rust-clippy/clippy_lints/src/is_unit_expr.rs

use rustc::lint::*;
use syntax::ast::*;
use syntax::ext::quote::rt::Span;
use utils::span_note_and_lint;

/// **What it does:** Checks for
///  - () being assigned to a variable
///  - () being passed to a function
///
/// **Why is this bad?** It is extremely unlikely that a user intended to
/// assign '()' to valiable. Instead,
/// Unit is what a block evaluates to when it returns nothing. This is
/// typically caused by a trailing
///   unintended semicolon.
///
/// **Known problems:** None.
///
/// **Example:**
/// * `let x = {"foo" ;}` when the user almost certainly intended `let x
/// ={"foo"}`
declare_lint! {
    pub UNIT_EXPR,
    Warn,
    "unintended assignment or use of a unit typed value"
}

#[derive(Copy, Clone)]
pub struct UnitExpr;

impl LintPass for UnitExpr {
    fn get_lints(&self) -> LintArray {
        lint_array!(UNIT_EXPR)
    }
}

impl EarlyLintPass for UnitExpr {
    fn check_expr(&mut self, cx: &EarlyContext, expr: &Expr) {
        if let ExprKind::Assign(ref _left, ref right) = expr.node {
            if let Some(span) = is_unit_expr(right) {
                span_note_and_lint(
                    cx,
                    UNIT_EXPR,
                    expr.span,
                    "This expression evaluates to the Unit type ()",
                    span,
                    "Consider removing the trailing semicolon",
                );
            }
        }
        if let ExprKind::MethodCall(ref _left, ref args) = expr.node {
            for arg in args {
                if let Some(span) = is_unit_expr(arg) {
                    span_note_and_lint(
                        cx,
                        UNIT_EXPR,
                        expr.span,
                        "This expression evaluates to the Unit type ()",
                        span,
                        "Consider removing the trailing semicolon",
                    );
                }
            }
        }
        if let ExprKind::Call(_, ref args) = expr.node {
            for arg in args {
                if let Some(span) = is_unit_expr(arg) {
                    span_note_and_lint(
                        cx,
                        UNIT_EXPR,
                        expr.span,
                        "This expression evaluates to the Unit type ()",
                        span,
                        "Consider removing the trailing semicolon",
                    );
                }
            }
        }
    }

    fn check_stmt(&mut self, cx: &EarlyContext, stmt: &Stmt) {
        if let StmtKind::Local(ref local) = stmt.node {
            if local.pat.node == PatKind::Wild {
                return;
            }
            if let Some(ref expr) = local.init {
                if let Some(span) = is_unit_expr(expr) {
                    span_note_and_lint(
                        cx,
                        UNIT_EXPR,
                        expr.span,
                        "This expression evaluates to the Unit type ()",
                        span,
                        "Consider removing the trailing semicolon",
                    );
                }
            }
        }
    }
}

fn is_unit_expr(expr: &Expr) -> Option<Span> {
    match expr.node {
        ExprKind::Block(ref block) => if check_last_stmt_in_block(block) {
            Some(block.stmts[block.stmts.len() - 1].span)
        } else {
            None
        },
        ExprKind::If(_, ref then, ref else_) => {
            let check_then = check_last_stmt_in_block(then);
            if let Some(ref else_) = *else_ {
                let check_else = is_unit_expr(else_);
                if let Some(ref expr_else) = check_else {
                    return Some(*expr_else);
                }
            }
            if check_then {
                Some(expr.span)
            } else {
                None
            }
        },
        ExprKind::Match(ref _pattern, ref arms) => {
            for arm in arms {
                if let Some(expr) = is_unit_expr(&arm.body) {
                    return Some(expr);
                }
            }
            None
        },
        _ => None,
    }
}

fn check_last_stmt_in_block(block: &Block) -> bool {
    let final_stmt = &block.stmts[block.stmts.len() - 1];


    // Made a choice here to risk false positives on divergent macro invocations
    // like `panic!()`
    match final_stmt.node {
        StmtKind::Expr(_) => false,
        StmtKind::Semi(ref expr) => match expr.node {
            ExprKind::Break(_, _) | ExprKind::Continue(_) | ExprKind::Ret(_) => false,
            _ => true,
        },
        _ => true,
    }
}