Add the new lint same_item_push

CHANGELOG.md

@@ -1687,6 +1687,7 @@ Released 2018-09-13
 [`result_map_unit_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#result_map_unit_fn
 [`reversed_empty_ranges`]: https://rust-lang.github.io/rust-clippy/master/index.html#reversed_empty_ranges
 [`same_functions_in_if_condition`]: https://rust-lang.github.io/rust-clippy/master/index.html#same_functions_in_if_condition
+[`same_item_push`]: https://rust-lang.github.io/rust-clippy/master/index.html#same_item_push
 [`search_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#search_is_some
 [`serde_api_misuse`]: https://rust-lang.github.io/rust-clippy/master/index.html#serde_api_misuse
 [`shadow_reuse`]: https://rust-lang.github.io/rust-clippy/master/index.html#shadow_reuse

clippy_lints/src/lib.rs

@@ -607,6 +607,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &loops::NEEDLESS_COLLECT,
         &loops::NEEDLESS_RANGE_LOOP,
         &loops::NEVER_LOOP,
+        &loops::SAME_ITEM_PUSH,
         &loops::WHILE_IMMUTABLE_CONDITION,
         &loops::WHILE_LET_LOOP,
         &loops::WHILE_LET_ON_ITERATOR,
@@ -1292,6 +1293,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::NEEDLESS_COLLECT),
         LintId::of(&loops::NEEDLESS_RANGE_LOOP),
         LintId::of(&loops::NEVER_LOOP),
+        LintId::of(&loops::SAME_ITEM_PUSH),
         LintId::of(&loops::WHILE_IMMUTABLE_CONDITION),
         LintId::of(&loops::WHILE_LET_LOOP),
         LintId::of(&loops::WHILE_LET_ON_ITERATOR),
@@ -1493,6 +1495,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::EMPTY_LOOP),
         LintId::of(&loops::FOR_KV_MAP),
         LintId::of(&loops::NEEDLESS_RANGE_LOOP),
+        LintId::of(&loops::SAME_ITEM_PUSH),
         LintId::of(&loops::WHILE_LET_ON_ITERATOR),
         LintId::of(&main_recursion::MAIN_RECURSION),
         LintId::of(&manual_async_fn::MANUAL_ASYNC_FN),

clippy_lints/src/loops.rs

@@ -5,8 +5,9 @@ use crate::utils::usage::{is_unused, mutated_variables};
 use crate::utils::{
     get_enclosing_block, get_parent_expr, get_trait_def_id, has_iter_method, higher, implements_trait,
     is_integer_const, is_no_std_crate, is_refutable, is_type_diagnostic_item, last_path_segment, match_trait_method,
-    match_type, match_var, multispan_sugg, qpath_res, snippet, snippet_opt, snippet_with_applicability, span_lint,
-    span_lint_and_help, span_lint_and_sugg, span_lint_and_then, sugg, SpanlessEq,
+    match_type, match_var, multispan_sugg, qpath_res, snippet, snippet_opt, snippet_with_applicability,
+    snippet_with_macro_callsite, span_lint, span_lint_and_help, span_lint_and_sugg, span_lint_and_then, sugg,
+    SpanlessEq,
 };
 use if_chain::if_chain;
 use rustc_ast::ast;
@@ -419,6 +420,39 @@ declare_clippy_lint! {
     "variables used within while expression are not mutated in the body"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks whether a for loop is being used to push a constant
+    /// value into a Vec.
+    ///
+    /// **Why is this bad?** This kind of operation can be expressed more succinctly with
+    /// `vec![item;SIZE]` or `vec.resize(NEW_SIZE, item)` and using these alternatives may also
+    /// have better performance.
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let item1 = 2;
+    /// let item2 = 3;
+    /// let mut vec: Vec<u8> = Vec::new();
+    /// for _ in 0..20 {
+    ///    vec.push(item1);
+    /// }
+    /// for _ in 0..30 {
+    ///     vec.push(item2);
+    /// }
+    /// ```
+    /// could be written as
+    /// ```rust
+    /// let item1 = 2;
+    /// let item2 = 3;
+    /// let mut vec: Vec<u8> = vec![item1; 20];
+    /// vec.resize(20 + 30, item2);
+    /// ```
+    pub SAME_ITEM_PUSH,
+    style,
+    "the same item is pushed inside of a for loop"
+}
+
 declare_lint_pass!(Loops => [
     MANUAL_MEMCPY,
     NEEDLESS_RANGE_LOOP,
@@ -435,6 +469,7 @@ declare_lint_pass!(Loops => [
     NEVER_LOOP,
     MUT_RANGE_BOUND,
     WHILE_IMMUTABLE_CONDITION,
+    SAME_ITEM_PUSH,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Loops {
@@ -740,6 +775,7 @@ fn check_for_loop<'tcx>(
     check_for_loop_over_map_kv(cx, pat, arg, body, expr);
     check_for_mut_range_bound(cx, arg, body);
     detect_manual_memcpy(cx, pat, arg, body, expr);
+    detect_same_item_push(cx, pat, arg, body, expr);
 }
 
 fn same_var<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, var: HirId) -> bool {
@@ -1016,6 +1052,117 @@ fn detect_manual_memcpy<'tcx>(
     }
 }
 
+// Scans the body of the for loop and determines whether lint should be given
+struct SameItemPushVisitor<'a, 'tcx> {
+    should_lint: bool,
+    // this field holds the last vec push operation visited, which should be the only push seen
+    vec_push: Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)>,
+    cx: &'a LateContext<'tcx>,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for SameItemPushVisitor<'a, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
+        match &expr.kind {
+            // Non-determinism may occur ... don't give a lint
+            ExprKind::Loop(_, _, _) | ExprKind::Match(_, _, _) => self.should_lint = false,
+            ExprKind::Block(block, _) => self.visit_block(block),
+            _ => {},
+        }
+    }
+
+    fn visit_block(&mut self, b: &'tcx Block<'_>) {
+        for stmt in b.stmts.iter() {
+            self.visit_stmt(stmt);
+        }
+    }
+
+    fn visit_stmt(&mut self, s: &'tcx Stmt<'_>) {
+        let vec_push_option = get_vec_push(self.cx, s);
+        if vec_push_option.is_none() {
+            // Current statement is not a push so visit inside
+            match &s.kind {
+                StmtKind::Expr(expr) | StmtKind::Semi(expr) => self.visit_expr(&expr),
+                _ => {},
+            }
+        } else {
+            // Current statement is a push ...check whether another
+            // push had been previously done
+            if self.vec_push.is_none() {
+                self.vec_push = vec_push_option;
+            } else {
+                // There are multiple pushes ... don't lint
+                self.should_lint = false;
+            }
+        }
+    }
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::None
+    }
+}
+
+// Given some statement, determine if that statement is a push on a Vec. If it is, return
+// the Vec being pushed into and the item being pushed
+fn get_vec_push<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) -> Option<(&'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
+    if_chain! {
+            // Extract method being called
+            if let StmtKind::Semi(semi_stmt) = &stmt.kind;
+            if let ExprKind::MethodCall(path, _, args, _) = &semi_stmt.kind;
+            // Figure out the parameters for the method call
+            if let Some(self_expr) = args.get(0);
+            if let Some(pushed_item) = args.get(1);
+            // Check that the method being called is push() on a Vec
+            if match_type(cx, cx.typeck_results().expr_ty(self_expr), &paths::VEC);
+            if path.ident.name.as_str() == "push";
+            then {
+                return Some((self_expr, pushed_item))
+            }
+    }
+    None
+}
+
+/// Detects for loop pushing the same item into a Vec
+fn detect_same_item_push<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &'tcx Pat<'_>,
+    _: &'tcx Expr<'_>,
+    body: &'tcx Expr<'_>,
+    _: &'tcx Expr<'_>,
+) {
+    // Determine whether it is safe to lint the body
+    let mut same_item_push_visitor = SameItemPushVisitor {
+        should_lint: true,
+        vec_push: None,
+        cx,
+    };
+    walk_expr(&mut same_item_push_visitor, body);
+    if same_item_push_visitor.should_lint {
+        if let Some((vec, pushed_item)) = same_item_push_visitor.vec_push {
+            // Make sure that the push does not involve possibly mutating values
+            if mutated_variables(pushed_item, cx).map_or(false, |mutvars| mutvars.is_empty()) {
+                if let PatKind::Wild = pat.kind {
+                    let vec_str = snippet_with_macro_callsite(cx, vec.span, "");
+                    let item_str = snippet_with_macro_callsite(cx, pushed_item.span, "");
+
+                    span_lint_and_help(
+                        cx,
+                        SAME_ITEM_PUSH,
+                        vec.span,
+                        "it looks like the same item is being pushed into this Vec",
+                        None,
+                        &format!(
+                            "try using vec![{};SIZE] or {}.resize(NEW_SIZE, {})",
+                            item_str, vec_str, item_str
+                        ),
+                    )
+                }
+            }
+        }
+    }
+}
+
 /// Checks for looping over a range and then indexing a sequence with it.
 /// The iteratee must be a range literal.
 #[allow(clippy::too_many_lines)]

src/lintlist/mod.rs

@@ -1935,6 +1935,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "copies",
     },
+    Lint {
+        name: "same_item_push",
+        group: "style",
+        desc: "the same item is pushed inside of a for loop",
+        deprecation: None,
+        module: "loops",
+    },
     Lint {
         name: "search_is_some",
         group: "complexity",

tests/ui/same_item_push.rs

@@ -0,0 +1,89 @@
+#![warn(clippy::same_item_push)]
+
+fn mutate_increment(x: &mut u8) -> u8 {
+    *x += 1;
+    *x
+}
+
+fn increment(x: u8) -> u8 {
+    x + 1
+}
+
+fn main() {
+    // Test for basic case
+    let mut spaces = Vec::with_capacity(10);
+    for _ in 0..10 {
+        spaces.push(vec![b' ']);
+    }
+
+    let mut vec2: Vec<u8> = Vec::new();
+    let item = 2;
+    for _ in 5..=20 {
+        vec2.push(item);
+    }
+
+    let mut vec3: Vec<u8> = Vec::new();
+    for _ in 0..15 {
+        let item = 2;
+        vec3.push(item);
+    }
+
+    let mut vec4: Vec<u8> = Vec::new();
+    for _ in 0..15 {
+        vec4.push(13);
+    }
+
+    // Suggestion should not be given as pushed variable can mutate
+    let mut vec5: Vec<u8> = Vec::new();
+    let mut item: u8 = 2;
+    for _ in 0..30 {
+        vec5.push(mutate_increment(&mut item));
+    }
+
+    let mut vec6: Vec<u8> = Vec::new();
+    let mut item: u8 = 2;
+    let mut item2 = &mut mutate_increment(&mut item);
+    for _ in 0..30 {
+        vec6.push(mutate_increment(item2));
+    }
+
+    let mut vec7: Vec<usize> = Vec::new();
+    for (a, b) in [0, 1, 4, 9, 16].iter().enumerate() {
+        vec7.push(a);
+    }
+
+    let mut vec8: Vec<u8> = Vec::new();
+    for i in 0..30 {
+        vec8.push(increment(i));
+    }
+
+    let mut vec9: Vec<u8> = Vec::new();
+    for i in 0..30 {
+        vec9.push(i + i * i);
+    }
+
+    // Suggestion should not be given as there are multiple pushes that are not the same
+    let mut vec10: Vec<u8> = Vec::new();
+    let item: u8 = 2;
+    for _ in 0..30 {
+        vec10.push(item);
+        vec10.push(item * 2);
+    }
+
+    // Suggestion should not be given as Vec is not involved
+    for _ in 0..5 {
+        println!("Same Item Push");
+    }
+
+    struct A {
+        kind: u32,
+    }
+    let mut vec_a: Vec<A> = Vec::new();
+    for i in 0..30 {
+        vec_a.push(A { kind: i });
+    }
+    let mut vec12: Vec<u8> = Vec::new();
+    for a in vec_a {
+        vec12.push(2u8.pow(a.kind));
+    }
+}

tests/ui/same_item_push.stderr

@@ -0,0 +1,35 @@
+error: it looks like the same item is being pushed into this Vec
+  --> $DIR/same_item_push.rs:16:9
+   |
+LL |         spaces.push(vec![b' ']);
+   |         ^^^^^^
+   |
+   = note: `-D clippy::same-item-push` implied by `-D warnings`
+   = help: try using vec![vec![b' '];SIZE] or spaces.resize(NEW_SIZE, vec![b' '])
+
+error: it looks like the same item is being pushed into this Vec
+  --> $DIR/same_item_push.rs:22:9
+   |
+LL |         vec2.push(item);
+   |         ^^^^
+   |
+   = help: try using vec![item;SIZE] or vec2.resize(NEW_SIZE, item)
+
+error: it looks like the same item is being pushed into this Vec
+  --> $DIR/same_item_push.rs:28:9
+   |
+LL |         vec3.push(item);
+   |         ^^^^
+   |
+   = help: try using vec![item;SIZE] or vec3.resize(NEW_SIZE, item)
+
+error: it looks like the same item is being pushed into this Vec
+  --> $DIR/same_item_push.rs:33:9
+   |
+LL |         vec4.push(13);
+   |         ^^^^
+   |
+   = help: try using vec![13;SIZE] or vec4.resize(NEW_SIZE, 13)
+
+error: aborting due to 4 previous errors
+