support using the in-tree compiler_builtins

Author: RalfJung

src/lib.rs

@@ -301,18 +301,20 @@ impl<'a> SysrootBuilder<'a> {
     fn gen_manifest(&self, src_dir: &Path) -> String {
         let have_sysroot_crate = src_dir.join("sysroot").exists();
         let crates = match &self.config {
+            // For some reason, the NoStd variant needs an explicit `compiler_builtins` here...
             SysrootConfig::NoStd => format!(
                 r#"
                 [dependencies.core]
                 path = {src_dir_core:?}
                 [dependencies.alloc]
                 path = {src_dir_alloc:?}
                 [dependencies.compiler_builtins]
+                path = {src_dir_builtins:?}
                 features = ["rustc-dep-of-std", "mem"]
-                version = "*"
                 "#,
                 src_dir_core = src_dir.join("core"),
                 src_dir_alloc = src_dir.join("alloc"),
+                src_dir_builtins = src_dir.join("compiler-builtins").join("compiler-builtins"),
             ),
             SysrootConfig::WithStd { std_features } if have_sysroot_crate => format!(
                 r#"