Exhaustiveness: reveal opaque types properly

[Nadrieril]

Previously, exhaustiveness had no clear policy around opaque types. In this PR I propose the following policy: within the body of an item that defines the hidden type of some opaque type, exhaustiveness checking on a value of that opaque type is performed using the concrete hidden type inferred in this body.

I'm not sure how consistent this is with other operations allowed on opaque types; I believe this will require FCP.

From what I can tell, this doesn't change anything for non-empty types.

The observable changes are:

• when the real type is uninhabited, matches within the defining scopes can now rely on that for exhaustiveness, e.g.:

#[derive(Copy, Clone)]
enum Void {}
fn return_never_rpit(x: Void) -> impl Copy {
    if false {
        match return_never_rpit(x) {}
    }
    x
}

• this properly fixes ICEs like ICE: Unexpected type for 'Single' constructor #117100 that occurred because a same match could have some patterns where the type is revealed and some where it is not.

Bonus subtle point: if x is opaque, a match like match x { ("", "") => {} ... } will constrain its type (playground). This is not the case for match x {}: this will not constain the type, and will only compile if something else constrains the type to be empty.

Fixes #117100

r? @oli-obk

Edited for precision of the wording

Included in the FCP on this PR is this rule:

Within the body of an item that defines the hidden type of some opaque type, exhaustiveness checking on a value of that opaque type is performed using the concrete hidden type inferred in this body.

[Nadrieril]

@oli-obk I explained what I'm proposing in the OP, and added a bunch of tests.

@rustbot ready

[oli-obk]

Nominating for T-lang discussion, as this allows more code to compile (both on stable, but especially on nightly around type-alias-impl-trait). See the main post for an explanation of this change along with a code example that now compiles, but didn't before

[Nadrieril]

I added a remark to the OP regarding the fact that match x {} is not used by type inference to constrain an opaque type (as one could perhaps expect).

[Nadrieril]

At the suggestion of @oli-obk I now reveal opaque types in depth, which solves an inconsistency I had. I've updated the OP.

[bors]

☔ The latest upstream changes (presumably #117564) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

r? @compiler-errors

[compiler-errors]

Implementation looks fine to me. Marking as waiting-on-fcp.

[bors]

⌛ Testing commit 2a87bae with merge 649f17f...

[rust-log-analyzer]

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

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

failures:

---- [rustdoc] tests\rustdoc\index-page.rs stdout ----

error: rustdoc failed!
status: exit code: 0xc00000fd
command: PATH="C:\a\rust\rust\build\x86_64-pc-windows-gnu\stage2\bin;C:\a\rust\rust\build\x86_64-pc-windows-gnu\stage0-bootstrap-tools\x86_64-pc-windows-gnu\release\deps;C:\a\rust\rust\build\x86_64-pc-windows-gnu\stage0\bin;C:\a\rust\rust\ninja;C:\a\rust\rust\mingw64\bin;C:\hostedtoolcache\windows\Python\3.12.1\x64\Scripts;C:\hostedtoolcache\windows\Python\3.12.1\x64;C:\msys64\usr\bin;C:\a\rust\rust\sccache;C:\Program Files\MongoDB\Server\5.0\bin;C:\aliyun-cli;C:\vcpkg;C:\cf-cli;C:\Program Files (x86)\NSIS;C:\tools\zstd;C:\Program Files\Mercurial;C:\hostedtoolcache\windows\stack\2.13.1\x64;C:\cabal\bin;C:\ghcup\bin;C:\mingw64\bin;C:\Program Files\dotnet;C:\Program Files\MySQL\MySQL Server 5.7\bin;C:\Program Files\R\R-4.3.2\bin\x64;C:\SeleniumWebDrivers\GeckoDriver;C:\SeleniumWebDrivers\EdgeDriver;C:\SeleniumWebDrivers\ChromeDriver;C:\Program Files (x86)\sbt\bin;C:\Program Files (x86)\GitHub CLI;C:\Program Files\Git\bin;C:\Program Files (x86)\pipx_bin;C:\npm\prefix;C:\hostedtoolcache\windows\go\1.20.12\x64\bin;C:\hostedtoolcache\windows\Python\3.7.9\x64\Scripts;C:\hostedtoolcache\windows\Python\3.7.9\x64;C:\hostedtoolcache\windows\Ruby\2.5.9\x64\bin;C:\Program Files\OpenSSL\bin;C:\tools\kotlinc\bin;C:\hostedtoolcache\windows\Java_Temurin-Hotspot_jdk\8.0.392-8\x64\bin;C:\Program Files\ImageMagick-7.1.1-Q16-HDRI;C:\Program Files\Microsoft SDKs\Azure\CLI2\wbin;C:\ProgramData\kind;C:\Program Files\Eclipse Foundation\jdk-8.0.302.8-hotspot\bin;C:\ProgramData\Chocolatey\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0;C:\Windows\System32\OpenSSH;C:\Program Files\PowerShell\7;C:\Program Files\Microsoft\Web Platform Installer;C:\Program Files\Microsoft SQL Server\130\Tools\Binn;C:\Program Files\Microsoft SQL Server\Client SDK\ODBC\170\Tools\Binn;C:\Program Files (x86)\Windows Kits\10\Windows Performance Toolkit;C:\Program Files (x86)\Microsoft SQL Server\110\DTS\Binn;C:\Program Files (x86)\Microsoft SQL Server\120\DTS\Binn;C:\Program Files (x86)\Microsoft SQL Server\130\DTS\Binn;C:\Program Files (x86)\Microsoft SQL Server\140\DTS\Binn;C:\Program Files (x86)\Microsoft SQL Server\150\DTS\Binn;C:\Program Files (x86)\Microsoft SQL Server\160\DTS\Binn;C:\Strawberry\c\bin;C:\Strawberry\perl\site\bin;C:\Strawberry\perl\bin;C:\ProgramData\chocolatey\lib\pulumi\tools\Pulumi\bin;C:\Program Files\TortoiseSVN\bin;C:\Program Files\CMake\bin;C:\ProgramData\chocolatey\lib\maven\apache-maven-3.8.7\bin;C:\Program Files\Microsoft Service Fabric\bin\Fabric\Fabric.Code;C:\Program Files\Microsoft SDKs\Service Fabric\Tools\ServiceFabricLocalClusterManager;C:\Program Files\nodejs;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program Files\Git\usr\bin;C:\Program Files\GitHub CLI;C:\tools\php;C:\Program Files (x86)\sbt\bin;C:\Program Files\Amazon\AWSCLIV2;C:\Program Files\Amazon\SessionManagerPlugin\bin;C:\Program Files\Amazon\AWSSAMCLI\bin;C:\Program Files (x86)\Google\Cloud SDK\google-cloud-sdk\bin;C:\Program Files (x86)\Microsoft BizTalk Server;C:\Program Files\LLVM\bin;C:\Users\runneradmin\.dotnet\tools;C:\Users\runneradmin\.cargo\bin;C:\Users\runneradmin\AppData\Local\Microsoft\WindowsApps" "C:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\stage2\\bin\\rustdoc.exe" "-L" "C:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\stage2\\lib\\rustlib\\x86_64-pc-windows-gnu\\lib" "-L" "C:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\test\\rustdoc\\index-page\\auxiliary" "-o" "C:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\test\\rustdoc\\index-page" "--deny" "warnings" "C:\\a\\rust\\rust\\tests\\rustdoc\\index-page.rs" "-A" "internal_features" "-Z" "unstable-options" "--enable-index-page"
--- stderr -------------------------------

thread 'main' has overflowed its stack
------------------------------------------

[bors]

💔 Test failed - checks-actions

[Nadrieril]

The error does not seem related to this PR (and I can't reproduce it)

@bors retry

[bors]

⌛ Testing commit 2a87bae with merge c1fc1d1...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing c1fc1d1 to master...

[rust-timer]

Finished benchmarking commit (c1fc1d1): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

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	mean	range	count
Regressions ❌ (primary)	3.9%	[3.9%, 3.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.9%	[3.9%, 3.9%]	1

Cycles

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Binary size

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Bootstrap: 673.496s -> 674.428s (0.14%)
Artifact size: 312.79 MiB -> 312.81 MiB (0.00%)