Emit error instead of ICE when optimized MIR is missing

Closes 51388.

Author: Enselic

compiler/rustc_monomorphize/messages.ftl

@@ -14,6 +14,9 @@ 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 =
+    Optimized MIR unavailable for `{$def}`. Was it compiled with just `--emit=metadata` and without `-Zalways-encode-mir`?
+
 monomorphize_recursion_limit =
     reached the recursion limit while instantiating `{$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 +951,7 @@ 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 { def: tcx.def_path_str(def_id) });
     }
 
     true

compiler/rustc_monomorphize/src/errors.rs

@@ -33,6 +33,12 @@ pub struct TypeLengthLimit {
     pub type_length: usize,
 }
 
+#[derive(Diagnostic)]
+#[diag(monomorphize_no_optimized_mir)]
+pub struct NoOptimizedMir {
+    pub def: String,
+}
+
 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 +1,4 @@
+error: Optimized MIR unavailable for `missing_optimized_mir`. Was it compiled with just `--emit=metadata` and without `-Zalways-encode-mir`?
+
+error: aborting due to previous error
+