Emit error instead of ICE when optimized MIR is missing

compiler/rustc_monomorphize/messages.ftl

@@ -14,6 +14,10 @@ monomorphize_large_assignments =
     .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_no_optimized_mir =
+    missing optimized MIR for an item in the crate `{$crate_name}`
+    .note = missing optimized MIR for this item (was the crate `{$crate_name}` compiled with `--emit=metadata`?)
+
 monomorphize_recursion_limit =
     reached the recursion limit while instantiating `{$shrunk}`
     .note = `{$def_path_str}` defined here

compiler/rustc_monomorphize/src/collector.rs

@@ -192,7 +192,8 @@ use rustc_target::abi::Size;
 use std::path::PathBuf;
 
 use crate::errors::{
-    EncounteredErrorWhileInstantiating, LargeAssignmentsLint, RecursionLimit, TypeLengthLimit,
+    EncounteredErrorWhileInstantiating, LargeAssignmentsLint, NoOptimizedMir, RecursionLimit,
+    TypeLengthLimit,
 };
 
 #[derive(PartialEq)]
@@ -950,7 +951,10 @@ fn should_codegen_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: &Instance<'tcx>) ->
     }
 
     if !tcx.is_mir_available(def_id) {
-        bug!("no MIR available for {:?}", def_id);
+        tcx.sess.emit_fatal(NoOptimizedMir {
+            span: tcx.def_span(def_id),
+            crate_name: tcx.crate_name(def_id.krate),
+        });
     }
 
     true

compiler/rustc_monomorphize/src/errors.rs

@@ -4,7 +4,7 @@ use crate::fluent_generated as fluent;
 use rustc_errors::ErrorGuaranteed;
 use rustc_errors::IntoDiagnostic;
 use rustc_macros::{Diagnostic, LintDiagnostic};
-use rustc_span::Span;
+use rustc_span::{Span, Symbol};
 
 #[derive(Diagnostic)]
 #[diag(monomorphize_recursion_limit)]
@@ -33,6 +33,14 @@ pub struct TypeLengthLimit {
     pub type_length: usize,
 }
 
+#[derive(Diagnostic)]
+#[diag(monomorphize_no_optimized_mir)]
+pub struct NoOptimizedMir {
+    #[note]
+    pub span: Span,
+    pub crate_name: Symbol,
+}
+
 pub struct UnusedGenericParamsHint {
     pub span: Span,
     pub param_spans: Vec<Span>,

tests/ui/rmeta/auxiliary/rmeta-meta.rs

@@ -6,3 +6,7 @@
 pub struct Foo {
     pub field: i32,
 }
+
+pub fn missing_optimized_mir() {
+    println!("indeed");
+}

tests/ui/rmeta/no_optitimized_mir.rs

@@ -0,0 +1,11 @@
+// aux-build:rmeta-meta.rs
+// no-prefer-dynamic
+// build-fail
+
+// Check that we do not ICE when we need optimized MIR but it is missing.
+
+extern crate rmeta_meta;
+
+fn main() {
+    rmeta_meta::missing_optimized_mir();
+}

tests/ui/rmeta/no_optitimized_mir.stderr

@@ -0,0 +1,10 @@
+error: missing optimized MIR for an item in the crate `rmeta_meta`
+   |
+note: missing optimized MIR for this item (was the crate `rmeta_meta` compiled with `--emit=metadata`?)
+  --> $DIR/auxiliary/rmeta-meta.rs:10:1
+   |
+LL | pub fn missing_optimized_mir() {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+