Add note if RUST_SRC_PATH is likely to be wrong

Author: jonas-schievink

crates/project_model/src/sysroot.rs

@@ -90,9 +90,15 @@ impl Sysroot {
         }
 
         if sysroot.by_name("core").is_none() {
+            let var_note = if env::var_os("RUST_SRC_PATH").is_some() {
+                " (`RUST_SRC_PATH` might be set incorrectly)"
+            } else {
+                ""
+            };
             anyhow::bail!(
-                "could not find libcore in sysroot path `{}`",
-                sysroot_src_dir.as_ref().display()
+                "could not find libcore in sysroot path `{}`{}",
+                sysroot_src_dir.as_ref().display(),
+                var_note,
             );
         }