Fix ICE on offsetted ZST pointer

[Mark-Simulacrum]

I'm not sure this is the right fix, but it's simple enough and does roughly what I'd expect. Like with the previous optimization to codegen usize rather than a zero-sized static, there's no guarantee that we continue returning a particular value from the offsetting.

A grep for const_usize.*align found the same code copied to rustc_codegen_gcc and cranelift but a quick skim didn't find other cases of similar 'optimization'. That said, I'm not convinced I caught everything, it's not trivial to search for this.

Closes #147516

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[rustbot]

r? @nnethercote

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[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 11.835
[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_codegen_cranelift/src/constant.rs:138:
                 let base_addr = match fx.tcx.global_alloc(alloc_id) {
                     GlobalAlloc::Memory(alloc) => {
                         if alloc.inner().len() == 0 {
-                            fx.bcx.ins().iconst(fx.pointer_type, alloc.inner().align.bytes().wrapping_add(offset.bytes()) as i64)
+                            fx.bcx.ins().iconst(
+                                fx.pointer_type,
+                                alloc.inner().align.bytes().wrapping_add(offset.bytes()) as i64,
+                            )
                         } else {
                             let data_id = data_id_for_alloc_id(
                                 &mut fx.constants_cx,
fmt: checked 6467 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:56
  local time: Sat Oct 11 02:02:35 UTC 2025
  network time: Sat, 11 Oct 2025 02:02:36 GMT

[nnethercote]

I don't understand this at all. Can you explain a little? Perhaps this test needs an explanatory comment at the top?