Fix ICE in peekable with unsized types

[charmitro]

This commit resolves an Internal Compiler Error (ICE) caused by calling peekable on iterators with unsized item types. The issue arises due to MIR optimizations (Inline and JumpThreading) encountering invalid states when handling unsized types.

Changes:

• Added a Self::Item: Sized bound to the peekable method in Iterator trait to prevent usage with unsized types.
• Introduced a new test case (tests/ui/iterators/unsized-peekable.rs) to verify that attempting to use peekable with unsized types results in a proper compile-time error instead of an ICE.

This fix ensures that the compiler provides a clear and actionable error message (E0277) when peekable is used with unsized types, avoiding invalid intermediate representations during MIR optimization.

Closes #136442

[rustbot]

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[cyrgani]

This is not the right fix for this issue: for one, it would break some existing uses of peekable, and secondly, the error is not specific to Peekable and could be caused with other types too.
The actual fix would need to update the jump threading or inlining optimization logic to prevent any such errors.

[rust-log-analyzer]

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#22 exporting to docker image format
#22 sending tarball 27.9s done
#22 DONE 34.2s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
---- [ui] tests/ui/layout/unexpected-unsized-field-issue-135020.rs stdout ----

error: 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/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/layout/unexpected-unsized-field-issue-135020.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/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--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/x86_64-unknown-linux-gnu/test/ui/layout/unexpected-unsized-field-issue-135020" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
--- stderr -------------------------------
error[E0277]: the size for values of type `str` cannot be known at compilation time
##[error]  --> /checkout/tests/ui/layout/unexpected-unsized-field-issue-135020.rs:4:11
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