[Perf experiment] Don't hash spans - #162729

note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
##[endgroup]
[2026-09-13T15:02:18.920Z INFO  opt_dist::timer] Section `Stage 1 (Rustc + rustdoc + cargo + clippy PGO)` starts
[2026-09-13T15:02:18.920Z INFO  opt_dist::timer] Section `Stage 1 (Rustc + rustdoc + cargo + clippy PGO) > Build PGO instrumented rustc and LLVM` starts
[2026-09-13T15:02:18.920Z INFO  opt_dist::exec] Executing `RUST_BACKTRACE=full python3 /checkout/x.py build --target x86_64-unknown-linux-gnu --host x86_64-unknown-linux-gnu --stage 2 library/std --set rust.llvm-bitcode-linker=false --set build.extended=false --set rust.codegen-backends=['llvm'] --set rust.deny-warnings=false rustdoc cargo --set pgo.rustc.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set pgo.cargo.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set pgo.rustdoc.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set llvm.thin-lto=false --set llvm.link-shared=true [at /checkout/obj]`
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.02s
##[endgroup]
[TIMING:start] compile::Assemble { target_compiler: Compiler { stage: 2, host: x86_64-unknown-linux-gnu, forced_compiler: false } }
[TIMING:start] builder::Libdir { compiler: Compiler { stage: 2, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu }
---
    |
 90 |           return;
    |           ------ any code following this expression is unreachable
...
109 | /         if !self.stable_hash_controls().hash_spans {
110 | |             return;
111 | |         }
    | |_________^ unreachable statement
    |
    = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default

warning: unused variable: `raw_span`
  --> /rustc-dev/79287b2e2eef8de08c5b268ad238d61b9985ed1c/compiler/rustc_middle/src/ich.rs:89:36
   |
89 |     fn stable_hash_span(&mut self, raw_span: RawSpan, hasher: &mut StableHasher) {
   |                                    ^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_raw_span`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

warning: unused variable: `hasher`
  --> /rustc-dev/79287b2e2eef8de08c5b268ad238d61b9985ed1c/compiler/rustc_middle/src/ich.rs:89:55
   |
89 |     fn stable_hash_span(&mut self, raw_span: RawSpan, hasher: &mut StableHasher) {
   |                                                       ^^^^^^ help: if this is intentional, prefix it with an underscore: `_hasher`

[RUSTC-TIMING] rustc_middle test:false 31.744
   Compiling rustc_infer v0.0.0 (/checkout/compiler/rustc_infer)
   Compiling rustc_transmute v0.0.0 (/checkout/compiler/rustc_transmute)
   Compiling rustc_symbol_mangling v0.0.0 (/checkout/compiler/rustc_symbol_mangling)
---
   Compiling profiler_builtins v0.0.0 (/checkout/library/profiler_builtins)
[RUSTC-TIMING] build_script_build test:false 0.065
[RUSTC-TIMING] build_script_build test:false 0.130
[RUSTC-TIMING] profiler_builtins test:false 0.012
error[E0658]: use of unstable library feature `panic_internals`: internal details of the implementation of the `panic!` and related macros
   --> /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/library/compiler-builtins/compiler-builtins/src/macros.rs:49:1
    |
 49 | / macro_rules! intrinsics {
 50 | |     () => ();
...   |
440 | |     );
441 | | }
    | |_^ in this expansion of `intrinsics!`
    |
   ::: /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/library/compiler-builtins/compiler-builtins/src/float/add.rs:192:1
    |
192 | / intrinsics! {
193 | |     #[cfg(f16_enabled)]
194 | |     pub extern "C" fn __addhf3(a: f16, b: f16) -> f16 {
195 | |         add(a, b)
...   |
213 | | }
    | |_^ in this macro invocation
    |
    = help: add `#![feature(panic_internals)]` to the crate attributes to enable
    = note: this compiler was built on 2026-09-13; consider upgrading it if it is out of date

For more information about this error, try `rustc --explain E0658`.
[RUSTC-TIMING] compiler_builtins test:false 0.522
error: could not compile `compiler_builtins` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] core test:false 10.921
Bootstrap failed while executing `build --target x86_64-unknown-linux-gnu --host x86_64-unknown-linux-gnu --stage 2 library/std --set rust.llvm-bitcode-linker=false --set build.extended=false --set rust.codegen-backends=['llvm'] --set rust.deny-warnings=false rustdoc cargo --set pgo.rustc.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set pgo.cargo.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set pgo.rustdoc.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set llvm.thin-lto=false --set llvm.link-shared=true`
Currently active steps:
compile::Assemble { target_compiler: Compiler { stage: 2, host: x86_64-unknown-linux-gnu, forced_compiler: false } } at src/bootstrap/src/core/build_steps/compile.rs:143
compile::Rustc { target: x86_64-unknown-linux-gnu, build_compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu, forced_compiler: false }, crates: [] } at src/bootstrap/src/core/build_steps/compile.rs:2328
compile::Std { target: x86_64-unknown-linux-gnu, build_compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu, forced_compiler: false }, crates: [], force_recompile: false, extra_rust_args: [], is_for_mir_opt_tests: false } at src/bootstrap/src/core/build_steps/compile.rs:1064
Build completed unsuccessfully in 0:05:20
---
[2026-09-13T15:07:39.422Z INFO  opt_dist::utils] Free disk space: 2.33 TiB out of total 2.55 TiB (8.35% used)
Error: Optimized build pipeline has failed

Caused by:
    Command RUST_BACKTRACE=full python3 /checkout/x.py build --target x86_64-unknown-linux-gnu --host x86_64-unknown-linux-gnu --stage 2 library/std --set rust.llvm-bitcode-linker=false --set build.extended=false --set rust.codegen-backends=['llvm'] --set rust.deny-warnings=false rustdoc cargo --set pgo.rustc.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set pgo.cargo.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set pgo.rustdoc.generate="/tmp/tmp-multistage/opt-artifacts/rustc-pgo" --set llvm.thin-lto=false --set llvm.link-shared=true [at /checkout/obj] has failed with exit code Some(1)

Stack backtrace:
   0: <anyhow::Error>::msg::<alloc::string::String>
             at /cargo/registry/25cdd57fae9f0462/anyhow-1.0.102/src/backtrace.rs:10:14
   1: <opt_dist::exec::CmdBuilder>::run
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/exec.rs:83:17
   2: <opt_dist::exec::Bootstrap>::run
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/exec.rs:274:18
   3: opt_dist::execute_pipeline::{closure#1}::{closure#0}
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/main.rs:270:25
   4: <opt_dist::timer::TimerSection>::section::<opt_dist::execute_pipeline::{closure#1}::{closure#0}, ()>
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/timer.rs:111:22
   5: opt_dist::execute_pipeline::{closure#1}
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/main.rs:248:19
   6: <opt_dist::timer::TimerSection>::section::<opt_dist::execute_pipeline::{closure#1}, (opt_dist::training::RustcPGOProfile, opt_dist::training::RustdocPGOProfile, core::option::Option<opt_dist::training::ClippyPGOProfile>)>
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/timer.rs:111:22
   7: opt_dist::execute_pipeline
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/main.rs:245:15
   8: opt_dist::main
             at /rustc/79287b2e2eef8de08c5b268ad238d61b9985ed1c/src/tools/opt-dist/src/main.rs:534:18
   9: <fn() -> core::result::Result<(), anyhow::Error> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/core/src/ops/function.rs:250:5
  10: std::sys::backtrace::__rust_begin_short_backtrace::<fn() -> core::result::Result<(), anyhow::Error>, core::result::Result<(), anyhow::Error>>
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/sys/backtrace.rs:166:18
  11: std::rt::lang_start::<core::result::Result<(), anyhow::Error>>::{closure#0}
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/rt.rs:206:18
  12: <&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync as core::ops::function::FnOnce<()>>::call_once
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/core/src/ops/function.rs:287:21
  13: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/panicking.rs:576:43
  14: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync>
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/panicking.rs:544:19
  15: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/panic.rs:359:14
  16: std::rt::lang_start_internal::{closure#0}
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/rt.rs:175:24
  17: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/cbae9b4cae2b108f6a3d18cfe6075714bb739463/library/std/src/panicking.rs:576:43