rust-clippy/clippy_lints/src/is_unit_expr.rs

use rustc::lint::*;
use syntax::ast::*;
use std::ops::Deref;
use syntax::ext::quote::rt::Span;


/// **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){
                    cx.span_lint(UNIT_EXPR, span, "Consider removing the trailing semicolon");
            }
        }
        if let ExprKind::MethodCall(ref _left, ref args) = expr.node {
            for ref arg in args{
                if let Some(span) = is_unit_expr(arg){
                    cx.span_lint(UNIT_EXPR, span, "Consider removing the trailing semicolon");
                }            
            }
        }
        if let ExprKind::Call( _, ref args) = expr.node{
            for ref arg in args{
                if let Some(span) = is_unit_expr(arg){
                       cx.span_lint(UNIT_EXPR, 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){
                    cx.span_lint(UNIT_EXPR, 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){
                 return Some(block.stmts[block.stmts.len()-1].span.clone());
             } else{
                 return 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_.deref());
                if let Some(ref expr_else) = check_else{
                    return Some(expr_else.clone());
                }else{
                    return Some(expr.span.clone());
                }
            } 
            if check_then { 
                return Some(expr.span.clone());

            } else{
                return Some(expr.span.clone());
            }
        },
        ExprKind::Match(ref _pattern, ref arms ) =>{
            for ref arm in arms{
                if let Some(expr) = is_unit_expr(&arm.body){
                    return Some(expr);
                }
            }
            return None;
        }
        _ => return None,
    }
}

fn check_last_stmt_in_block(block: &Block)->bool{
    let ref final_stmt = &block.stmts[block.stmts.len()-1];
        if let StmtKind::Expr(_) = final_stmt.node{
                return false;
            }
            else{
              return true; 
            }
}