Semi-spurious ui test fluctuation

[oli-obk]

Since a few weeks we are getting an unrelated failure in a ui test where the order of two diagnostics changes. It is not entirely random, since it only changes if there are compiler changes, but we should still find and fix it

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

---- [ui] ui/panic-handler/weak-lang-item.rs stdout ----
diff of stderr:

10 LL | extern crate core as other_core;
12 
12 
- error: language item required, but not found: `eh_personality`
- 
15 error: `#[panic_handler]` function required, but not found
+ 
+ error: language item required, but not found: `eh_personality`
17 error: aborting due to 3 previous errors
18 



The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/panic-handler/weak-lang-item/weak-lang-item.stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args panic-handler/weak-lang-item.rs`
error: 1 errors occurred comparing output.
status: exit code: 1
status: exit code: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/panic-handler/weak-lang-item.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zemit-future-incompat-report" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/panic-handler/weak-lang-item" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/panic-handler/weak-lang-item/auxiliary"
------------------------------------------

------------------------------------------
stderr:
stderr:
------------------------------------------
error[E0259]: the name `core` is defined multiple times
   |
LL | extern crate core;
LL | extern crate core;
   | ^^^^^^^^^^^^^^^^^^ `core` reimported here
   |
   = note: `core` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
   |
LL | extern crate core as other_core;


error: `#[panic_handler]` function required, but not found

error: language item required, but not found: `eh_personality`
error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0259`.

---

Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--suite" "ui" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-9/bin/FileCheck" "--nodejs" "/usr/bin/node" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python3" "--lldb-python" "/usr/bin/python3" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "9.0.0" "--llvm-components" "aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo codegen core coroutines coverage debuginfocodeview debuginfodwarf debuginfogsym debuginfomsf debuginfopdb demangle dlltooldriver engine executionengine fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo instcombine instrumentation interpreter ipo irreader jitlink lanai lanaiasmparser lanaicodegen lanaidesc lanaidisassembler lanaiinfo libdriver lineeditor linker lto mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts object objectyaml option orcjit passes perfjitevents powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvutils runtimedyld scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target textapi transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86utils xcore xcorecodegen xcoredesc xcoredisassembler xcoreinfo xray" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"


failed to run: /checkout/obj/build/bootstrap/debug/bootstrap --stage 2 test --exclude src/tools/tidy
Build completed unsuccessfully in 0:16:22

Originally posted by @rust-log-analyzer in #81294 (comment)

[oli-obk]

The first time I've seen it was in #80418

The second time in #81294

[camelid]

This happened to me in #80965 (comment) too.

[pnkfelix]

This could be happening because WEAK_ITEMS_REFS, declared here:

rust/compiler/rustc_hir/src/weak_lang_items.rs

Line 15 in 84864bf

pub static WEAK_ITEMS_REFS: SyncLazy<FxHashMap<Symbol, LangItem>> = SyncLazy::new(|| {

is being traversed here:

rust/compiler/rustc_passes/src/weak_lang_items.rs

Lines 62 to 72 in 84864bf

	for (name, &item) in WEAK_ITEMS_REFS.iter() {
	if missing.contains(&item) && required(tcx, item) && items.require(item).is_err() {
	if item == LangItem::PanicImpl {
	tcx.sess.err("`#[panic_handler]` function required, but not found");
	} else if item == LangItem::Oom {
	if !tcx.features().default_alloc_error_handler {
	tcx.sess.err("`#[alloc_error_handler]` function required, but not found.");
	tcx.sess.note_without_error("Use `#![feature(default_alloc_error_handler)]` for a default error handler.");
	}
	} else {
	tcx.sess.err(&format!("language item required, but not found: `{}`", name));

and thus we get non-determinism in the output...

Two obvious solutions:

1. change to a sorted map structure in the declaration, or
2. normalize the order before traversing in the loop in rustc_passes::weak_lang_items

[pnkfelix]

cc #63713