clippy_lints/src/temporary_assignment.rs

use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::hir::{Expr, ExprAssign, ExprField, ExprStruct, ExprTup, ExprTupField};
use utils::is_adjusted;
use utils::span_lint;

/// **What it does:** Checks for construction of a structure or tuple just to
/// assign a value in it.
///
/// **Why is this bad?** Readability. If the structure is only created to be
/// updated, why not write the structure you want in the first place?
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// (0, 0).0 = 1
/// ```
declare_lint! {
    pub TEMPORARY_ASSIGNMENT,
    Warn,
    "assignments to temporaries"
}

fn is_temporary(expr: &Expr) -> bool {
    match expr.node {
        ExprStruct(..) | ExprTup(..) => true,
        _ => false,
    }
}

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

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

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        if let ExprAssign(ref target, _) = expr.node {
            match target.node {
                ExprField(ref base, _) | ExprTupField(ref base, _) => if is_temporary(base) && !is_adjusted(cx, base) {
                    span_lint(cx, TEMPORARY_ASSIGNMENT, expr.span, "assignment to temporary");
                },
                _ => (),
            }
        }
    }
}