rust-clippy/clippy_lints/src/map_unit_fn.rs

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::snippet;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::{iter_input_pats, method_chain_args};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, Ty};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;
use rustc_span::sym;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of `option.map(f)` where f is a function
    /// or closure that returns the unit type `()`.
    ///
    /// ### Why is this bad?
    /// Readability, this can be written more clearly with
    /// an if let statement
    ///
    /// ### Example
    /// ```rust
    /// # fn do_stuff() -> Option<String> { Some(String::new()) }
    /// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
    /// # fn format_msg(foo: String) -> String { String::new() }
    /// let x: Option<String> = do_stuff();
    /// x.map(log_err_msg);
    /// # let x: Option<String> = do_stuff();
    /// x.map(|msg| log_err_msg(format_msg(msg)));
    /// ```
    ///
    /// The correct use would be:
    ///
    /// ```rust
    /// # fn do_stuff() -> Option<String> { Some(String::new()) }
    /// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
    /// # fn format_msg(foo: String) -> String { String::new() }
    /// let x: Option<String> = do_stuff();
    /// if let Some(msg) = x {
    ///     log_err_msg(msg);
    /// }
    ///
    /// # let x: Option<String> = do_stuff();
    /// if let Some(msg) = x {
    ///     log_err_msg(format_msg(msg));
    /// }
    /// ```
    pub OPTION_MAP_UNIT_FN,
    complexity,
    "using `option.map(f)`, where `f` is a function or closure that returns `()`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for usage of `result.map(f)` where f is a function
    /// or closure that returns the unit type `()`.
    ///
    /// ### Why is this bad?
    /// Readability, this can be written more clearly with
    /// an if let statement
    ///
    /// ### Example
    /// ```rust
    /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
    /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
    /// # fn format_msg(foo: String) -> String { String::new() }
    /// let x: Result<String, String> = do_stuff();
    /// x.map(log_err_msg);
    /// # let x: Result<String, String> = do_stuff();
    /// x.map(|msg| log_err_msg(format_msg(msg)));
    /// ```
    ///
    /// The correct use would be:
    ///
    /// ```rust
    /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
    /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
    /// # fn format_msg(foo: String) -> String { String::new() }
    /// let x: Result<String, String> = do_stuff();
    /// if let Ok(msg) = x {
    ///     log_err_msg(msg);
    /// };
    /// # let x: Result<String, String> = do_stuff();
    /// if let Ok(msg) = x {
    ///     log_err_msg(format_msg(msg));
    /// };
    /// ```
    pub RESULT_MAP_UNIT_FN,
    complexity,
    "using `result.map(f)`, where `f` is a function or closure that returns `()`"
}

declare_lint_pass!(MapUnit => [OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN]);

fn is_unit_type(ty: Ty<'_>) -> bool {
    match ty.kind() {
        ty::Tuple(slice) => slice.is_empty(),
        ty::Never => true,
        _ => false,
    }
}

fn is_unit_function(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
    let ty = cx.typeck_results().expr_ty(expr);

    if let ty::FnDef(id, _) = *ty.kind() {
        if let Some(fn_type) = cx.tcx.fn_sig(id).no_bound_vars() {
            return is_unit_type(fn_type.output());
        }
    }
    false
}

fn is_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
    is_unit_type(cx.typeck_results().expr_ty(expr))
}

/// The expression inside a closure may or may not have surrounding braces and
/// semicolons, which causes problems when generating a suggestion. Given an
/// expression that evaluates to '()' or '!', recursively remove useless braces
/// and semi-colons until is suitable for including in the suggestion template
fn reduce_unit_expression<'a>(cx: &LateContext<'_>, expr: &'a hir::Expr<'_>) -> Option<Span> {
    if !is_unit_expression(cx, expr) {
        return None;
    }

    match expr.kind {
        hir::ExprKind::Call(_, _) | hir::ExprKind::MethodCall(_, _, _, _) => {
            // Calls can't be reduced any more
            Some(expr.span)
        },
        hir::ExprKind::Block(block, _) => {
            match (block.stmts, block.expr.as_ref()) {
                (&[], Some(inner_expr)) => {
                    // If block only contains an expression,
                    // reduce `{ X }` to `X`
                    reduce_unit_expression(cx, inner_expr)
                },
                (&[ref inner_stmt], None) => {
                    // If block only contains statements,
                    // reduce `{ X; }` to `X` or `X;`
                    match inner_stmt.kind {
                        hir::StmtKind::Local(local) => Some(local.span),
                        hir::StmtKind::Expr(e) => Some(e.span),
                        hir::StmtKind::Semi(..) => Some(inner_stmt.span),
                        hir::StmtKind::Item(..) => None,
                    }
                },
                _ => {
                    // For closures that contain multiple statements
                    // it's difficult to get a correct suggestion span
                    // for all cases (multi-line closures specifically)
                    //
                    // We do not attempt to build a suggestion for those right now.
                    None
                },
            }
        },
        _ => None,
    }
}

fn unit_closure<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &hir::Expr<'_>,
) -> Option<(&'tcx hir::Param<'tcx>, &'tcx hir::Expr<'tcx>)> {
    if_chain! {
        if let hir::ExprKind::Closure(_, decl, inner_expr_id, _, _) = expr.kind;
        let body = cx.tcx.hir().body(inner_expr_id);
        let body_expr = &body.value;
        if decl.inputs.len() == 1;
        if is_unit_expression(cx, body_expr);
        if let Some(binding) = iter_input_pats(decl, body).next();
        then {
            return Some((binding, body_expr));
        }
    }
    None
}

/// Builds a name for the let binding variable (`var_arg`)
///
/// `x.field` => `x_field`
/// `y` => `_y`
///
/// Anything else will return `a`.
fn let_binding_name(cx: &LateContext<'_>, var_arg: &hir::Expr<'_>) -> String {
    match &var_arg.kind {
        hir::ExprKind::Field(_, _) => snippet(cx, var_arg.span, "_").replace(".", "_"),
        hir::ExprKind::Path(_) => format!("_{}", snippet(cx, var_arg.span, "")),
        _ => "a".to_string(),
    }
}

#[must_use]
fn suggestion_msg(function_type: &str, map_type: &str) -> String {
    format!(
        "called `map(f)` on an `{0}` value where `f` is a {1} that returns the unit type `()`",
        map_type, function_type
    )
}

fn lint_map_unit_fn(cx: &LateContext<'_>, stmt: &hir::Stmt<'_>, expr: &hir::Expr<'_>, map_args: &[hir::Expr<'_>]) {
    let var_arg = &map_args[0];

    let (map_type, variant, lint) =
        if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym::option_type) {
            ("Option", "Some", OPTION_MAP_UNIT_FN)
        } else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym::result_type) {
            ("Result", "Ok", RESULT_MAP_UNIT_FN)
        } else {
            return;
        };
    let fn_arg = &map_args[1];

    if is_unit_function(cx, fn_arg) {
        let msg = suggestion_msg("function", map_type);
        let suggestion = format!(
            "if let {0}({binding}) = {1} {{ {2}({binding}) }}",
            variant,
            snippet(cx, var_arg.span, "_"),
            snippet(cx, fn_arg.span, "_"),
            binding = let_binding_name(cx, var_arg)
        );

        span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
            diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::MachineApplicable);
        });
    } else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
        let msg = suggestion_msg("closure", map_type);

        span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
            if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
                let suggestion = format!(
                    "if let {0}({1}) = {2} {{ {3} }}",
                    variant,
                    snippet(cx, binding.pat.span, "_"),
                    snippet(cx, var_arg.span, "_"),
                    snippet(cx, reduced_expr_span, "_")
                );
                diag.span_suggestion(
                    stmt.span,
                    "try this",
                    suggestion,
                    Applicability::MachineApplicable, // snippet
                );
            } else {
                let suggestion = format!(
                    "if let {0}({1}) = {2} {{ ... }}",
                    variant,
                    snippet(cx, binding.pat.span, "_"),
                    snippet(cx, var_arg.span, "_"),
                );
                diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::HasPlaceholders);
            }
        });
    }
}

impl<'tcx> LateLintPass<'tcx> for MapUnit {
    fn check_stmt(&mut self, cx: &LateContext<'_>, stmt: &hir::Stmt<'_>) {
        if stmt.span.from_expansion() {
            return;
        }

        if let hir::StmtKind::Semi(expr) = stmt.kind {
            if let Some(arglists) = method_chain_args(expr, &["map"]) {
                lint_map_unit_fn(cx, stmt, expr, arglists[0]);
            }
        }
    }
}