Replace num_cpus with std:🧵:available_parallelism

I think this might help startup time a little bit, since it looks like
on linux at least the std implementation uses syscalls to determine the
parallelism, whereas num_cpus was processing the /proc/cpuinfo file.

It also removes another dependency.

Cargo.lock

@@ -312,7 +312,6 @@ dependencies = [
  "nix 0.26.4",
  "normpath",
  "nu-ansi-term",
- "num_cpus",
  "regex",
  "regex-syntax 0.7.5",
  "tempfile",
@@ -357,12 +356,6 @@ version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "95505c38b4572b2d910cecb0281560f54b440a19336cbbcb27bf6ce6adc6f5a8"
 
-[[package]]
-name = "hermit-abi"
-version = "0.3.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d77f7ec81a6d05a3abb01ab6eb7590f6083d08449fe5a1c8b1e620283546ccb7"
-
 [[package]]
 name = "home"
 version = "0.5.5"
@@ -546,16 +539,6 @@ dependencies = [
  "autocfg",
 ]
 
-[[package]]
-name = "num_cpus"
-version = "1.16.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4161fcb6d602d4d2081af7c3a45852d875a03dd337a6bfdd6e06407b61342a43"
-dependencies = [
- "hermit-abi",
- "libc",
-]
-
 [[package]]
 name = "once_cell"
 version = "1.18.0"

Cargo.toml

@@ -38,7 +38,6 @@ aho-corasick = "1.0"
 nu-ansi-term = "0.49"
 argmax = "0.3.1"
 ignore = "0.4.20"
-num_cpus = "1.16"
 regex = "1.9.6"
 regex-syntax = "0.7"
 ctrlc = "3.2"

src/cli.rs

@@ -1,3 +1,4 @@
+use std::num::NonZeroUsize;
 use std::path::{Path, PathBuf};
 use std::time::Duration;
 
@@ -693,11 +694,9 @@ impl Opts {
         // unlikely fd will be running in such an environment, and even more unlikely someone would
         // be trying to use that many threads on such an environment, so I think panicing is an
         // appropriate way to handle that.
-        std::cmp::max(
-            self.threads
-                .map_or_else(num_cpus::get, |n| n.try_into().expect("too many threads")),
-            1,
-        )
+        self.threads.map_or_else(default_num_threads, |n| {
+            std::cmp::max(n.try_into().expect("too many threads"), 1)
+        })
     }
 
     pub fn max_results(&self) -> Option<usize> {
@@ -719,6 +718,16 @@ impl Opts {
     }
 }
 
+/// Get the default number of threads to use, if not explicitly specified.
+fn default_num_threads() -> usize {
+    // If we can't get the amount of parallelism for some reason, then
+    // default to a single thread, because that is safe.
+    const FALLBACK_PARALLELISM: NonZeroUsize = NonZeroUsize::MIN;
+    std::thread::available_parallelism()
+        .unwrap_or(FALLBACK_PARALLELISM)
+        .get()
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, ValueEnum)]
 pub enum FileType {
     #[value(alias = "f")]