1 files changed, 31 insertions, 7 deletions

diff --git a/crates/project_model/src/rustc_cfg.rs b/crates/project_model/src/rustc_cfg.rs
index 312708575..6de40cfe2 100644
--- a/crates/project_model/src/rustc_cfg.rs
+++ b/crates/project_model/src/rustc_cfg.rs
@@ -2,9 +2,11 @@
 
 use std::process::Command;
 
+use paths::AbsPath;
+
 use crate::{cfg_flag::CfgFlag, utf8_stdout};
 
-pub(crate) fn get(target: Option<&str>) -> Vec<CfgFlag> {
+pub(crate) fn get(cargo_toml: Option<&AbsPath>, target: Option<&str>) -> Vec<CfgFlag> {
     let _p = profile::span("rustc_cfg::get");
     let mut res = Vec::with_capacity(6 * 2 + 1);
 
@@ -17,12 +19,34 @@ pub(crate) fn get(target: Option<&str>) -> Vec<CfgFlag> {
     }
 
     let rustc_cfgs = {
-        let mut cmd = Command::new(toolchain::rustc());
-        cmd.args(&["--print", "cfg", "-O"]);
-        if let Some(target) = target {
-            cmd.args(&["--target", target]);
-        }
-        utf8_stdout(cmd)
+        cargo_toml
+            .and_then(|cargo_toml| {
+                let mut cargo_config = Command::new(toolchain::cargo());
+                cargo_config.current_dir(cargo_toml.parent().unwrap()).args(&[
+                    "+nightly",
+                    "-Z",
+                    "unstable-options",
+                    "rustc",
+                    "--print",
+                    "cfg",
+                ]);
+                if let Some(target) = target {
+                    cargo_config.args(&["--target", target]);
+                }
+                utf8_stdout(cargo_config).ok()
+            })
+            .map_or_else(
+                || {
+                    // using unstable cargo features failed, fall back to using plain rustc
+                    let mut cmd = Command::new(toolchain::rustc());
+                    cmd.args(&["--print", "cfg", "-O"]);
+                    if let Some(target) = target {
+                        cmd.args(&["--target", target]);
+                    }
+                    utf8_stdout(cmd)
+                },
+                Ok,
+            )
     };
 
     match rustc_cfgs {