1 files changed, 81 insertions, 0 deletions
diff --git a/src/tools/rust-analyzer/crates/hir-expand/src/proc_macro.rs b/src/tools/rust-analyzer/crates/hir-expand/src/proc_macro.rs
new file mode 100644
index 000000000..5afdcc0e6
--- /dev/null
+++ b/src/tools/rust-analyzer/crates/hir-expand/src/proc_macro.rs
@@ -0,0 +1,81 @@
+//! Proc Macro Expander stub
+
+use base_db::{CrateId, ProcMacroExpansionError, ProcMacroId, ProcMacroKind};
+use stdx::never;
+
+use crate::{db::AstDatabase, ExpandError, ExpandResult};
+
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
+pub struct ProcMacroExpander {
+    krate: CrateId,
+    proc_macro_id: Option<ProcMacroId>,
+}
+
+impl ProcMacroExpander {
+    pub fn new(krate: CrateId, proc_macro_id: ProcMacroId) -> Self {
+        Self { krate, proc_macro_id: Some(proc_macro_id) }
+    }
+
+    pub fn dummy(krate: CrateId) -> Self {
+        // FIXME: Should store the name for better errors
+        Self { krate, proc_macro_id: None }
+    }
+
+    pub fn is_dummy(&self) -> bool {
+        self.proc_macro_id.is_none()
+    }
+
+    pub fn expand(
+        self,
+        db: &dyn AstDatabase,
+        calling_crate: CrateId,
+        tt: &tt::Subtree,
+        attr_arg: Option<&tt::Subtree>,
+    ) -> ExpandResult<tt::Subtree> {
+        match self.proc_macro_id {
+            Some(id) => {
+                let krate_graph = db.crate_graph();
+                let proc_macros = match &krate_graph[self.krate].proc_macro {
+                    Ok(proc_macros) => proc_macros,
+                    Err(_) => {
+                        never!("Non-dummy expander even though there are no proc macros");
+                        return ExpandResult::only_err(ExpandError::Other("Internal error".into()));
+                    }
+                };
+                let proc_macro = match proc_macros.get(id.0 as usize) {
+                    Some(proc_macro) => proc_macro,
+                    None => {
+                        never!(
+                            "Proc macro index out of bounds: the length is {} but the index is {}",
+                            proc_macros.len(),
+                            id.0
+                        );
+                        return ExpandResult::only_err(ExpandError::Other("Internal error".into()));
+                    }
+                };
+
+                // Proc macros have access to the environment variables of the invoking crate.
+                let env = &krate_graph[calling_crate].env;
+                match proc_macro.expander.expand(tt, attr_arg, env) {
+                    Ok(t) => ExpandResult::ok(t),
+                    Err(err) => match err {
+                        // Don't discard the item in case something unexpected happened while expanding attributes
+                        ProcMacroExpansionError::System(text)
+                            if proc_macro.kind == ProcMacroKind::Attr =>
+                        {
+                            ExpandResult {
+                                value: tt.clone(),
+                                err: Some(ExpandError::Other(text.into())),
+                            }
+                        }
+                        ProcMacroExpansionError::System(text)
+                        | ProcMacroExpansionError::Panic(text) => {
+                            ExpandResult::only_err(ExpandError::Other(text.into()))
+                        }
+                    },
+                }
+            }
+            None => ExpandResult::only_err(ExpandError::UnresolvedProcMacro(self.krate)),
+        }
+    }
+}