rust/src/eta_reduction.rs

use rustc::lint::*;
use rustc_front::hir::*;
use rustc::middle::ty;

use utils::{snippet_opt, span_lint_and_then, is_adjusted};


#[allow(missing_copy_implementations)]
pub struct EtaPass;


/// **What it does:** This lint checks for closures which just call another function where the function can be called directly. `unsafe` functions or calls where types get adjusted are ignored.
///
/// **Why is this bad?** Needlessly creating a closure just costs heap space and adds code for no benefit.
///
/// **Known problems:** None
///
/// **Example:** `xs.map(|x| foo(x))` where `foo(_)` is a plain function that takes the exact argument type of `x`.
declare_lint! {
    pub REDUNDANT_CLOSURE, Warn,
    "using redundant closures, i.e. `|a| foo(a)` (which can be written as just `foo`)"
}

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

impl LateLintPass for EtaPass {
    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
        match expr.node {
            ExprCall(_, ref args) |
            ExprMethodCall(_, _, ref args) => {
                for arg in args {
                    check_closure(cx, arg)
                }
            }
            _ => (),
        }
    }
}

fn check_closure(cx: &LateContext, expr: &Expr) {
    if let ExprClosure(_, ref decl, ref blk) = expr.node {
        if !blk.stmts.is_empty() {
            // || {foo(); bar()}; can't be reduced here
            return;
        }

        if let Some(ref ex) = blk.expr {
            if let ExprCall(ref caller, ref args) = ex.node {
                if args.len() != decl.inputs.len() {
                    // Not the same number of arguments, there
                    // is no way the closure is the same as the function
                    return;
                }
                if is_adjusted(cx, ex) || args.iter().any(|arg| is_adjusted(cx, arg)) {
                    // Are the expression or the arguments type-adjusted? Then we need the closure
                    return;
                }
                let fn_ty = cx.tcx.expr_ty(caller);
                if let ty::TyBareFn(_, fn_ty) = fn_ty.sty {
                    // Is it an unsafe function? They don't implement the closure traits
                    if fn_ty.unsafety == Unsafety::Unsafe {
                        return;
                    }
                }
                for (ref a1, ref a2) in decl.inputs.iter().zip(args) {
                    if let PatIdent(_, ident, _) = a1.pat.node {
                        // XXXManishearth Should I be checking the binding mode here?
                        if let ExprPath(None, ref p) = a2.node {
                            if p.segments.len() != 1 {
                                // If it's a proper path, it can't be a local variable
                                return;
                            }
                            if p.segments[0].identifier != ident.node {
                                // The two idents should be the same
                                return;
                            }
                        } else {
                            return;
                        }
                    } else {
                        return;
                    }
                }
                span_lint_and_then(cx, REDUNDANT_CLOSURE, expr.span, "redundant closure found", |db| {
                    if let Some(snippet) = snippet_opt(cx, caller.span) {
                        db.span_suggestion(expr.span, "remove closure as shown:", snippet);
                    }
                });
            }
        }
    }
}