rust/src/eta_reduction.rs

use syntax::ast::*;
use rustc::lint::{Context, LintPass, LintArray, Lint, Level};
use syntax::codemap::{Span, Spanned};
use syntax::print::pprust::expr_to_string;


#[allow(missing_copy_implementations)]
pub struct EtaPass;


declare_lint!(pub REDUNDANT_CLOSURE, Warn,
              "Warn on usage of redundant closures, i.e. `|a| foo(a)`");

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

    fn check_expr(&mut self, cx: &Context, 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;
                    }
                    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
                        }
                    }
                    cx.span_lint(REDUNDANT_CLOSURE, expr.span,
                                 &format!("Redundant closure found, consider using `{}` in its place",
                                          expr_to_string(caller))[..])
                }
            }
        }
    }
}