allow non-monomorphize modules to access hard-coded error message through new struct, use fluent message in monomorphize

compiler/rustc_hir/src/errors.rs

@@ -0,0 +1,9 @@
+use crate::LangItem;
+
+pub struct LangItemError(pub LangItem);
+
+impl ToString for LangItemError {
+    fn to_string(&self) -> String {
+        format!("requires `{}` lang_item", self.0.name())
+    }
+}

compiler/rustc_hir/src/lang_items.rs

@@ -8,6 +8,7 @@
 //! * Functions called by the compiler itself.
 
 use crate::def_id::DefId;
+use crate::errors::LangItemError;
 use crate::{MethodKind, Target};
 
 use rustc_ast as ast;
@@ -115,9 +116,9 @@ macro_rules! language_item_table {
 
             /// Requires that a given `LangItem` was bound and returns the corresponding `DefId`.
             /// If it wasn't bound, e.g. due to a missing `#[lang = "<it.name()>"]`,
-            /// returns an error message as a string.
-            pub fn require(&self, it: LangItem) -> Result<DefId, String> {
-                self.items[it as usize].ok_or_else(|| format!("requires `{}` lang_item", it.name()))
+            /// returns an error encapsulating the `LangItem`.
+            pub fn require(&self, it: LangItem) -> Result<DefId, LangItemError> {
+                self.items[it as usize].ok_or_else(|| LangItemError(it))
             }
 
             /// Returns the [`DefId`]s of all lang items in a group.

compiler/rustc_hir/src/lib.rs

@@ -27,6 +27,7 @@ pub mod def;
 pub mod def_path_hash_map;
 pub mod definitions;
 pub mod diagnostic_items;
+pub mod errors;
 pub use rustc_span::def_id;
 mod hir;
 pub mod hir_id;