Regeneralize the "-1 for None" change

[scottmcm]

Not planning on un-drafting this until after #158734 lands.

The "prefer the smaller magnitude" change (#155473) had been applied pretty narrowly: just to option-like niches and just to non-bool-like values. But now that llvm/llvm-project#208611 has been fixed, let's apply it more generally.

In doing so I ended up pretty substantially restructuring the function. I also greatly expanded the comments and added unit tests for it so it's easier to exercise all the paths (and much quicker than rebuilding all of rustc). I made sure to keep the previous "try to keep zero available for None" cleverness, however.

---

Or, if you prefer, this is contradicting the old comment in the code that

If niche zero is already reserved, the selection of bounds are of little interest.

🙂

[scottmcm]

:/

sccache: error: Timed out waiting for server startup. Maybe the remote service is unreachable?

[rust-log-analyzer]

The job x86_64-gnu-gcc-core-tests failed! Check out the build log: (web) (plain enhanced) (plain)

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

running 2793 tests
error: test failed, to rerun pass `-p coretests --test coretests`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/coretests/a8d1c5450ff3af52/out/coretests-a8d1c5450ff3af52 -Z unstable-options --format json` (signal: 11, SIGSEGV: invalid memory reference)
Bootstrap failed while executing `--stage 1 test library/coretests`
Currently active steps:
test::Crate { build_compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, mode: Std, crates: ["coretests"] } at src/bootstrap/src/core/build_steps/test.rs:3390
Build completed unsuccessfully in 0:14:46
  local time: Sun Aug  2 16:53:38 UTC 2026

Important

For more information how to resolve CI failures of this job, visit this link.

[scottmcm]

@bors try jobs=aarch64llvm-21

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

error: comptime fns can only be called at compile time
##[error]  --> example/example.rs:14:12
   |
14 |     if b { a * 2 } else { a * 3 }
   |            ^^^^^

error: comptime fns can only be called at compile time
##[error]  --> example/example.rs:14:27
   |
14 |     if b { a * 2 } else { a * 3 }
   |                           ^^^^^

error: comptime fns can only be called at compile time
##[error]  --> example/example.rs:47:6
   |
---

error: comptime fns can only be called at compile time
##[error]  --> example/example.rs:81:9
   |
81 |         intrinsics::copy::<u8>(src, dst, 1);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: comptime fns can only be called at compile time
##[error]  --> example/example.rs:88:9
   |
88 |         copy2(src, dst, 1);
   |         ^^^^^^^^^^^^^^^^^^

error: comptime fns can only be called at compile time
##[error]  --> example/example.rs:99:5
   |
99 |     (|_, _, _| {})(0u8, 42u16, 0u8)
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: comptime fns can only be called at compile time
##[error]   --> example/example.rs:103:5
    |
103 |     (|_, _| {})(0u8, 42u16)
    |     ^^^^^^^^^^^^^^^^^^^^^^^

error: comptime fns can only be called at compile time
##[error]   --> example/example.rs:113:9
    |
113 |         self.call_mut(arg)
    |         ^^^^^^^^^^^^^^^^^^

error: comptime fns can only be called at compile time
##[error]   --> example/example.rs:125:5
    |
125 |     IsNotEmpty.call_once((&(&[0u16] as &[_]),));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: comptime fns can only be called at compile time
##[error]   --> example/example.rs:129:5
    |
---
    |     ^^^^

error: aborting due to 16 previous errors

"/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustc-clif" "-Zannotate-moves" "-Zrandomize-layout" "-Zunstable-options" "--check-cfg=cfg(bootstrap)" "-Zmacro-backtrace" "-Csplit-debuginfo=off" "-Clink-arg=-L/usr/lib/llvm-21/lib" "-Cllvm-args=-import-instr-limit=10" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib" "-Alinker-messages" "-L" "crate=/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/example" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/example" "-Cdebuginfo=2" "--target" "aarch64-unknown-linux-gnu" "-Cpanic=abort" "--check-cfg=cfg(jit)" "--edition=2024" "example/example.rs" "--crate-type" "lib" exited with status ExitStatus(unix_wait_status(256))
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --manifest-path /checkout/compiler/rustc_codegen_cranelift/build_system/Cargo.toml -- test --download-dir /checkout/obj/build/cg_clif_download --out-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif --no-unstable-features --use-backend cranelift --sysroot llvm --skip-test testsuite.extended_sysroot [workdir=/checkout/compiler/rustc_codegen_cranelift]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:4270:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:4315:26

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip intrinsic-test --skip src/tools/rust-analyzer --skip tests --skip library --skip tidyselftest`
Currently active steps:
test::CodegenCranelift { compilers: RustcPrivateCompilers { build_compiler: Compiler { stage: 1, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target_compiler: Compiler { stage: 2, host: aarch64-unknown-linux-gnu, forced_compiler: false } }, target: aarch64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:4256
Build completed unsuccessfully in 0:24:33
  local time: Sun Aug  2 17:28:45 UTC 2026

[rust-bors]

💔 Test for 84c2f57 failed: CI. Failed jobs:

• try - aarch64-gnu-llvm-21-1 (web logs, enhanced plaintext logs)
• try - aarch64-gnu-llvm-21-2 (web logs, enhanced plaintext logs)