diff --git a/crates/base_db/src/input.rs b/crates/base_db/src/input.rs
index 671039e46b..c3730d0bbd 100644
--- a/crates/base_db/src/input.rs
+++ b/crates/base_db/src/input.rs
@@ -11,7 +11,7 @@ use std::{fmt, iter::FromIterator, ops, panic::RefUnwindSafe, str::FromStr, sync
 use cfg::CfgOptions;
 use rustc_hash::{FxHashMap, FxHashSet};
 use syntax::SmolStr;
-use tt::{ExpansionError, Subtree};
+use tt::Subtree;
 use vfs::{file_set::FileSet, FileId, VfsPath};
 
 /// Files are grouped into source roots. A source root is a directory on the
@@ -163,7 +163,13 @@ pub trait ProcMacroExpander: fmt::Debug + Send + Sync + RefUnwindSafe {
         subtree: &Subtree,
         attrs: Option<&Subtree>,
         env: &Env,
-    ) -> Result<Subtree, ExpansionError>;
+    ) -> Result<Subtree, ProcMacroExpansionError>;
+}
+
+pub enum ProcMacroExpansionError {
+    Panic(String),
+    /// Things like "proc macro server was killed by OOM".
+    System(String),
 }
 
 #[derive(Debug, Clone)]
diff --git a/crates/base_db/src/lib.rs b/crates/base_db/src/lib.rs
index d26f8f1808..c34a329540 100644
--- a/crates/base_db/src/lib.rs
+++ b/crates/base_db/src/lib.rs
@@ -12,7 +12,8 @@ pub use crate::{
     change::Change,
     input::{
         CrateData, CrateDisplayName, CrateGraph, CrateId, CrateName, Dependency, Edition, Env,
-        ProcMacro, ProcMacroExpander, ProcMacroId, ProcMacroKind, SourceRoot, SourceRootId,
+        ProcMacro, ProcMacroExpander, ProcMacroExpansionError, ProcMacroId, ProcMacroKind,
+        SourceRoot, SourceRootId,
     },
 };
 pub use salsa::{self, Cancelled};
diff --git a/crates/hir_expand/src/proc_macro.rs b/crates/hir_expand/src/proc_macro.rs
index 1e5fff07d5..e8e1d99e80 100644
--- a/crates/hir_expand/src/proc_macro.rs
+++ b/crates/hir_expand/src/proc_macro.rs
@@ -1,7 +1,7 @@
 //! Proc Macro Expander stub
 
 use crate::db::AstDatabase;
-use base_db::{CrateId, ProcMacroId};
+use base_db::{CrateId, ProcMacroExpansionError, ProcMacroId};
 use mbe::ExpandResult;
 
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
@@ -42,7 +42,14 @@ impl ProcMacroExpander {
                 // Proc macros have access to the environment variables of the invoking crate.
                 let env = &krate_graph[calling_crate].env;
 
-                proc_macro.expander.expand(tt, attr_arg, env).map_err(mbe::ExpandError::from).into()
+                proc_macro
+                    .expander
+                    .expand(tt, attr_arg, env)
+                    .map_err(|err| match err {
+                        ProcMacroExpansionError::Panic(text) => mbe::ExpandError::Other(text),
+                        ProcMacroExpansionError::System(text) => mbe::ExpandError::Other(text),
+                    })
+                    .into()
             }
             None => ExpandResult::only_err(mbe::ExpandError::UnresolvedProcMacro),
         }
diff --git a/crates/mbe/src/lib.rs b/crates/mbe/src/lib.rs
index 242d07c9ec..f490d75c3f 100644
--- a/crates/mbe/src/lib.rs
+++ b/crates/mbe/src/lib.rs
@@ -39,17 +39,11 @@ pub enum ExpandError {
     UnexpectedToken,
     BindingError(String),
     ConversionError,
-    ProcMacroError(tt::ExpansionError),
+    // FXME: no way mbe should know about proc macros.
     UnresolvedProcMacro,
     Other(String),
 }
 
-impl From<tt::ExpansionError> for ExpandError {
-    fn from(it: tt::ExpansionError) -> Self {
-        ExpandError::ProcMacroError(it)
-    }
-}
-
 impl fmt::Display for ExpandError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
@@ -57,7 +51,6 @@ impl fmt::Display for ExpandError {
             ExpandError::UnexpectedToken => f.write_str("unexpected token in input"),
             ExpandError::BindingError(e) => f.write_str(e),
             ExpandError::ConversionError => f.write_str("could not convert tokens"),
-            ExpandError::ProcMacroError(e) => e.fmt(f),
             ExpandError::UnresolvedProcMacro => f.write_str("unresolved proc macro"),
             ExpandError::Other(e) => f.write_str(e),
         }
diff --git a/crates/rust-analyzer/src/reload.rs b/crates/rust-analyzer/src/reload.rs
index 0a1226af5e..2cb5eb46b2 100644
--- a/crates/rust-analyzer/src/reload.rs
+++ b/crates/rust-analyzer/src/reload.rs
@@ -5,7 +5,8 @@ use flycheck::{FlycheckConfig, FlycheckHandle};
 use hir::db::DefDatabase;
 use ide::Change;
 use ide_db::base_db::{
-    CrateGraph, Env, ProcMacro, ProcMacroExpander, ProcMacroKind, SourceRoot, VfsPath,
+    CrateGraph, Env, ProcMacro, ProcMacroExpander, ProcMacroExpansionError, ProcMacroKind,
+    SourceRoot, VfsPath,
 };
 use proc_macro_api::{MacroDylib, ProcMacroServer};
 use project_model::{ProjectWorkspace, WorkspaceBuildScripts};
@@ -606,12 +607,12 @@ pub(crate) fn load_proc_macro(client: Option<&ProcMacroServer>, path: &AbsPath)
             subtree: &tt::Subtree,
             attrs: Option<&tt::Subtree>,
             env: &Env,
-        ) -> Result<tt::Subtree, tt::ExpansionError> {
+        ) -> Result<tt::Subtree, ProcMacroExpansionError> {
             let env = env.iter().map(|(k, v)| (k.to_string(), v.to_string())).collect();
             match self.0.expand(subtree, attrs, env) {
                 Ok(Ok(subtree)) => Ok(subtree),
-                Ok(Err(err)) => Err(tt::ExpansionError::ExpansionError(err.0)),
-                Err(err) => Err(tt::ExpansionError::Unknown(err.to_string())),
+                Ok(Err(err)) => Err(ProcMacroExpansionError::Panic(err.0)),
+                Err(err) => Err(ProcMacroExpansionError::System(err.to_string())),
             }
         }
     }
diff --git a/crates/tt/src/lib.rs b/crates/tt/src/lib.rs
index b2841370ec..f457881f2e 100644
--- a/crates/tt/src/lib.rs
+++ b/crates/tt/src/lib.rs
@@ -274,18 +274,3 @@ impl Subtree {
 }
 
 pub mod buffer;
-
-#[derive(Debug, PartialEq, Eq, Clone)]
-pub enum ExpansionError {
-    Unknown(String),
-    ExpansionError(String),
-}
-
-impl fmt::Display for ExpansionError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            ExpansionError::Unknown(e) => e.fmt(f),
-            ExpansionError::ExpansionError(e) => write!(f, "proc macro returned error: {}", e),
-        }
-    }
-}