Merge pull request #132 from birkenfeld/let_and_return

New lint: let and immediately return

README.md

@@ -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.

src/lib.rs

@@ -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,

src/misc.rs

@@ -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)]

src/returns.rs

@@ -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);
+    }
+}

tests/compile-fail/let_return.rs

@@ -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();
+}