Merge #10076

10076: Use struct init shorthand when applicable in fill struct fields assist r=matklad a=nathanwhit

This PR tweaks the fill struct fields assist to use the struct init shorthand when a local variable with a matching name and type is in scope.

For example:
```rust
struct Foo {
    a: usize,
    b: i32,
    c: char,
}

fn main() {
    let a = 1;
    let b = 2;
    let c = 3;
    let foo = Foo { <|> };
}
```
Before we would insert
```rust
Foo {
    a: (),
    b: (),
    c: (),
}
```
now we would insert
```rust
Foo {
    a,
    b,
    c: ()
}
```

Co-authored-by: nathan.whitaker <nathan.whitaker01@gmail.com>

crates/ide_diagnostics/src/handlers/missing_fields.rs

@@ -1,6 +1,7 @@
 use either::Either;
 use hir::{db::AstDatabase, InFile};
 use ide_db::{assists::Assist, source_change::SourceChange};
+use rustc_hash::FxHashMap;
 use stdx::format_to;
 use syntax::{algo, ast::make, AstNode, SyntaxNodePtr};
 use text_edit::TextEdit;
@@ -54,9 +55,27 @@ fn fixes(ctx: &DiagnosticsContext<'_>, d: &hir::MissingFields) -> Option<Vec<Ass
     };
     let old_field_list = field_list_parent.record_expr_field_list()?;
     let new_field_list = old_field_list.clone_for_update();
-    for f in d.missed_fields.iter() {
+    let mut locals = FxHashMap::default();
+    ctx.sema.scope(field_list_parent.syntax()).process_all_names(&mut |name, def| {
+        if let hir::ScopeDef::Local(local) = def {
+            locals.insert(name.clone(), local);
+        }
+    });
+    let missing_fields = ctx.sema.record_literal_missing_fields(&field_list_parent);
+    for (f, ty) in missing_fields.iter() {
+        let field_expr = if let Some(local_candidate) = locals.get(&f.name(ctx.sema.db)) {
+            cov_mark::hit!(field_shorthand);
+            let candidate_ty = local_candidate.ty(ctx.sema.db);
+            if ty.could_unify_with(ctx.sema.db, &candidate_ty) {
+                None
+            } else {
+                Some(make::expr_unit())
+            }
+        } else {
+            Some(make::expr_unit())
+        };
         let field =
-            make::record_expr_field(make::name_ref(&f.to_string()), Some(make::expr_unit()))
+            make::record_expr_field(make::name_ref(&f.name(ctx.sema.db).to_string()), field_expr)
                 .clone_for_update();
         new_field_list.add_field(field);
     }
@@ -224,7 +243,7 @@ enum Expr {
 
 impl Expr {
     fn new_bin(lhs: Box<Expr>, rhs: Box<Expr>) -> Expr {
-        Expr::Bin { lhs: (), rhs: () }
+        Expr::Bin { lhs, rhs }
     }
 }
 "#,
@@ -324,6 +343,94 @@ fn f() {
         );
     }
 
+    #[test]
+    fn test_fill_struct_fields_shorthand() {
+        cov_mark::check!(field_shorthand);
+        check_fix(
+            r#"
+struct S { a: &'static str, b: i32 }
+
+fn f() {
+    let a = "hello";
+    let b = 1i32;
+    S {
+        $0
+    };
+}
+"#,
+            r#"
+struct S { a: &'static str, b: i32 }
+
+fn f() {
+    let a = "hello";
+    let b = 1i32;
+    S {
+        a,
+        b,
+    };
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn test_fill_struct_fields_shorthand_ty_mismatch() {
+        check_fix(
+            r#"
+struct S { a: &'static str, b: i32 }
+
+fn f() {
+    let a = "hello";
+    let b = 1usize;
+    S {
+        $0
+    };
+}
+"#,
+            r#"
+struct S { a: &'static str, b: i32 }
+
+fn f() {
+    let a = "hello";
+    let b = 1usize;
+    S {
+        a,
+        b: (),
+    };
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn test_fill_struct_fields_shorthand_unifies() {
+        check_fix(
+            r#"
+struct S<T> { a: &'static str, b: T }
+
+fn f() {
+    let a = "hello";
+    let b = 1i32;
+    S {
+        $0
+    };
+}
+"#,
+            r#"
+struct S<T> { a: &'static str, b: T }
+
+fn f() {
+    let a = "hello";
+    let b = 1i32;
+    S {
+        a,
+        b,
+    };
+}
+"#,
+        );
+    }
+
     #[test]
     fn import_extern_crate_clash_with_inner_item() {
         // This is more of a resolver test, but doesn't really work with the hir_def testsuite.