diff --git a/crates/hir_expand/src/builtin_attr.rs b/crates/hir_expand/src/builtin_attr.rs
index 5764c682be..6257050d80 100644
--- a/crates/hir_expand/src/builtin_attr.rs
+++ b/crates/hir_expand/src/builtin_attr.rs
@@ -1,5 +1,6 @@
 //! Builtin attributes.
 
+use mbe::ExpandResult;
 use syntax::ast;
 
 use crate::{db::AstDatabase, name, AstId, CrateId, MacroCallId, MacroDefId, MacroDefKind};
@@ -18,7 +19,7 @@ macro_rules! register_builtin {
                 id: MacroCallId,
                 tt: &tt::Subtree,
                 item: &tt::Subtree,
-            ) -> Result<tt::Subtree, mbe::ExpandError> {
+            ) -> ExpandResult<tt::Subtree> {
                 let expander = match *self {
                     $( BuiltinAttrExpander::$variant => $expand, )*
                 };
@@ -64,6 +65,6 @@ fn dummy_attr_expand(
     _id: MacroCallId,
     _tt: &tt::Subtree,
     item: &tt::Subtree,
-) -> Result<tt::Subtree, mbe::ExpandError> {
-    Ok(item.clone())
+) -> ExpandResult<tt::Subtree> {
+    ExpandResult::ok(item.clone())
 }
diff --git a/crates/hir_expand/src/db.rs b/crates/hir_expand/src/db.rs
index e477ce8fff..f2b1895046 100644
--- a/crates/hir_expand/src/db.rs
+++ b/crates/hir_expand/src/db.rs
@@ -53,19 +53,16 @@ impl TokenExpander {
             TokenExpander::MacroRules { mac, .. } => mac.expand(tt),
             TokenExpander::MacroDef { mac, .. } => mac.expand(tt),
             TokenExpander::Builtin(it) => it.expand(db, id, tt),
-            // FIXME switch these to ExpandResult as well
             TokenExpander::BuiltinAttr(it) => match db.macro_arg(id) {
-                Some(macro_arg) => it.expand(db, id, tt, &macro_arg.0).into(),
-                None => mbe::ExpandResult::only_err(
-                    mbe::ExpandError::Other("No item argument for attribute".to_string()).into(),
-                ),
+                Some(macro_arg) => it.expand(db, id, tt, &macro_arg.0),
+                None => mbe::ExpandResult::str_err("No item argument for attribute".to_string()),
             },
             TokenExpander::BuiltinDerive(it) => it.expand(db, id, tt),
             TokenExpander::ProcMacro(_) => {
                 // We store the result in salsa db to prevent non-deterministic behavior in
                 // some proc-macro implementation
                 // See #4315 for details
-                db.expand_proc_macro(id).into()
+                db.expand_proc_macro(id)
             }
         }
     }
@@ -133,7 +130,7 @@ pub trait AstDatabase: SourceDatabase {
     /// proc macros, since they are not deterministic in general, and
     /// non-determinism breaks salsa in a very, very, very bad way. @edwin0cheng
     /// heroically debugged this once!
-    fn expand_proc_macro(&self, call: MacroCallId) -> Result<tt::Subtree, mbe::ExpandError>;
+    fn expand_proc_macro(&self, call: MacroCallId) -> ExpandResult<tt::Subtree>;
     /// Firewall query that returns the error from the `macro_expand` query.
     fn macro_expand_error(&self, macro_call: MacroCallId) -> Option<ExpandError>;
 
@@ -379,18 +376,11 @@ fn macro_expand_error(db: &dyn AstDatabase, macro_call: MacroCallId) -> Option<E
     db.macro_expand(macro_call).err
 }
 
-fn expand_proc_macro(
-    db: &dyn AstDatabase,
-    id: MacroCallId,
-) -> Result<tt::Subtree, mbe::ExpandError> {
+fn expand_proc_macro(db: &dyn AstDatabase, id: MacroCallId) -> ExpandResult<tt::Subtree> {
     let loc: MacroCallLoc = db.lookup_intern_macro(id);
     let macro_arg = match db.macro_arg(id) {
         Some(it) => it,
-        None => {
-            return Err(
-                tt::ExpansionError::Unknown("No arguments for proc-macro".to_string()).into()
-            )
-        }
+        None => return ExpandResult::str_err("No arguments for proc-macro".to_string()),
     };
 
     let expander = match loc.def.kind {
diff --git a/crates/hir_expand/src/proc_macro.rs b/crates/hir_expand/src/proc_macro.rs
index 025e10239b..1e5fff07d5 100644
--- a/crates/hir_expand/src/proc_macro.rs
+++ b/crates/hir_expand/src/proc_macro.rs
@@ -2,6 +2,7 @@
 
 use crate::db::AstDatabase;
 use base_db::{CrateId, ProcMacroId};
+use mbe::ExpandResult;
 
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
 pub struct ProcMacroExpander {
@@ -9,15 +10,6 @@ pub struct ProcMacroExpander {
     proc_macro_id: Option<ProcMacroId>,
 }
 
-macro_rules! err {
-    ($fmt:literal, $($tt:tt),*) => {
-        mbe::ExpandError::ProcMacroError(tt::ExpansionError::Unknown(format!($fmt, $($tt),*)))
-    };
-    ($fmt:literal) => {
-        mbe::ExpandError::ProcMacroError(tt::ExpansionError::Unknown($fmt.to_string()))
-    }
-}
-
 impl ProcMacroExpander {
     pub fn new(krate: CrateId, proc_macro_id: ProcMacroId) -> Self {
         Self { krate, proc_macro_id: Some(proc_macro_id) }
@@ -38,21 +30,21 @@ impl ProcMacroExpander {
         calling_crate: CrateId,
         tt: &tt::Subtree,
         attr_arg: Option<&tt::Subtree>,
-    ) -> Result<tt::Subtree, mbe::ExpandError> {
+    ) -> ExpandResult<tt::Subtree> {
         match self.proc_macro_id {
             Some(id) => {
                 let krate_graph = db.crate_graph();
-                let proc_macro = krate_graph[self.krate]
-                    .proc_macro
-                    .get(id.0 as usize)
-                    .ok_or_else(|| err!("No derive macro found."))?;
+                let proc_macro = match krate_graph[self.krate].proc_macro.get(id.0 as usize) {
+                    Some(proc_macro) => proc_macro,
+                    None => return ExpandResult::str_err("No derive macro found.".to_string()),
+                };
 
                 // 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)
+                proc_macro.expander.expand(tt, attr_arg, env).map_err(mbe::ExpandError::from).into()
             }
-            None => Err(mbe::ExpandError::UnresolvedProcMacro),
+            None => ExpandResult::only_err(mbe::ExpandError::UnresolvedProcMacro),
         }
     }
 }