Clean up generate_lto_work.

This function has some shared code for the thin LTO and fat LTO cases,
but those cases have so little in common that it's actually clearer to
treat them fully separately.

Author: nnethercote

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -380,32 +380,33 @@ fn generate_lto_work<B: ExtraBackendMethods>(
 ) -> Vec<(WorkItem<B>, u64)> {
     let _prof_timer = cgcx.prof.generic_activity("codegen_generate_lto_work");
 
-    let (lto_modules, copy_jobs) = if !needs_fat_lto.is_empty() {
+    if !needs_fat_lto.is_empty() {
         assert!(needs_thin_lto.is_empty());
-        let lto_module =
+        let module =
             B::run_fat_lto(cgcx, needs_fat_lto, import_only_modules).unwrap_or_else(|e| e.raise());
-        (vec![lto_module], vec![])
+        // We are adding a single work item, so the cost doesn't matter.
+        vec![(WorkItem::LTO(module), 0)]
     } else {
         assert!(needs_fat_lto.is_empty());
-        B::run_thin_lto(cgcx, needs_thin_lto, import_only_modules).unwrap_or_else(|e| e.raise())
-    };
-
-    lto_modules
-        .into_iter()
-        .map(|module| {
-            let cost = module.cost();
-            (WorkItem::LTO(module), cost)
-        })
-        .chain(copy_jobs.into_iter().map(|wp| {
-            (
-                WorkItem::CopyPostLtoArtifacts(CachedModuleCodegen {
-                    name: wp.cgu_name.clone(),
-                    source: wp,
-                }),
-                0,
-            )
-        }))
-        .collect()
+        let (lto_modules, copy_jobs) = B::run_thin_lto(cgcx, needs_thin_lto, import_only_modules)
+            .unwrap_or_else(|e| e.raise());
+        lto_modules
+            .into_iter()
+            .map(|module| {
+                let cost = module.cost();
+                (WorkItem::LTO(module), cost)
+            })
+            .chain(copy_jobs.into_iter().map(|wp| {
+                (
+                    WorkItem::CopyPostLtoArtifacts(CachedModuleCodegen {
+                        name: wp.cgu_name.clone(),
+                        source: wp,
+                    }),
+                    0, // copying is very cheap
+                )
+            }))
+            .collect()
+    }
 }
 
 pub struct CompiledModules {