm, with the custom if in output obligations rule

26403a2

Adjust self-type to require equality

a9903a2

Mark-Simulacrum force-pushed the never-type-fallback-new branch from f8b5a32 to 6fd5dee Compare August 30, 2021 21:56

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Mark-Simulacrum added 5 commits August 30, 2021 18:20

Rebase tests

a963d8d

Remove breakage example

73e9259

clippy

c4532c0

Relate type variables during predicate registration

5a8bcf9

This avoids select_obligations_where_possible having an effect, which future proofs the algorithm.

Utilize coercion information to guide never type fallback

40d1e69

This doesn't yet fully work -- running coercions seems to be prone to causing errors in the environment, which creates a little noise in some cases. It should be enough to let us re-run crater, though, I think.

Mark-Simulacrum force-pushed the never-type-fallback-new branch from 6fd5dee to 40d1e69 Compare August 30, 2021 22:20

jackh726 added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 3, 2021

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Mark-Simulacrum mentioned this pull request Sep 10, 2021

Revise never type fallback algorithm #88804

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[WIP] Never type experiments #84573

[WIP] Never type experiments #84573

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