Propagate deref coercion into block

[ldm0]

fn main() {
    // This already compiles
    let _: &i32 = &Box::new(1i32);
    // This doesn't compile until this PR.
    let _: &i32 = & { Box::new(1i32) };
}

Above code doesn't compiles because we apply check_expr_with_expectation() on { Box::new(1i32) } with ExpectHasType(i32). Which is a hard constraint and won't success without deref coercion.

This PR taks the same path as https://github.com/rust-lang/rust/pull/20083 (remove the hard constraint on check_expr_with_expectation when meets a unsized type). When the compiler see a AddrOf(Foo(...))(e.g. &{ Box::new(1i32) }), removes the hard constraint that this Foo have some type.

Fixes #26978 Fixes #57749 Fixes #83783

[rust-highfive]

r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

[petrochenkov]

This is no my area of expertise, and also a language change. r? @nikomatsakis @eddyb (because coercions)

[eddyb]

Wouldn't this lead to inference errors? I'm surprised we don't have tests that trip. But maybe it's special-cased enough to work. I'll defer to @nikomatsakis either way.

[nikomatsakis]

Sorry @ldm0 ! I'll try to give useful feedback ASAP

[lambda-fairy]

Wouldn't this lead to inference errors?

Is this something that we can figure out with a crater run?

[jyn514]

Hi @nikomatsakis , do you know when you'll get a chance to look at this?

[nikomatsakis]

I'm reviewing this now.

[nikomatsakis]

Apologies for the very long delay.

[bors]

:umbrella: The latest upstream changes (presumably #92007) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

:umbrella: The latest upstream changes (presumably #93148) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

Sorry @ldm0, got underwater again, but coming out of it :)

[ldm0]

fn f(_: &[i32]) {}

fn main() {
    f(&Box::new([1, 2]));
}

I guess it's failed because unsized coercion([i32; 2] => [i32]) doesn't happen during autoderef.

It still fails to compile even after this PR. Will add it to tests later.

[nikomatsakis]

Thanks. I'll take a look at the tests over the next few days! I'm trying to allocate some time every day for reviewing and notifications...

[Dylan-DPC]

@ldm0 if you can answer the question in the review above, we can proceed with this

[theemathas]

Will this fix #23014?

[Dylan-DPC]

Closing this due to inactivity

[pnkfelix]

reopening. ~~Even though the PR author didn't respond to the most recent question (which in hindsight I think was a confusing question, as I responded above),~~ I think that this PR either:

1. represents a change that we should make, or
2. represents a change that we shouldn't make, but: The constraints that would lead one to reach that conclusion aren't captured in our test suite currently.

So, as a first step, I propose that we put this through a crater run, in the hopes of identifying cases where this change causes inference failures. (And then we can take those cases and propagate them into the Rust test suite.)

[pnkfelix]

@bors try

[bors]

:hourglass: Trying commit ee3931b3099ab6c425053f338875e81d623c9cf9 with merge f6d7c613ae2d161ff37dbc63cc5abd809c878597...

[rust-log-analyzer]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

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

diff of stderr:

2   --> $DIR/coerce-block-tail-83850.rs:5:7
3    |
4 LL |     f(&Box::new([1, 2]));
-    |       ^^^^^^^^^^^^^^^^^ expected slice `[i32]`, found struct `Box`
+    |     - ^^^^^^^^^^^^^^^^^ expected slice `[i32]`, found struct `Box`
+    |     arguments to this function are incorrect
6    |
7    = note: expected reference `&[i32]`
7    = note: expected reference `&[i32]`
8               found reference `&Box<[{integer}; 2]>`
+ note: function defined here
+   --> $DIR/coerce-block-tail-83850.rs:2:4
+    |
+    |
+ LL | fn f(_: &[i32]) {}
+    |    ^ ---------
10 error: aborting due to previous error
11 



Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/coerce-block-tail-83850/coerce-block-tail-83850.stderr
To only update this specific test, also pass `--test-args coercion/coerce-block-tail-83850.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/coercion/coerce-block-tail-83850.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/coerce-block-tail-83850" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/coerce-block-tail-83850/auxiliary"
stdout: none
--- stderr -------------------------------
  --> /checkout/src/test/ui/coercion/coerce-block-tail-83850.rs:5:7
   |
   |
LL |     f(&Box::new([1, 2]));
   |     - ^^^^^^^^^^^^^^^^^ expected slice `[i32]`, found struct `Box`
   |     arguments to this function are incorrect
   |
   = note: expected reference `&[i32]`
   = note: expected reference `&[i32]`
              found reference `&Box<[{integer}; 2]>`
  --> /checkout/src/test/ui/coercion/coerce-block-tail-83850.rs:2:4
   |
   |
LL | fn f(_: &[i32]) {}
   |    ^ ---------
error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
------------------------------------------
---
25 error[E0308]: mismatched types
-   --> $DIR/brackets-to-braces-single-element.rs:7:27
+   --> $DIR/brackets-to-braces-single-element.rs:7:23
27    |
28 LL | const C: &&[u32; 1] = &&{ 1 };
-    |                           ^ expected array `[u32; 1]`, found integer
+    |                       ^^^^^^^ expected array `[u32; 1]`, found integer
30    |
+    = note: expected reference `&'static &'static [u32; 1]`
+               found reference `&&{integer}`
31 help: to create an array, use square brackets instead of curly braces
32    |
33 LL | const C: &&[u32; 1] = &&[ 1 ];

The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/did_you_mean/brackets-to-braces-single-element/brackets-to-braces-single-element.stderr
To update references, rerun the tests and pass the `--bless` flag
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args did_you_mean/brackets-to-braces-single-element.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/did_you_mean/brackets-to-braces-single-element.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/did_you_mean/brackets-to-braces-single-element" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/did_you_mean/brackets-to-braces-single-element/auxiliary"
stdout: none
--- stderr -------------------------------
  --> /checkout/src/test/ui/did_you_mean/brackets-to-braces-single-element.rs:1:24
   |
   |
LL | const A: [&str; 1] = { "hello" };
   |                        ^^^^^^^ expected array `[&'static str; 1]`, found `&str`
   |
help: to create an array, use square brackets instead of curly braces
   |
LL | const A: [&str; 1] = [ "hello" ];

error[E0308]: mismatched types
  --> /checkout/src/test/ui/did_you_mean/brackets-to-braces-single-element.rs:4:19
   |
   |
LL | const B: &[u32] = &{ 1 };
   |                   ^^^^^^ expected slice `[u32]`, found integer
   = note: expected reference `&'static [u32]`
              found reference `&{integer}`
              found reference `&{integer}`
help: to create an array, use square brackets instead of curly braces
   |
LL | const B: &[u32] = &[ 1 ];

error[E0308]: mismatched types
  --> /checkout/src/test/ui/did_you_mean/brackets-to-braces-single-element.rs:7:23
   |
   |
LL | const C: &&[u32; 1] = &&{ 1 };
   |                       ^^^^^^^ expected array `[u32; 1]`, found integer
   |
   = note: expected reference `&'static &'static [u32; 1]`
              found reference `&&{integer}`
help: to create an array, use square brackets instead of curly braces
   |
LL | const C: &&[u32; 1] = &&[ 1 ];

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`.

[bors]

:sunny: Try build successful - checks-actions Build commit: f6d7c613ae2d161ff37dbc63cc5abd809c878597 (f6d7c613ae2d161ff37dbc63cc5abd809c878597)

[pnkfelix]

@craterbot check f6d7c613ae2d161ff37dbc63cc5abd809c878597

[craterbot]

:rotating_light: Error: failed to parse the command

:sos: If you have any trouble with Crater please ping @rust-lang/infra! :information_source: Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[Dylan-DPC]

@pnkfelix your crater command didn't run, did you intend to do craterbot run <commit> because check doesn't support a commit as an arg

[pnkfelix]

@craterbot run f6d7c613ae2d161ff37dbc63cc5abd809c878597

[craterbot]

:rotating_light: Error: failed to parse the command

:sos: If you have any trouble with Crater please ping @rust-lang/infra! :information_source: Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[pnkfelix]

@craterbot run start=master#1ab98933fa75d72e882b86feac1a0be3a5b02cb0 end=try#f6d7c613ae2d161ff37dbc63cc5abd809c878597 mode=check-only

[craterbot]

:ok_hand: Experiment pr-83850 created and queued. :mag: You can check out the queue and this experiment's details.

:information_source: Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

:construction: Experiment pr-83850 is now running

:information_source: Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

:tada: Experiment pr-83850 is completed! :bar_chart: 4194 regressed and 11 fixed (237111 total) :newspaper: Open the full report.

:warning: If you notice any spurious failure please add them to the blacklist! :information_source: Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more