Issue-141806: Remove sizedness_fast_path in fulfill.rs

[Shunpoco]

Context

Fix #141806

If Sized type doesn't exist, the ICE happens. This ICE is from sizedness_fast_path() call in FulfillProcessor.process_abligation.
Originally the function call was added for performance improvement in #139577 .

Change

• Remove the function call
• Add test for the ICE

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

---- [ui] tests/ui/sized-hierarchy/incomplete-inference-issue-143992.rs#current stdout ----

error in revision `current`: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/sized-hierarchy/incomplete-inference-issue-143992.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--cfg" "current" "--check-cfg" "cfg(test,FALSE,current,next,current_sized_hierarchy,next_sized_hierarchy)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/sized-hierarchy/incomplete-inference-issue-143992.current" "-A" "unused" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--crate-type=lib"
stdout: none
--- stderr -------------------------------
error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/sized-hierarchy/incomplete-inference-issue-143992.rs:30:28
   |
LL |     let _x = T::Assoc::new(());
   |              ------------- ^^ expected `[u32; 1]`, found `()`
   |              |
   |              arguments to this function are incorrect
   |
note: associated function defined here
  --> /checkout/tests/ui/sized-hierarchy/incomplete-inference-issue-143992.rs:21:8
   |
LL |     fn new(r: R) -> R {
   |        ^^^ ----

error: aborting due to 1 previous error

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

[compiler-errors]

This solution does not look correct and also has a performance implication. This needs a lot more detailed explanation for why the ICE occurs in the first place.

[Shunpoco]

Yes, at first I thought that the problem is solved by deleting the block, but the actual problem is much deeper.
I will close this PR at the moment, thanks for the comment