Add f16 and f128 const eval for casting operations

[tgross35]

This is a continuation of #126429 that should add missing operations.

[rustbot]

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[tgross35]

Still a WIP with the tests

@rustbot author
r? @RalfJung

[rust-log-analyzer]

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

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

## Running ui tests in tests/pass for x86_64-unknown-linux-gnu
   Compiler: "MIRI_ENV_VAR_TEST"="0" "MIRI_TEMP"="/tmp/miri-uitest-Esh5E1" "RUST_BACKTRACE"="1" /checkout/obj/build/x86_64-unknown-linux-gnu/stage1/bin/miri "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" OUT_DIR

FAILED TEST: tests/pass/float.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-Esh5E1" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1/bin/miri" "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--target" "x86_64-unknown-linux-gnu" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/ui/tests/pass" "tests/pass/float.rs" "--edition" "2021"
error: pass test got exit status: 101, but expected 0

error: actual output differed from expected
Execute `./miri test --bless` to update `tests/pass/float.stderr` to the actual output
Execute `./miri test --bless` to update `tests/pass/float.stderr` to the actual output
--- tests/pass/float.stderr
+++ <stderr output>
+thread 'main' panicked at $DIR/float.rs:LL:CC:
+assertion `left == right` failed
+  left: $HEX
+ right: $HEX
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+note: in Miri, you may have to set `-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect

full stderr:
thread 'main' panicked at tests/pass/float.rs:495:5:
assertion `left == right` failed
assertion `left == right` failed
  left: 0x7419
 right: 0x7c00
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
note: in Miri, you may have to set `-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect
full stdout:


FAILURES:
---
Error: 
   0: ui tests in tests/pass for x86_64-unknown-linux-gnu failed
   1: tests failed

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`
Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/release/deps/ui-ff8e5f7317669711 --quiet` (exit status: 1)
Build completed unsuccessfully in 0:03:28
  local time: Thu Jun 20 00:33:42 UTC 2024

[RalfJung]

Does this match rustc tests, or is rustc not ready yet to run all of these?
I can't really check whether your values make sense; so far I always did this by just running the test with rustc and then making sure Miri behavior matches rustc behavior.

[tgross35]

rustc should be able to run the f16/f128 on at least Linux x86 and aarch64, but many other platforms are missing the f16/f128 symbols until compiler builtins gets updated. Not sure if you meant fully testing in CI or just locally.

However, I still don't have the constants needed to fully test casting on the new types. So I moved only the test refactoring to its own PR (#126727), and am just going to close this one until #126608 can land.

[tgross35]

Also there are some ABI issues on x86 without SSE2, which might explain the failure at https://github.com/rust-lang/rust/actions/runs/9589520480/job/26443432727?pr=126718#step:25:3379 (haven't looked too much into it)