Allow benchmark allocator features together (#21905)

## Which issue does this PR close?

Split from #21637.

## Rationale for this change

The benchmark binaries currently reject enabling both `snmalloc` and
`mimalloc`. However, workspace-wide checks such as `cargo clippy
--all-targets --all-features` enable both feature flags, which makes
these binaries fail before clippy can run.

## What changes are included in this PR?

This PR removes the explicit compile error from the benchmark binaries
and makes allocator selection deterministic: `snmalloc` is used when
enabled, otherwise `mimalloc` is used when enabled.

## Are these changes tested?



## Are there any user-facing changes?

No. This only affects benchmark binary feature combinations used by
development checks.

Author: xudong963

benchmarks/benches/sql.rs

@@ -43,16 +43,13 @@ static SQL_BENCHMARK_DIRECTORY: LazyLock<String> = LazyLock::new(|| {
     )
 });
 
-#[cfg(all(feature = "snmalloc", feature = "mimalloc"))]
-compile_error!(
-    "feature \"snmalloc\" and feature \"mimalloc\" cannot be enabled at the same time"
-);
-
 #[cfg(feature = "snmalloc")]
 #[global_allocator]
 static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
 
-#[cfg(feature = "mimalloc")]
+// `cargo clippy --all-features` enables both allocator features, so prefer
+// `snmalloc` in that case and fall back to `mimalloc` otherwise.
+#[cfg(all(not(feature = "snmalloc"), feature = "mimalloc"))]
 #[global_allocator]
 static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
 

benchmarks/src/bin/benchmark_runner.rs

@@ -17,16 +17,13 @@
 
 use datafusion_benchmarks::benchmark_runner::run_cli;
 
-#[cfg(all(feature = "snmalloc", feature = "mimalloc"))]
-compile_error!(
-    "feature \"snmalloc\" and feature \"mimalloc\" cannot be enabled at the same time"
-);
-
 #[cfg(feature = "snmalloc")]
 #[global_allocator]
 static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
 
-#[cfg(feature = "mimalloc")]
+// `cargo clippy --all-features` enables both allocator features, so prefer
+// `snmalloc` in that case and fall back to `mimalloc` otherwise.
+#[cfg(all(not(feature = "snmalloc"), feature = "mimalloc"))]
 #[global_allocator]
 static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
 

benchmarks/src/bin/dfbench.rs

@@ -20,16 +20,13 @@ use datafusion::error::Result;
 
 use clap::{Parser, Subcommand};
 
-#[cfg(all(feature = "snmalloc", feature = "mimalloc"))]
-compile_error!(
-    "feature \"snmalloc\" and feature \"mimalloc\" cannot be enabled at the same time"
-);
-
 #[cfg(feature = "snmalloc")]
 #[global_allocator]
 static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
 
-#[cfg(feature = "mimalloc")]
+// `cargo clippy --all-features` enables both allocator features, so prefer
+// `snmalloc` in that case and fall back to `mimalloc` otherwise.
+#[cfg(all(not(feature = "snmalloc"), feature = "mimalloc"))]
 #[global_allocator]
 static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
 

benchmarks/src/bin/imdb.rs

@@ -21,16 +21,13 @@ use clap::{Parser, Subcommand};
 use datafusion::error::Result;
 use datafusion_benchmarks::imdb;
 
-#[cfg(all(feature = "snmalloc", feature = "mimalloc"))]
-compile_error!(
-    "feature \"snmalloc\" and feature \"mimalloc\" cannot be enabled at the same time"
-);
-
 #[cfg(feature = "snmalloc")]
 #[global_allocator]
 static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
 
-#[cfg(feature = "mimalloc")]
+// `cargo clippy --all-features` enables both allocator features, so prefer
+// `snmalloc` in that case and fall back to `mimalloc` otherwise.
+#[cfg(all(not(feature = "snmalloc"), feature = "mimalloc"))]
 #[global_allocator]
 static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;