Use fenix from NIX_PATH if it is present

Sciencentistguy · Sciencentistguy · commit 48aadebd3c37 · 2022-06-10T14:51:06.000+01:00
diff --git a/src/main.rs b/src/main.rs
@@ -1,4 +1,4 @@
-use std::process::Command;
+use std::process::{Command, Stdio};
 
 use clap::Parser;
 use indoc::indoc;
@@ -26,22 +26,49 @@ fn main() {
         cmd.arg(args.shell);
     }
 
+    let toolchain_attrs = match args.channel {
+        RustChannel::Stable => r#"{channel = "stable";}"#.to_owned(),
+        RustChannel::Beta => r#"{channel = "beta";}"#.to_owned(),
+        RustChannel::Nightly => r#"{channel = "nightly";}"#.to_owned(),
+        RustChannel::DatedNightly(date) => format!(r#"{{channel = "nightly"; date = "{date}";}}"#),
+        RustChannel::Version(version) => format!(r#"{{channel = "{version}";}}"#),
+    };
+
+    let fenix_expr = {
+        let fenix_in_nix_path = {
+            let mut cmd = Command::new("nix");
+            cmd.args([
+                "--extra-experimental-features",
+                "nix-command flakes",
+                "eval",
+                "--impure",
+                "--expr",
+                "<fenix>",
+            ]);
+            cmd.stdout(Stdio::null());
+            let ret = cmd.status().expect("Nix command should not fail");
+            ret.code().unwrap() == 0
+        };
+
+        match fenix_in_nix_path {
+            true if !args.fresh_fenix => "<fenix>",
+            _ => r#"fetchTarball "https://github.com/nix-community/fenix/archive/main.tar.gz""#,
+        }
+    };
+
     let expression = format!(
         indoc! {"
-        ({}) {{
-            toolchainAttrs = {};
-            otherDeps = with import <nixpkgs> {{}}; [{}];
+        ({rust_nix}) {{
+            toolchainAttrs = {toolchain_attrs};
+            otherDeps = with import <nixpkgs> {{}}; [{extra_packages}];
+            fenix = import ({fenix_expr}) {{}};
+            name = \"{drv_name}\";
         }}"},
-        include_str!("rust.nix"),
-        match args.channel {
-            RustChannel::Stable => r#"{channel = "stable";}"#.to_owned(),
-            RustChannel::Beta => r#"{channel = "beta";}"#.to_owned(),
-            RustChannel::Nightly => r#"{channel = "nightly";}"#.to_owned(),
-            RustChannel::DatedNightly(date) =>
-                format!(r#"{{channel = "nightly"; date = "{date}";}}"#),
-            RustChannel::Version(version) => format!(r#"{{channel = "{version}";}}"#),
-        },
-        args.packages.join(" ")
+        rust_nix = include_str!("rust.nix"),
+        toolchain_attrs = toolchain_attrs,
+        extra_packages = args.packages.join(" "),
+        drv_name = args.name,
+        fenix_expr = fenix_expr,
     );
 
     if args.verbose {
@@ -100,6 +127,14 @@ struct Opt {
     /// Output debug information
     #[clap(long)]
     verbose: bool,
+
+    /// The name for the shell
+    #[clap(long, default_value = "rust-nix-shell")]
+    name: String,
+
+    /// Force the use of a fresh fenix
+    #[clap(long)]
+    fresh_fenix: bool,
 }
 
 #[cfg(test)]
diff --git a/src/rust.nix b/src/rust.nix
@@ -1,6 +1,8 @@
 {
   toolchainAttrs,
   otherDeps,
+  fenix,
+  name,
 }: let
   rust-nix-shell = {
     mkShell,
@@ -10,7 +12,7 @@
     rustPlatform,
   }:
     mkShell {
-      name = "rust-nix-shell";
+      inherit name;
       buildInputs =
         [
           (toolchain.withComponents ["cargo" "rustc" "rust-src" "rustfmt" "clippy"])
@@ -24,7 +26,8 @@
       RUST_SRC_PATH = "${toolchain.rust-src}";
     };
   pkgs = import <nixpkgs> {};
-  fenix = import (fetchTarball "https://github.com/nix-community/fenix/archive/main.tar.gz") {};
+  inherit fenix;
+  # fenix = import (fetchTarball "https://github.com/nix-community/fenix/archive/main.tar.gz") {};
   toolchain = fenix.toolchainOf toolchainAttrs;
 in
   pkgs.callPackage rust-nix-shell {
