mir: Improve size_of handling when arg is unsized

[osa1]

As discussed on Zulip with @RalfJung.

[rust-highfive]

r? @davidtwco

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[osa1]

@RalfJung this seems to make the error worse though. Should I maybe try to find out why we try to evaluate this stuff even though the program has type errors? (problem described in https://rust-lang.zulipchat.com/#narrow/stream/269128-miri/topic/Help.20fixing.20.2380742/near/223243016 IIUC)

[osa1]

r? @RalfJung

[rust-log-analyzer]

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Successfully built a3f694a571ee
Successfully tagged rust-ci:latest
Built container sha256:a3f694a571ee696b1d4d2cb8d9fd3bdfaa0af46712fe44ee9e1f5aaff9d14507
Uploading finished image to https://ci-caches.rust-lang.org/docker/aa85b52f727783ce661c5275c3edac7e8b4fbba025fc0a8e6392d6566595f3d157bb7da4c70e2eda4e5e6e7455d8337e7dfb6de6b288fcd6e68cf37f838a32b6
upload failed: - to s3://rust-lang-ci-sccache2/docker/aa85b52f727783ce661c5275c3edac7e8b4fbba025fc0a8e6392d6566595f3d157bb7da4c70e2eda4e5e6e7455d8337e7dfb6de6b288fcd6e68cf37f838a32b6 Unable to locate credentials
[CI_JOB_NAME=mingw-check]
---
configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/compiler/rustc_middle/src/mir/interpret/error.rs at line 146:
                 "transmuting `{}` to `{}` is not possible, because these types do not have the same size",
                 from_ty, to_ty
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/compiler/rustc_middle/src/mir/interpret/error.rs"` failed.
             ),
-            SizeOfUnsizedType(ty) => write!(
-                f,
-                "size_of called on unsized type `{}`",
-            ),
-            ),
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
+            SizeOfUnsizedType(ty) => write!(f, "size_of called on unsized type `{}`", ty),
     }
 }
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --stage 2 src/tools/tidy
Build completed unsuccessfully in 0:00:18

[RalfJung]

this seems to make the error worse though

You mean because there's two errors now? Hm. Still that seems better than SizeOf just making up a nonsensical return value.^^

[osa1]

You mean because there's two errors now? Hm. Still that seems better than SizeOf just making up a nonsensical return value.^^

Yeah. I started looking into this but it's going slow as I don't have much experience with the code base. If anyone have any tips that would be helpful.

[RalfJung]

Thanks!
@bors r+

[bors]

📌 Commit e3faeb4 has been approved by RalfJung