project-model: when using rust-project.json, prefer the sysroot-defined rustc over an env-based one

crates/project-model/src/rustc_cfg.rs

@@ -4,10 +4,11 @@ use std::process::Command;
 
 use rustc_hash::FxHashMap;
 
-use crate::{cfg_flag::CfgFlag, utf8_stdout, ManifestPath};
+use crate::{cfg_flag::CfgFlag, utf8_stdout, ManifestPath, Sysroot};
 
 pub(crate) fn get(
     cargo_toml: Option<&ManifestPath>,
+    sysroot: Option<&Sysroot>,
     target: Option<&str>,
     extra_env: &FxHashMap<String, String>,
 ) -> Vec<CfgFlag> {
@@ -25,7 +26,7 @@ pub(crate) fn get(
     // Add miri cfg, which is useful for mir eval in stdlib
     res.push(CfgFlag::Atom("miri".into()));
 
-    match get_rust_cfgs(cargo_toml, target, extra_env) {
+    match get_rust_cfgs(cargo_toml, sysroot, target, extra_env) {
         Ok(rustc_cfgs) => {
             tracing::debug!(
                 "rustc cfgs found: {:?}",
@@ -44,6 +45,7 @@ pub(crate) fn get(
 
 fn get_rust_cfgs(
     cargo_toml: Option<&ManifestPath>,
+    sysroot: Option<&Sysroot>,
     target: Option<&str>,
     extra_env: &FxHashMap<String, String>,
 ) -> anyhow::Result<String> {
@@ -62,8 +64,22 @@ fn get_rust_cfgs(
             Err(e) => tracing::debug!("{e:?}: falling back to querying rustc for cfgs"),
         }
     }
+
+    let rustc = match sysroot {
+        Some(sysroot) => {
+            let rustc = sysroot.discover_rustc()?.into();
+            tracing::debug!(?rustc, "using rustc from sysroot");
+            rustc
+        }
+        None => {
+            let rustc = toolchain::rustc();
+            tracing::debug!(?rustc, "using rustc from env");
+            rustc
+        }
+    };
+
     // using unstable cargo features failed, fall back to using plain rustc
-    let mut cmd = Command::new(toolchain::rustc());
+    let mut cmd = Command::new(rustc);
     cmd.envs(extra_env);
     cmd.args(["--print", "cfg", "-O"]);
     if let Some(target) = target {

crates/project-model/src/sysroot.rs

@@ -115,10 +115,19 @@ impl Sysroot {
         Ok(Sysroot::load(sysroot_dir, src))
     }
 
-    pub fn discover_rustc(&self) -> Option<ManifestPath> {
+    pub fn discover_rustc_src(&self) -> Option<ManifestPath> {
         get_rustc_src(&self.root)
     }
 
+    pub fn discover_rustc(&self) -> Result<AbsPathBuf, std::io::Error> {
+        let rustc = self.root.join("bin/rustc");
+        tracing::debug!(?rustc, "checking for rustc binary at location");
+        match fs::metadata(&rustc) {
+            Ok(_) => Ok(rustc),
+            Err(e) => Err(e),
+        }
+    }
+
     pub fn with_sysroot_dir(sysroot_dir: AbsPathBuf) -> Result<Sysroot> {
         let sysroot_src_dir = discover_sysroot_src_dir(&sysroot_dir).ok_or_else(|| {
             format_err!("can't load standard library from sysroot path {sysroot_dir}")

crates/project-model/src/workspace.rs

@@ -240,9 +240,9 @@ impl ProjectWorkspace {
                     Some(RustLibSource::Path(path)) => ManifestPath::try_from(path.clone())
                         .map_err(|p| Some(format!("rustc source path is not absolute: {p}"))),
                     Some(RustLibSource::Discover) => {
-                        sysroot.as_ref().ok().and_then(Sysroot::discover_rustc).ok_or_else(|| {
-                            Some(format!("Failed to discover rustc source for sysroot."))
-                        })
+                        sysroot.as_ref().ok().and_then(Sysroot::discover_rustc_src).ok_or_else(
+                            || Some(format!("Failed to discover rustc source for sysroot.")),
+                        )
                     }
                     None => Err(None),
                 };
@@ -279,8 +279,12 @@ impl ProjectWorkspace {
                     }
                 });
 
-                let rustc_cfg =
-                    rustc_cfg::get(Some(&cargo_toml), config.target.as_deref(), &config.extra_env);
+                let rustc_cfg = rustc_cfg::get(
+                    Some(&cargo_toml),
+                    None,
+                    config.target.as_deref(),
+                    &config.extra_env,
+                );
 
                 let cfg_overrides = config.cfg_overrides.clone();
                 let data_layout = target_data_layout::get(
@@ -335,7 +339,7 @@ impl ProjectWorkspace {
             tracing::info!(src_root = %sysroot.src_root(), root = %sysroot.root(), "Using sysroot");
         }
 
-        let rustc_cfg = rustc_cfg::get(None, target, extra_env);
+        let rustc_cfg = rustc_cfg::get(None, sysroot.as_ref().ok(), target, extra_env);
         ProjectWorkspace::Json { project: project_json, sysroot, rustc_cfg, toolchain }
     }
 
@@ -360,7 +364,7 @@ impl ProjectWorkspace {
         if let Ok(sysroot) = &sysroot {
             tracing::info!(src_root = %sysroot.src_root(), root = %sysroot.root(), "Using sysroot");
         }
-        let rustc_cfg = rustc_cfg::get(None, None, &Default::default());
+        let rustc_cfg = rustc_cfg::get(None, sysroot.as_ref().ok(), None, &Default::default());
         Ok(ProjectWorkspace::DetachedFiles { files: detached_files, sysroot, rustc_cfg })
     }
 
@@ -756,7 +760,7 @@ fn project_json_to_crate_graph(
                 let target_cfgs = match target.as_deref() {
                     Some(target) => cfg_cache
                         .entry(target)
-                        .or_insert_with(|| rustc_cfg::get(None, Some(target), extra_env)),
+                        .or_insert_with(|| rustc_cfg::get(None, sysroot, Some(target), extra_env)),
                     None => &rustc_cfg,
                 };