Add config to unconditionally prefer core imports over std

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

crates/ide-completion/src/completions.rs

@@ -551,7 +551,9 @@ fn enum_variants_with_paths(
     }
 
     for variant in variants {
-        if let Some(path) = ctx.module.find_use_path(ctx.db, hir::ModuleDef::from(variant)) {
+        if let Some(path) =
+            ctx.module.find_use_path(ctx.db, hir::ModuleDef::from(variant), ctx.config.prefer_core)
+        {
             // Variants with trivial paths are already added by the existing completion logic,
             // so we should avoid adding these twice
             if path.segments().len() > 1 {

crates/ide-completion/src/completions/expr.rs

@@ -165,7 +165,11 @@ pub(crate) fn complete_expr_path(
                     hir::Adt::Struct(strukt) => {
                         let path = ctx
                             .module
-                            .find_use_path(ctx.db, hir::ModuleDef::from(strukt))
+                            .find_use_path(
+                                ctx.db,
+                                hir::ModuleDef::from(strukt),
+                                ctx.config.prefer_core,
+                            )
                             .filter(|it| it.len() > 1);
 
                         acc.add_struct_literal(ctx, path_ctx, strukt, path, None);
@@ -183,7 +187,7 @@ pub(crate) fn complete_expr_path(
                     hir::Adt::Union(un) => {
                         let path = ctx
                             .module
-                            .find_use_path(ctx.db, hir::ModuleDef::from(un))
+                            .find_use_path(ctx.db, hir::ModuleDef::from(un), ctx.config.prefer_core)
                             .filter(|it| it.len() > 1);
 
                         acc.add_union_literal(ctx, un, path, None);

crates/ide-completion/src/completions/flyimport.rs

@@ -262,7 +262,11 @@ fn import_on_the_fly(
 
     acc.add_all(
         import_assets
-            .search_for_imports(&ctx.sema, ctx.config.insert_use.prefix_kind)
+            .search_for_imports(
+                &ctx.sema,
+                ctx.config.insert_use.prefix_kind,
+                ctx.config.prefer_core,
+            )
             .into_iter()
             .filter(ns_filter)
             .filter(|import| {
@@ -306,7 +310,11 @@ fn import_on_the_fly_pat_(
 
     acc.add_all(
         import_assets
-            .search_for_imports(&ctx.sema, ctx.config.insert_use.prefix_kind)
+            .search_for_imports(
+                &ctx.sema,
+                ctx.config.insert_use.prefix_kind,
+                ctx.config.prefer_core,
+            )
             .into_iter()
             .filter(ns_filter)
             .filter(|import| {
@@ -344,7 +352,7 @@ fn import_on_the_fly_method(
     let user_input_lowercased = potential_import_name.to_lowercase();
 
     import_assets
-        .search_for_imports(&ctx.sema, ctx.config.insert_use.prefix_kind)
+        .search_for_imports(&ctx.sema, ctx.config.insert_use.prefix_kind, ctx.config.prefer_core)
         .into_iter()
         .filter(|import| {
             !ctx.is_item_hidden(&import.item_to_import)

crates/ide-completion/src/config.rs

@@ -17,6 +17,7 @@ pub struct CompletionConfig {
     pub callable: Option<CallableSnippets>,
     pub snippet_cap: Option<SnippetCap>,
     pub insert_use: InsertUseConfig,
+    pub prefer_core: bool,
     pub snippets: Vec<Snippet>,
 }
 

crates/ide-completion/src/lib.rs

@@ -234,7 +234,12 @@ pub fn resolve_completion_edits(
         );
         let import = items_with_name
             .filter_map(|candidate| {
-                current_module.find_use_path_prefixed(db, candidate, config.insert_use.prefix_kind)
+                current_module.find_use_path_prefixed(
+                    db,
+                    candidate,
+                    config.insert_use.prefix_kind,
+                    config.prefer_core,
+                )
             })
             .find(|mod_path| mod_path.to_string() == full_import_path);
         if let Some(import_path) = import {

crates/ide-completion/src/snippet.rs

@@ -174,8 +174,12 @@ fn import_edits(ctx: &CompletionContext<'_>, requires: &[GreenNode]) -> Option<V
             hir::PathResolution::Def(def) => def.into(),
             _ => return None,
         };
-        let path =
-            ctx.module.find_use_path_prefixed(ctx.db, item, ctx.config.insert_use.prefix_kind)?;
+        let path = ctx.module.find_use_path_prefixed(
+            ctx.db,
+            item,
+            ctx.config.insert_use.prefix_kind,
+            ctx.config.prefer_core,
+        )?;
         Some((path.len() > 1).then(|| LocatedImport::new(path.clone(), item, item, None)))
     };
     let mut res = Vec::with_capacity(requires.len());

crates/ide-completion/src/tests.rs

@@ -66,6 +66,7 @@ pub(crate) const TEST_CONFIG: CompletionConfig = CompletionConfig {
     enable_private_editable: false,
     callable: Some(CallableSnippets::FillArguments),
     snippet_cap: SnippetCap::new(true),
+    prefer_core: false,
     insert_use: InsertUseConfig {
         granularity: ImportGranularity::Crate,
         prefix_kind: PrefixKind::Plain,