rust/clippy_lints/src/returns.rs
Michael Wright fe8068c41b Fix compilation for nightly 2018-04-06
Breakages for introduced by rust pull request
'AST: Give spans to all identifies' - rust-lang/rust/pull#49154

Closes #2639
2018-04-07 07:32:30 +02:00

154 lines
5.6 KiB
Rust

use rustc::lint::*;
use syntax::ast;
use syntax::codemap::Span;
use syntax::visit::FnKind;
use 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.name.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().map_or(false, |n| n == "cfg")
}