feat: Use cargo-config2 to read rustflags from the config to append to

cargo-dist/Cargo.toml

@@ -72,6 +72,7 @@ current_platform.workspace = true
 color-backtrace.workspace = true
 backtrace.workspace = true
 schemars.workspace = true
+cargo-config2 = "0.1.36"
 
 [dev-dependencies]
 insta.workspace = true

cargo-dist/src/build/cargo.rs

@@ -70,7 +70,22 @@ impl<'a> DistGraphBuilder<'a> {
         let mut builds = vec![];
         for (target_triple, binaries) in targets {
             let target = target_triple.parse()?;
-            let mut rustflags = std::env::var("RUSTFLAGS").unwrap_or_default();
+
+            // If RUSTFLAGS is set, use that.
+            // Otherwise, read .cargo/config.toml.
+            let mut rustflags = match std::env::var("RUSTFLAGS") {
+                Ok(rustflags) => rustflags,
+                Err(_) => {
+                    let config = cargo_config2::Config::load()?;
+                    let flags = config
+                        .rustflags(target_triple.as_str())?
+                        .unwrap_or_default();
+                    // Convert from Vec<String> to a single space-separated String.
+                    flags.flags.join(" ")
+                }
+            };
+
+            // let mut rustflags = std::env::var("RUSTFLAGS").unwrap_or_default();
 
             // FIXME: is there a more principled way for us to add things to RUSTFLAGS
             // without breaking everything. Cargo has some builtin ways like keys

cargo-dist/src/errors.rs

@@ -69,6 +69,10 @@ pub enum DistError {
     #[error(transparent)]
     TripleError(#[from] cargo_dist_schema::target_lexicon::ParseError),
 
+    /// random cargo_config2 error
+    #[error(transparent)]
+    ConfigError(#[from] cargo_config2::Error),
+
     /// A problem with a jinja template, which is always a dist bug
     #[error("Failed to render template")]
     #[diagnostic(