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Merge pull request #132 from birkenfeld/let_and_return 

New lint: let and immediately return
This commit is contained in:
Manish Goregaokar 2015-08-12 02:27:48 +05:30
parent 1d948705bf f6dc48fe3a
commit 7c14b5b67f
5 changed files with 148 additions and 71 deletions
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1
README.md
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@ -31,6 +31,7 @@ Lints included in this crate:
- `zero_width_space`: Warns on encountering a unicode zero-width space
- `string_add_assign`: Warns on `x = x + ..` where `x` is a `String` and suggests using `push_str(..)` instead.
- `needless_return`: Warns on using `return expr;` when a simple `expr` would suffice.
- `let_and_return`: Warns on doing `let x = expr; x` at the end of a function.
- `option_unwrap_used`: Warns when `Option.unwrap()` is used, and suggests `.expect()`.
- `result_unwrap_used`: Warns when `Result.unwrap()` is used (silent by default).
- `modulo_one`: Warns on taking a number modulo 1, which always has a result of 0.

5
src/lib.rs
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@ -30,6 +30,7 @@ pub mod unicode;
pub mod utils;
pub mod strings;
pub mod methods;
pub mod returns;
#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
@ -55,7 +56,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
reg.register_lint_pass(box misc::ModuloOne as LintPassObject);
reg.register_lint_pass(box unicode::Unicode as LintPassObject);
reg.register_lint_pass(box strings::StringAdd as LintPassObject);
reg.register_lint_pass(box misc::NeedlessReturn as LintPassObject);
reg.register_lint_pass(box returns::ReturnPass as LintPassObject);
reg.register_lint_pass(box methods::MethodsPass as LintPassObject);
reg.register_lint_group("clippy", vec![types::BOX_VEC, types::LINKEDLIST,
@ -77,7 +78,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
collapsible_if::COLLAPSIBLE_IF,
unicode::ZERO_WIDTH_SPACE,
strings::STRING_ADD_ASSIGN,
misc::NEEDLESS_RETURN,
returns::NEEDLESS_RETURN,
misc::MODULO_ONE,
methods::OPTION_UNWRAP_USED,
methods::RESULT_UNWRAP_USED,

69
src/misc.rs
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@ -270,75 +270,6 @@ fn is_str_arg(cx: &Context, args: &[P<Expr>]) -> bool {
walk_ptrs_ty(cx.tcx.expr_ty(&*args[0])).sty { true } else { false }
}
declare_lint!(pub NEEDLESS_RETURN, Warn,
"Warn on using a return statement where an expression would be enough");
#[derive(Copy,Clone)]
pub struct NeedlessReturn;
impl NeedlessReturn {
// Check the final stmt or expr in a block for unnecessary return.
fn check_block_return(&mut self, cx: &Context, block: &Block) {
if let Some(ref expr) = block.expr {
self.check_final_expr(cx, expr);
} else if let Some(stmt) = block.stmts.last() {
if let StmtSemi(ref expr, _) = stmt.node {
if let ExprRet(Some(ref inner)) = expr.node {
self.emit_lint(cx, (expr.span, inner.span));
}
}
}
}
// Check a the final expression in a block if it's a return.
fn check_final_expr(&mut self, cx: &Context, expr: &Expr) {
match expr.node {
// simple return is always "bad"
ExprRet(Some(ref inner)) => {
self.emit_lint(cx, (expr.span, inner.span));
}
// a whole block? check it!
ExprBlock(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
ExprIf(_, ref ifblock, Some(ref elsexpr)) |
ExprIfLet(_, _, ref ifblock, Some(ref elsexpr)) => {
self.check_block_return(cx, ifblock);
self.check_final_expr(cx, elsexpr);
}
// a match expr, check all arms
ExprMatch(_, ref arms, _) => {
for arm in arms {
self.check_final_expr(cx, &*arm.body);
}
}
_ => { }
}
}
fn emit_lint(&mut self, cx: &Context, spans: (Span, Span)) {
span_lint(cx, NEEDLESS_RETURN, spans.0, &format!(
"unneeded return statement. Consider using {} \
without the trailing semicolon",
snippet(cx, spans.1, "..")))
}
}
impl LintPass for NeedlessReturn {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_RETURN)
}
fn check_fn(&mut self, cx: &Context, _: FnKind, _: &FnDecl,
block: &Block, _: Span, _: ast::NodeId) {
self.check_block_return(cx, block);
}
}
declare_lint!(pub MODULO_ONE, Warn, "Warn on expressions that include % 1, which is always 0");
#[derive(Copy,Clone)]

110
src/returns.rs Normal file
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@ -0,0 +1,110 @@
use syntax::ast;
use syntax::ast::*;
use syntax::codemap::{Span, Spanned};
use syntax::visit::FnKind;
use rustc::lint::{Context, LintPass, LintArray, Level};
use utils::{span_lint, snippet, match_path};
declare_lint!(pub NEEDLESS_RETURN, Warn,
"Warn on using a return statement where an expression would be enough");
declare_lint!(pub LET_AND_RETURN, Warn,
"Warn on creating a let-binding and then immediately returning it");
#[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: &Context, block: &Block) {
if let Some(ref expr) = block.expr {
self.check_final_expr(cx, expr);
} else if let Some(stmt) = block.stmts.last() {
if let StmtSemi(ref expr, _) = stmt.node {
if let ExprRet(Some(ref inner)) = expr.node {
self.emit_return_lint(cx, (expr.span, inner.span));
}
}
}
}
// Check a the final expression in a block if it's a return.
fn check_final_expr(&mut self, cx: &Context, expr: &Expr) {
match expr.node {
// simple return is always "bad"
ExprRet(Some(ref inner)) => {
self.emit_return_lint(cx, (expr.span, inner.span));
}
// a whole block? check it!
ExprBlock(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
ExprIf(_, ref ifblock, Some(ref elsexpr)) |
ExprIfLet(_, _, ref ifblock, Some(ref elsexpr)) => {
self.check_block_return(cx, ifblock);
self.check_final_expr(cx, elsexpr);
}
// a match expr, check all arms
ExprMatch(_, ref arms, _) => {
for arm in arms {
self.check_final_expr(cx, &*arm.body);
}
}
_ => { }
}
}
fn emit_return_lint(&mut self, cx: &Context, spans: (Span, Span)) {
span_lint(cx, NEEDLESS_RETURN, spans.0, &format!(
"unneeded return statement. Consider using {} \
without the trailing semicolon",
snippet(cx, spans.1, "..")))
}
// Check for "let x = EXPR; x"
fn check_let_return(&mut self, cx: &Context, block: &Block) {
// we need both a let-binding stmt and an expr
if let Some(stmt) = block.stmts.last() {
if let StmtDecl(ref decl, _) = stmt.node {
if let DeclLocal(ref local) = decl.node {
if let Some(ref initexpr) = local.init {
if let PatIdent(_, Spanned { node: id, .. }, _) = local.pat.node {
if let Some(ref retexpr) = block.expr {
if let ExprPath(_, ref path) = retexpr.node {
if match_path(path, &[&*id.name.as_str()]) {
self.emit_let_lint(cx, retexpr.span, initexpr.span);
}
}
}
}
}
}
}
}
}
fn emit_let_lint(&mut self, cx: &Context, lint_span: Span, note_span: Span) {
span_lint(cx, LET_AND_RETURN, lint_span,
"returning the result of a let binding. \
Consider returning the expression directly.");
if cx.current_level(LET_AND_RETURN) != Level::Allow {
cx.sess().span_note(note_span,
"this expression can be directly returned");
}
}
}
impl LintPass for ReturnPass {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
}
fn check_fn(&mut self, cx: &Context, _: FnKind, _: &FnDecl,
block: &Block, _: Span, _: ast::NodeId) {
self.check_block_return(cx, block);
self.check_let_return(cx, block);
}
}

34
tests/compile-fail/let_return.rs Executable file
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@ -0,0 +1,34 @@
#![feature(plugin)]
#![plugin(clippy)]
#![deny(let_and_return)]
fn test() -> i32 {
let _y = 0; // no warning
let x = 5; //~NOTE
x //~ERROR:
}
fn test_nowarn_1() -> i32 {
let mut x = 5;
x += 1;
x
}
fn test_nowarn_2() -> i32 {
let x = 5;
x + 1
}
fn test_nowarn_3() -> (i32, i32) {
// this should technically warn, but we do not compare complex patterns
let (x, y) = (5, 9);
(x, y)
}
fn main() {
test();
test_nowarn_1();
test_nowarn_2();
test_nowarn_3();
}