Add config to unconditionally prefer core imports over std

Fixes https://github.com/rust-lang/rust-analyzer/issues/12979

crates/ide-diagnostics/src/handlers/json_is_not_rust.rs

@@ -137,6 +137,7 @@ pub(crate) fn json_in_items(
                                         sema.db,
                                         it,
                                         config.insert_use.prefix_kind,
+                                        config.prefer_core,
                                     ) {
                                         insert_use(
                                             &scope,
@@ -152,6 +153,7 @@ pub(crate) fn json_in_items(
                                         sema.db,
                                         it,
                                         config.insert_use.prefix_kind,
+                                        config.prefer_core,
                                     ) {
                                         insert_use(
                                             &scope,

crates/ide-diagnostics/src/handlers/missing_fields.rs

@@ -124,6 +124,7 @@ fn fixes(ctx: &DiagnosticsContext<'_>, d: &hir::MissingFields) -> Option<Vec<Ass
                         let type_path = current_module?.find_use_path(
                             ctx.sema.db,
                             item_for_path_search(ctx.sema.db, item_in_ns)?,
+                            ctx.config.prefer_core,
                         )?;
 
                         use_trivial_constructor(

crates/ide-diagnostics/src/lib.rs

@@ -150,6 +150,7 @@ pub struct DiagnosticsConfig {
     pub expr_fill_default: ExprFillDefaultMode,
     // FIXME: We may want to include a whole `AssistConfig` here
     pub insert_use: InsertUseConfig,
+    pub prefer_core: bool,
 }
 
 impl DiagnosticsConfig {
@@ -170,6 +171,7 @@ impl DiagnosticsConfig {
                 group: false,
                 skip_glob_imports: false,
             },
+            prefer_core: false,
         }
     }
 }