clippy_lints/src/manual_string_new.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::sym;
use rustc_ast::LitKind;
use rustc_errors::Applicability::MachineApplicable;
use rustc_hir::{Expr, ExprKind, PathSegment, QPath, TyKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::declare_lint_pass;
use rustc_span::Span;

declare_clippy_lint! {
    /// ### What it does
    ///
    /// Checks for usage of `""` to create a `String`, such as `"".to_string()`, `"".to_owned()`,
    /// `String::from("")` and others.
    ///
    /// ### Why is this bad?
    ///
    /// Different ways of creating an empty string makes your code less standardized, which can
    /// be confusing.
    ///
    /// ### Example
    /// ```no_run
    /// let a = "".to_string();
    /// let b: String = "".into();
    /// ```
    /// Use instead:
    /// ```no_run
    /// let a = String::new();
    /// let b = String::new();
    /// ```
    #[clippy::version = "1.65.0"]
    pub MANUAL_STRING_NEW,
    pedantic,
    "empty String is being created manually"
}

declare_lint_pass!(ManualStringNew => [MANUAL_STRING_NEW]);

impl LateLintPass<'_> for ManualStringNew {
    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
        if expr.span.from_expansion() {
            return;
        }

        let ty = cx.typeck_results().expr_ty(expr);
        match ty.kind() {
            ty::Adt(adt_def, _) if adt_def.is_struct() => {
                if cx.tcx.lang_items().string() != Some(adt_def.did()) {
                    return;
                }
            },
            _ => return,
        }

        match expr.kind {
            ExprKind::Call(func, [arg]) => {
                parse_call(cx, expr.span, func, arg);
            },
            ExprKind::MethodCall(path_segment, receiver, ..) => {
                parse_method_call(cx, expr.span, path_segment, receiver);
            },
            _ => (),
        }
    }
}

/// Checks if an expression's kind corresponds to an empty &str.
fn is_expr_kind_empty_str(expr_kind: &ExprKind<'_>) -> bool {
    if let ExprKind::Lit(lit) = expr_kind
        && let LitKind::Str(value, _) = lit.node
        && value == sym::empty
    {
        return true;
    }

    false
}

fn warn_then_suggest(cx: &LateContext<'_>, span: Span) {
    span_lint_and_sugg(
        cx,
        MANUAL_STRING_NEW,
        span,
        "empty String is being created manually",
        "consider using",
        "String::new()".into(),
        MachineApplicable,
    );
}

/// Tries to parse an expression as a method call, emitting the warning if necessary.
fn parse_method_call(cx: &LateContext<'_>, span: Span, path_segment: &PathSegment<'_>, receiver: &Expr<'_>) {
    let method_arg_kind = &receiver.kind;
    if matches!(path_segment.ident.name, sym::to_string | sym::to_owned | sym::into)
        && is_expr_kind_empty_str(method_arg_kind)
    {
        warn_then_suggest(cx, span);
    } else if let ExprKind::Call(func, [arg]) = method_arg_kind {
        // If our first argument is a function call itself, it could be an `unwrap`-like function.
        // E.g. String::try_from("hello").unwrap(), TryFrom::try_from("").expect("hello"), etc.
        parse_call(cx, span, func, arg);
    }
}

/// Tries to parse an expression as a function call, emitting the warning if necessary.
fn parse_call(cx: &LateContext<'_>, span: Span, func: &Expr<'_>, arg: &Expr<'_>) {
    if let ExprKind::Path(qpath) = &func.kind {
        // String::from(...) or String::try_from(...)
        if let QPath::TypeRelative(ty, path_seg) = qpath
            && [sym::from, sym::try_from].contains(&path_seg.ident.name)
            && let TyKind::Path(qpath) = &ty.kind
            && let QPath::Resolved(_, path) = qpath
            && let [path_seg] = path.segments
            && path_seg.ident.name == sym::String
            && is_expr_kind_empty_str(&arg.kind)
        {
            warn_then_suggest(cx, span);
        } else if let QPath::Resolved(_, path) = qpath
            // From::from(...) or TryFrom::try_from(...)
            && let [path_seg1, path_seg2] = path.segments
            && is_expr_kind_empty_str(&arg.kind)
            && ((path_seg1.ident.name == sym::From && path_seg2.ident.name == sym::from)
                || (path_seg1.ident.name == sym::TryFrom && path_seg2.ident.name == sym::try_from))
        {
            warn_then_suggest(cx, span);
        }
    }
}