Implement "Extract type alias" assist

Author: Jonas Schievink

crates/ide_assists/src/handlers/extract_type_alias.rs

@@ -0,0 +1,165 @@
+use syntax::{
+    ast::{self, AstNode},
+    SyntaxKind,
+};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: extract_type_alias
+//
+// Extracts the selected type as a type alias.
+//
+// ```
+// struct S {
+//     field: $0(u8, u8, u8)$0,
+// }
+// ```
+// ->
+// ```
+// type Type = (u8, u8, u8);
+//
+// struct S {
+//     field: Type,
+// }
+// ```
+pub(crate) fn extract_type_alias(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    if ctx.frange.range.is_empty() {
+        return None;
+    }
+
+    let node = match ctx.covering_element() {
+        syntax::NodeOrToken::Node(node) => node,
+        syntax::NodeOrToken::Token(tok) => tok.parent()?,
+    };
+    let range = node.text_range();
+    let mut type_like_node = None;
+    for node in node.ancestors() {
+        if node.text_range() != range {
+            break;
+        }
+
+        let kind = node.kind();
+        if ast::Type::can_cast(kind) || kind == SyntaxKind::TYPE_ARG {
+            type_like_node = Some(node);
+            break;
+        }
+    }
+
+    let node = type_like_node?;
+
+    let insert = ctx.find_node_at_offset::<ast::Item>()?.syntax().text_range().start();
+    let target = node.text_range();
+
+    acc.add(
+        AssistId("extract_type_alias", AssistKind::RefactorExtract),
+        "Extract type as type alias",
+        target,
+        |builder| {
+            builder.edit_file(ctx.frange.file_id);
+            // FIXME: add snippet support
+            builder.replace(target, "Type");
+            builder.insert(insert, format!("type Type = {};\n\n", node));
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn test_not_applicable_without_selection() {
+        check_assist_not_applicable(
+            extract_type_alias,
+            r"
+struct S {
+    field: $0(u8, u8, u8),
+}
+            ",
+        );
+    }
+
+    #[test]
+    fn test_simple_types() {
+        check_assist(
+            extract_type_alias,
+            r"
+struct S {
+    field: $0u8$0,
+}
+            ",
+            r#"
+type Type = u8;
+
+struct S {
+    field: Type,
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn test_generic_type_arg() {
+        check_assist(
+            extract_type_alias,
+            r"
+fn generic<T>() {}
+
+fn f() {
+    generic::<$0()$0>();
+}
+            ",
+            r#"
+fn generic<T>() {}
+
+type Type = ();
+
+fn f() {
+    generic::<Type>();
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn test_inner_type_arg() {
+        check_assist(
+            extract_type_alias,
+            r"
+struct Vec<T> {}
+struct S {
+    v: Vec<Vec<$0Vec<u8>$0>>,
+}
+            ",
+            r#"
+struct Vec<T> {}
+type Type = Vec<u8>;
+
+struct S {
+    v: Vec<Vec<Type>>,
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn test_extract_inner_type() {
+        check_assist(
+            extract_type_alias,
+            r"
+struct S {
+    field: ($0u8$0,),
+}
+            ",
+            r#"
+type Type = u8;
+
+struct S {
+    field: (Type,),
+}
+            "#,
+        );
+    }
+}

crates/ide_assists/src/lib.rs

@@ -121,6 +121,7 @@ mod handlers {
     mod expand_glob_import;
     mod extract_function;
     mod extract_struct_from_enum_variant;
+    mod extract_type_alias;
     mod extract_variable;
     mod fill_match_arms;
     mod fix_visibility;
@@ -187,6 +188,7 @@ mod handlers {
             early_return::convert_to_guarded_return,
             expand_glob_import::expand_glob_import,
             extract_struct_from_enum_variant::extract_struct_from_enum_variant,
+            extract_type_alias::extract_type_alias,
             fill_match_arms::fill_match_arms,
             fix_visibility::fix_visibility,
             flip_binexpr::flip_binexpr,

crates/ide_assists/src/tests/generated.rs

@@ -328,6 +328,25 @@ enum A { One(One) }
     )
 }
 
+#[test]
+fn doctest_extract_type_alias() {
+    check_doc_test(
+        "extract_type_alias",
+        r#####"
+struct S {
+    field: $0(u8, u8, u8)$0,
+}
+"#####,
+        r#####"
+type Type = (u8, u8, u8);
+
+struct S {
+    field: Type,
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_extract_variable() {
     check_doc_test(