Fix debugger stepping for consts assigned to local vars

[vlad20012]

Fixes #141344

rustc should not optimize assignments to local vars (with debuginfo) in debug mode.

r? @scottmcm
r? @saethlin

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[saethlin]

Please add a debuginfo test that checks for the desired behavior.

@rustbot author

[scottmcm]

I don't think this is the right approach. There not being an assignment is perfectly fine here, and important for simplified codegen.

That assembly you're looking at is the debug spill for the debug information. Maybe this wants to emit those spills in a smarter order instead?

[vlad20012]

@scottmcm The problem is not an absence of an assignment in disassembly, but rather a reorder of existing assignments

[scottmcm]

There's no assignment in MIR, so seeing an assignment in assembly is caused by the debug info. If that debug info artifact is being emitted poorly, that to me is a debug info problem, not a MIR statements problem.

[vlad20012]

Okay, that makes sense

[rust-log-analyzer]

The job x86_64-gnu-llvm-19 failed! Check out the build log: (web) (plain)

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

#18 exporting to docker image format
#18 sending tarball 20.2s done
#18 DONE 27.5s
##[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-19]
[CI_JOB_NAME=x86_64-gnu-llvm-19]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-19', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'build.print-step-timings', '--enable-verbose-tests', '--set', 'build.metrics', '--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', '--set', 'gcc.download-ci-gcc=true', '--enable-new-symbol-mangling']
configure: build.build          := x86_64-unknown-linux-gnu
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-19/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---

warning: static `C_STR` is never used
##[warning] --> alloc_input.rs:8:12
  |
8 |     static C_STR: &std::ffi::CStr = c"cstr";
  |            ^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `check_type_id` is never used