Auto merge of #131816 - Zalathar:profiler-feature, r=Kobzol

Make `profiler_builtins` an optional dependency of sysroot, not std

This avoids unnecessary rebuilds of std (and the compiler) when `build.profiler` is toggled off or on.

Fixes #131812.

---

Background: The `profiler_builtins` crate has been an optional dependency of std (behind a cargo feature) ever since it was added back in #42433. But as far as I can tell that has only ever been a convenient way to force the crate to be built, not a genuine dependency.

The side-effect of this false dependency is that toggling `build.profiler` causes a rebuild of std and the compiler, which shouldn't be necessary. This PR therefore makes `profiler_builtins` an optional dependency of the dummy sysroot crate (#108865), rather than a dependency of std.

What makes this change so small is that all of the necessary infrastructure already exists. Previously, bootstrap would enable the `profiler` feature on the sysroot crate, which would forward that feature to std. Now, enabling that feature directly enables sysroot's `profiler_builtins` dependency instead.

---

I believe this is more of a bootstrap change than a libs change, so tentatively:
r? bootstrap

library/Cargo.lock

@@ -340,7 +340,6 @@ dependencies = [
  "object",
  "panic_abort",
  "panic_unwind",
- "profiler_builtins",
  "r-efi",
  "r-efi-alloc",
  "rand",
@@ -368,6 +367,7 @@ name = "sysroot"
 version = "0.0.0"
 dependencies = [
  "proc_macro",
+ "profiler_builtins",
  "std",
  "test",
 ]

library/std/Cargo.toml

@@ -18,7 +18,6 @@ panic_unwind = { path = "../panic_unwind", optional = true }
 panic_abort = { path = "../panic_abort" }
 core = { path = "../core", public = true }
 compiler_builtins = { version = "0.1.133" }
-profiler_builtins = { path = "../profiler_builtins", optional = true }
 unwind = { path = "../unwind" }
 hashbrown = { version = "0.15", default-features = false, features = [
     'rustc-dep-of-std',
@@ -98,7 +97,6 @@ backtrace = [
 ]
 
 panic-unwind = ["panic_unwind"]
-profiler = ["profiler_builtins"]
 compiler-builtins-c = ["alloc/compiler-builtins-c"]
 compiler-builtins-mem = ["alloc/compiler-builtins-mem"]
 compiler-builtins-no-asm = ["alloc/compiler-builtins-no-asm"]

library/sysroot/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 # this is a dummy crate to ensure that all required crates appear in the sysroot
 [dependencies]
 proc_macro = { path = "../proc_macro" }
+profiler_builtins = { path = "../profiler_builtins", optional = true }
 std = { path = "../std" }
 test = { path = "../test" }
 
@@ -23,7 +24,7 @@ system-llvm-libunwind = ["std/system-llvm-libunwind"]
 panic-unwind = ["std/panic_unwind"]
 panic_immediate_abort = ["std/panic_immediate_abort"]
 optimize_for_size = ["std/optimize_for_size"]
-profiler = ["std/profiler"]
+profiler = ["dep:profiler_builtins"]
 std_detect_file_io = ["std/std_detect_file_io"]
 std_detect_dlsym_getauxval = ["std/std_detect_dlsym_getauxval"]
 std_detect_env_override = ["std/std_detect_env_override"]