rust/clippy_lints/src/returns.rs

use rustc::lint::*;
use rustc::{declare_lint, lint_array};
use if_chain::if_chain;
use syntax::ast;
use syntax::codemap::Span;
use syntax::visit::FnKind;

use crate::utils::{in_external_macro, in_macro, match_path_ast, snippet_opt, span_lint_and_then, span_note_and_lint};

/// **What it does:** Checks for return statements at the end of a block.
///
/// **Why is this bad?** Removing the `return` and semicolon will make the code
/// more rusty.
///
/// **Known problems:** If the computation returning the value borrows a local
/// variable, removing the `return` may run afoul of the borrow checker.
///
/// **Example:**
/// ```rust
/// fn foo(x: usize) { return x; }
/// ```
declare_clippy_lint! {
    pub NEEDLESS_RETURN,
    style,
    "using a return statement like `return expr;` where an expression would suffice"
}

/// **What it does:** Checks for `let`-bindings, which are subsequently
/// returned.
///
/// **Why is this bad?** It is just extraneous code. Remove it to make your code
/// more rusty.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// { let x = ..; x }
/// ```
declare_clippy_lint! {
    pub LET_AND_RETURN,
    style,
    "creating a let-binding and then immediately returning it like `let x = expr; x` at \
     the end of a block"
}

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

impl ReturnPass {
    // Check the final stmt or expr in a block for unnecessary return.
    fn check_block_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
        if let Some(stmt) = block.stmts.last() {
            match stmt.node {
                ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
                    self.check_final_expr(cx, expr, Some(stmt.span));
                },
                _ => (),
            }
        }
    }

    // Check a the final expression in a block if it's a return.
    fn check_final_expr(&mut self, cx: &EarlyContext, expr: &ast::Expr, span: Option<Span>) {
        match expr.node {
            // simple return is always "bad"
            ast::ExprKind::Ret(Some(ref inner)) => {
                // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
                if !expr.attrs.iter().any(attr_is_cfg) {
                    self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.span);
                }
            },
            // a whole block? check it!
            ast::ExprKind::Block(ref block, _) => {
                self.check_block_return(cx, block);
            },
            // an if/if let expr, check both exprs
            // note, if without else is going to be a type checking error anyways
            // (except for unit type functions) so we don't match it
            ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
                self.check_block_return(cx, ifblock);
                self.check_final_expr(cx, elsexpr, None);
            },
            // a match expr, check all arms
            ast::ExprKind::Match(_, ref arms) => for arm in arms {
                self.check_final_expr(cx, &arm.body, Some(arm.body.span));
            },
            _ => (),
        }
    }

    fn emit_return_lint(&mut self, cx: &EarlyContext, ret_span: Span, inner_span: Span) {
        if in_external_macro(cx, inner_span) || in_macro(inner_span) {
            return;
        }
        span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
            if let Some(snippet) = snippet_opt(cx, inner_span) {
                db.span_suggestion(ret_span, "remove `return` as shown", snippet);
            }
        });
    }

    // Check for "let x = EXPR; x"
    fn check_let_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
        let mut it = block.stmts.iter();

        // we need both a let-binding stmt and an expr
        if_chain! {
            if let Some(retexpr) = it.next_back();
            if let ast::StmtKind::Expr(ref retexpr) = retexpr.node;
            if let Some(stmt) = it.next_back();
            if let ast::StmtKind::Local(ref local) = stmt.node;
            // don't lint in the presence of type inference
            if local.ty.is_none();
            if !local.attrs.iter().any(attr_is_cfg);
            if let Some(ref initexpr) = local.init;
            if let ast::PatKind::Ident(_, ident, _) = local.pat.node;
            if let ast::ExprKind::Path(_, ref path) = retexpr.node;
            if match_path_ast(path, &[&ident.as_str()]);
            if !in_external_macro(cx, initexpr.span);
            then {
                    span_note_and_lint(cx,
                                       LET_AND_RETURN,
                                       retexpr.span,
                                       "returning the result of a let binding from a block. \
                                       Consider returning the expression directly.",
                                       initexpr.span,
                                       "this expression can be directly returned");
            }
        }
    }
}

impl LintPass for ReturnPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
    }
}

impl EarlyLintPass for ReturnPass {
    fn check_fn(&mut self, cx: &EarlyContext, kind: FnKind, _: &ast::FnDecl, _: Span, _: ast::NodeId) {
        match kind {
            FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
            FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
        }
    }

    fn check_block(&mut self, cx: &EarlyContext, block: &ast::Block) {
        self.check_let_return(cx, block);
    }
}

fn attr_is_cfg(attr: &ast::Attribute) -> bool {
    attr.meta_item_list().is_some() && attr.name() == "cfg"
}