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bors bd53aa3bf7 Auto merge of #129317 - compiler-errors:expectation-subtyping, r=lcnr 
Use equality when relating formal and expected type in arg checking

#129059 uncovered an interesting issue in argument checking. When we check arguments, we create three sets of types:
* Formals
* Expected
* Actuals

The **actuals** are the types of the argument expressions themselves. The **formals** are the types from the signature that we're checking. The **expected** types are the formal types, but passed through `expected_inputs_for_expected_outputs`:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs (L691-L725)

This method attempts to constrain the formal inputs by relating the [expectation](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/expectation/enum.Expectation.html) of the call expression and the formal output.

When we check an argument, we get the expression's actual type, and then we first attempt to coerce it to the expected type:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs (L280-L293)

Then we subtype the expected type and the formal type:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs (L299-L305)

However, since we are now recording the right coercion target (since #129059), we now end up recording the expected type to the typeck results, rather than the actual.

Since that expected type was [fudged](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.fudge_inference_if_ok), it has fresh variables. And since the expected type is only subtyped against the formal type, if that expected type has a bivariant parameter, it will likely remain unconstrained since `Covariant * Bivariant = Bivariant` according to [xform](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.Variance.html#method.xform). This leads to an unconstrained type variable in writeback.

AFAICT, there's no reason for us to be using subtyping here, though. The expected output is already related to the expectation by subtyping:

a971212545/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs (L713)

So the formals don't need "another" indirection of subtyping in the argument checking... So I've changed it to use equality here. We could alternatively fix this by requiring WF for all the expected types to constrain their bivariant parameters, but this seems a bit overkill.

Fixes #129286
2024-09-02 16:08:50 +00:00
.github Update library/Cargo.toml in weekly job 2024-08-19 07:30:11 -05:00
compiler Auto merge of #129317 - compiler-errors:expectation-subtyping, r=lcnr 2024-09-02 16:08:50 +00:00
library Auto merge of #129873 - matthiaskrgr:rollup-bv849ud, r=matthiaskrgr 2024-09-02 13:41:42 +00:00
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src Auto merge of #129873 - matthiaskrgr:rollup-bv849ud, r=matthiaskrgr 2024-09-02 13:41:42 +00:00
tests Auto merge of #129317 - compiler-errors:expectation-subtyping, r=lcnr 2024-09-02 16:08:50 +00:00
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.git-blame-ignore-revs Exclude the copy blessing from git blame 2024-08-18 16:09:03 -07:00
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.gitignore gitignore: ignore ICE reports regardless of directory 2024-08-25 14:19:05 +03:00
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Cargo.lock Rollup merge of #129619 - sunshowers:update-stacker, r=Mark-Simulacrum 2024-09-01 03:58:04 +02:00
Cargo.toml Switch to using the v2 resolver in most workspaces 2024-08-18 13:59:09 -05:00
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config.example.toml document miri and cargo-miri in build.tools 2024-08-22 08:32:39 +03:00
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RELEASES.md Remove the relnote for rustdoc's *unstable* --remap-path-prefix 2024-08-30 17:56:17 -07:00
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triagebot.toml Mark myself as on vacation for triagebot 2024-08-31 21:24:47 -05:00
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README.md

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