src/handlers/promote_local_to_const.rs

use hir::{HirDisplay, ModuleDef, PathResolution, Semantics};
use ide_db::{
    assists::{AssistId, AssistKind},
    defs::Definition,
    helpers::node_ext::preorder_expr,
    RootDatabase,
};
use stdx::to_upper_snake_case;
use syntax::{
    ast::{self, make, HasName},
    AstNode, WalkEvent,
};

use crate::{
    assist_context::{AssistContext, Assists},
    utils::{render_snippet, Cursor},
};

// Assist: promote_local_to_const
//
// Promotes a local variable to a const item changing its name to a `SCREAMING_SNAKE_CASE` variant
// if the local uses no non-const expressions.
//
// ```
// fn main() {
//     let foo$0 = true;
//
//     if foo {
//         println!("It's true");
//     } else {
//         println!("It's false");
//     }
// }
// ```
// ->
// ```
// fn main() {
//     const $0FOO: bool = true;
//
//     if FOO {
//         println!("It's true");
//     } else {
//         println!("It's false");
//     }
// }
// ```
pub(crate) fn promote_local_to_const(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
    let pat = ctx.find_node_at_offset::<ast::IdentPat>()?;
    let name = pat.name()?;
    if !pat.is_simple_ident() {
        cov_mark::hit!(promote_local_non_simple_ident);
        return None;
    }
    let let_stmt = pat.syntax().parent().and_then(ast::LetStmt::cast)?;

    let module = ctx.sema.scope(pat.syntax()).module()?;
    let local = ctx.sema.to_def(&pat)?;
    let ty = ctx.sema.type_of_pat(&pat.into())?.original;

    if ty.contains_unknown() || ty.is_closure() {
        cov_mark::hit!(promote_lcoal_not_applicable_if_ty_not_inferred);
        return None;
    }
    let ty = ty.display_source_code(ctx.db(), module.into()).ok()?;

    let initializer = let_stmt.initializer()?;
    if !is_body_const(&ctx.sema, &initializer) {
        cov_mark::hit!(promote_local_non_const);
        return None;
    }
    let target = let_stmt.syntax().text_range();
    acc.add(
        AssistId("promote_local_to_const", AssistKind::Refactor),
        "Promote local to constant",
        target,
        |builder| {
            let name = to_upper_snake_case(&name.to_string());
            let usages = Definition::Local(local).usages(&ctx.sema).all();
            if let Some(usages) = usages.references.get(&ctx.file_id()) {
                for usage in usages {
                    builder.replace(usage.range, &name);
                }
            }

            let item = make::item_const(None, make::name(&name), make::ty(&ty), initializer);
            match ctx.config.snippet_cap.zip(item.name()) {
                Some((cap, name)) => builder.replace_snippet(
                    cap,
                    target,
                    render_snippet(cap, item.syntax(), Cursor::Before(name.syntax())),
                ),
                None => builder.replace(target, item.to_string()),
            }
        },
    )
}

fn is_body_const(sema: &Semantics<RootDatabase>, expr: &ast::Expr) -> bool {
    let mut is_const = true;
    preorder_expr(expr, &mut |ev| {
        let expr = match ev {
            WalkEvent::Enter(_) if !is_const => return true,
            WalkEvent::Enter(expr) => expr,
            WalkEvent::Leave(_) => return false,
        };
        match expr {
            ast::Expr::CallExpr(call) => {
                if let Some(ast::Expr::PathExpr(path_expr)) = call.expr() {
                    if let Some(PathResolution::Def(ModuleDef::Function(func))) =
                        path_expr.path().and_then(|path| sema.resolve_path(&path))
                    {
                        is_const &= func.is_const(sema.db);
                    }
                }
            }
            ast::Expr::MethodCallExpr(call) => {
                is_const &=
                    sema.resolve_method_call(&call).map(|it| it.is_const(sema.db)).unwrap_or(true)
            }
            ast::Expr::BoxExpr(_)
            | ast::Expr::ForExpr(_)
            | ast::Expr::ReturnExpr(_)
            | ast::Expr::TryExpr(_)
            | ast::Expr::YieldExpr(_)
            | ast::Expr::AwaitExpr(_) => is_const = false,
            _ => (),
        }
        !is_const
    });
    is_const
}

#[cfg(test)]
mod tests {
    use crate::tests::{check_assist, check_assist_not_applicable};

    use super::*;

    #[test]
    fn simple() {
        check_assist(
            promote_local_to_const,
            r"
fn foo() {
    let x$0 = 0;
    let y = x;
}
",
            r"
fn foo() {
    const $0X: i32 = 0;
    let y = X;
}
",
        );
    }

    #[test]
    fn not_applicable_non_const_meth_call() {
        cov_mark::check!(promote_local_non_const);
        check_assist_not_applicable(
            promote_local_to_const,
            r"
struct Foo;
impl Foo {
    fn foo(self) {}
}
fn foo() {
    let x$0 = Foo.foo();
}
",
        );
    }

    #[test]
    fn not_applicable_non_const_call() {
        check_assist_not_applicable(
            promote_local_to_const,
            r"
fn bar(self) {}
fn foo() {
    let x$0 = bar();
}
",
        );
    }

    #[test]
    fn not_applicable_unknown_ty() {
        cov_mark::check!(promote_lcoal_not_applicable_if_ty_not_inferred);
        check_assist_not_applicable(
            promote_local_to_const,
            r"
fn foo() {
    let x$0 = bar();
}
",
        );
    }

    #[test]
    fn not_applicable_non_simple_ident() {
        cov_mark::check!(promote_local_non_simple_ident);
        check_assist_not_applicable(
            promote_local_to_const,
            r"
fn foo() {
    let ref x$0 = ();
}
",
        );
        check_assist_not_applicable(
            promote_local_to_const,
            r"
fn foo() {
    let mut x$0 = ();
}
",
        );
    }
}