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

compiler/rustc_error_messages/locales/en-US/monomorphize.ftl

@@ -21,3 +21,6 @@ monomorphize_large_assignments =
     moving {$size} bytes
     .label = value moved from here
     .note = The current maximum size is {$limit}, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`
+
+monomorphize_requires_lang_item =
+    requires `{$lang_item}` lang_item

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;

compiler/rustc_middle/src/middle/lang_items.rs

@@ -18,11 +18,11 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Returns the `DefId` for a given `LangItem`.
     /// If not found, fatally aborts compilation.
     pub fn require_lang_item(self, lang_item: LangItem, span: Option<Span>) -> DefId {
-        self.lang_items().require(lang_item).unwrap_or_else(|msg| {
+        self.lang_items().require(lang_item).unwrap_or_else(|err| {
             if let Some(span) = span {
-                self.sess.span_fatal(span, &msg)
+                self.sess.span_fatal(span, err.to_string())
             } else {
-                self.sess.fatal(&msg)
+                self.sess.fatal(err.to_string())
             }
         })
     }

compiler/rustc_monomorphize/src/collector.rs

@@ -207,7 +207,7 @@ use std::iter;
 use std::ops::Range;
 use std::path::PathBuf;
 
-use crate::errors::{FatalError, LargeAssignmentsLint, RecursionLimit, TypeLengthLimit};
+use crate::errors::{LargeAssignmentsLint, RecursionLimit, RequiresLangItem, TypeLengthLimit};
 
 #[derive(PartialEq)]
 pub enum MonoItemCollectionMode {
@@ -1328,8 +1328,10 @@ impl<'v> RootCollector<'_, 'v> {
 
         let start_def_id = match self.tcx.lang_items().require(LangItem::Start) {
             Ok(s) => s,
-            Err(error_message) => {
-                self.tcx.sess.emit_fatal(FatalError { error_message: error_message.clone() });
+            Err(lang_item_err) => {
+                self.tcx
+                    .sess
+                    .emit_fatal(RequiresLangItem { lang_item: lang_item_err.0.name().to_string() });
             }
         };
         let main_ret_ty = self.tcx.fn_sig(main_def_id).output();

compiler/rustc_monomorphize/src/errors.rs

@@ -33,9 +33,9 @@ pub struct TypeLengthLimit {
 }
 
 #[derive(SessionDiagnostic)]
-#[diag(monomorphize::fatal_error)]
-pub struct FatalError {
-    pub error_message: String,
+#[diag(monomorphize::requires_lang_item)]
+pub struct RequiresLangItem {
+    pub lang_item: String,
 }
 
 pub struct UnusedGenericParams {

compiler/rustc_typeck/src/coherence/builtin.rs

@@ -359,7 +359,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUn
     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 
     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
-        tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
+        tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
     });
 
     let source = tcx.type_of(impl_did);