Rust: Pass --ignore-rust-version to Cargo

Author: paldepind

rust/extractor/src/config.rs

@@ -189,6 +189,14 @@ impl Config {
 
     pub fn to_cargo_config(&self, dir: &AbsPath) -> (CargoConfig, LoadCargoConfig) {
         let sysroot = self.sysroot(dir);
+        let mut extra_args = self.cargo_extra_args.clone();
+        // The fixed toolchain may be older than the project's declared minimum. If that's the case
+        // `cargo` will error. Passing the flag below ensures that the project's specified Rust
+        // version is ignored. With this failures will only occur if the project actually uses Rust
+        // features that rust-analyzer can't understand (which is the best we can hope for).
+        if !extra_args.iter().any(|arg| arg == "--ignore-rust-version") {
+            extra_args.push("--ignore-rust-version".to_owned());
+        }
         (
             CargoConfig {
                 all_targets: self.cargo_all_targets,
@@ -205,7 +213,7 @@ impl Config {
                     .map(RustLibSource::Path),
 
                 extra_env: self.get_extra_env(),
-                extra_args: self.cargo_extra_args.clone(),
+                extra_args,
                 extra_includes: self
                     .extra_includes
                     .iter()