130 lines
5.0 KiB
Rust
130 lines
5.0 KiB
Rust
use rustc::lint::*;
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use syntax::ast::*;
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use syntax::codemap::{Span, Spanned};
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use syntax::visit::FnKind;
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use utils::{span_note_and_lint, span_lint_and_then, snippet_opt, match_path_ast, in_external_macro};
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/// **What it does:** This lint checks for return statements at the end of a block.
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///
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/// **Why is this bad?** Removing the `return` and semicolon will make the code more rusty.
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///
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/// **Known problems:** Following this lint's advice may currently run afoul of Rust issue [#31439](https://github.com/rust-lang/rust/issues/31439), so if you get lifetime errors, please roll back the change until that issue is fixed.
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///
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/// **Example:** `fn foo(x: usize) { return x; }`
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declare_lint! {
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pub NEEDLESS_RETURN, Warn,
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"using a return statement like `return expr;` where an expression would suffice"
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}
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/// **What it does:** This lint checks for `let`-bindings, which are subsequently returned.
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///
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/// **Why is this bad?** It is just extraneous code. Remove it to make your code more rusty.
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///
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/// **Known problems:** Following this lint's advice may currently run afoul of Rust issue [#31439](https://github.com/rust-lang/rust/issues/31439), so if you get lifetime errors, please roll back the change until that issue is fixed.
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///
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/// **Example:** `{ let x = ..; x }`
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declare_lint! {
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pub LET_AND_RETURN, Warn,
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"creating a let-binding and then immediately returning it like `let x = expr; x` at \
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the end of a block"
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}
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#[derive(Copy, Clone)]
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pub struct ReturnPass;
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impl ReturnPass {
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// Check the final stmt or expr in a block for unnecessary return.
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fn check_block_return(&mut self, cx: &EarlyContext, block: &Block) {
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if let Some(stmt) = block.stmts.last() {
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match stmt.node {
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StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => {
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self.check_final_expr(cx, expr);
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}
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_ => (),
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}
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}
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}
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// Check a the final expression in a block if it's a return.
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fn check_final_expr(&mut self, cx: &EarlyContext, expr: &Expr) {
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match expr.node {
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// simple return is always "bad"
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ExprKind::Ret(Some(ref inner)) => {
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self.emit_return_lint(cx, (expr.span, inner.span));
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}
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// a whole block? check it!
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ExprKind::Block(ref block) => {
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self.check_block_return(cx, block);
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}
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// an if/if let expr, check both exprs
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// note, if without else is going to be a type checking error anyways
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// (except for unit type functions) so we don't match it
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ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
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self.check_block_return(cx, ifblock);
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self.check_final_expr(cx, elsexpr);
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}
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// a match expr, check all arms
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ExprKind::Match(_, ref arms) => {
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for arm in arms {
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self.check_final_expr(cx, &arm.body);
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}
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}
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_ => (),
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}
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}
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fn emit_return_lint(&mut self, cx: &EarlyContext, spans: (Span, Span)) {
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if in_external_macro(cx, spans.1) {
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return;
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}
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span_lint_and_then(cx, NEEDLESS_RETURN, spans.0, "unneeded return statement", |db| {
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if let Some(snippet) = snippet_opt(cx, spans.1) {
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db.span_suggestion(spans.0, "remove `return` as shown:", snippet);
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}
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});
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}
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// Check for "let x = EXPR; x"
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fn check_let_return(&mut self, cx: &EarlyContext, block: &Block) {
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let mut it = block.stmts.iter();
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// we need both a let-binding stmt and an expr
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if_let_chain! {[
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let Some(ref retexpr) = it.next_back(),
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let StmtKind::Expr(ref retexpr) = retexpr.node,
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let Some(stmt) = it.next_back(),
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let StmtKind::Local(ref local) = stmt.node,
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let Some(ref initexpr) = local.init,
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let PatKind::Ident(_, Spanned { node: id, .. }, _) = local.pat.node,
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let ExprKind::Path(_, ref path) = retexpr.node,
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match_path_ast(path, &[&id.name.as_str()]),
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!in_external_macro(cx, initexpr.span),
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], {
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span_note_and_lint(cx,
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LET_AND_RETURN,
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retexpr.span,
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"returning the result of a let binding from a block. \
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Consider returning the expression directly.",
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initexpr.span,
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"this expression can be directly returned");
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}}
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}
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}
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impl LintPass for ReturnPass {
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fn get_lints(&self) -> LintArray {
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lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
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}
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}
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impl EarlyLintPass for ReturnPass {
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fn check_fn(&mut self, cx: &EarlyContext, _: FnKind, _: &FnDecl, block: &Block, _: Span, _: NodeId) {
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self.check_block_return(cx, block);
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}
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fn check_block(&mut self, cx: &EarlyContext, block: &Block) {
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self.check_let_return(cx, block);
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}
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}
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