Auto merge of rust-lang#8953 - DevAccentor:slow_vector_initialization, r=Manishearth

add vec.capacity() to [`slow_vec_initialization`] detection

fix rust-lang#8800

for example
```rust
let mut vec1 = Vec::with_capacity(len);
vec1.resize(vec1.capacity(), 0);

let mut vec2 = Vec::with_capacity(len);
vec2.extend(repeat(0).take(vec2.capacity()));
```
will trigger the lint

---

changelog: add `vec.capacity()` to [`slow_vec_initialization`] detection

Author: bors

clippy_lints/src/slow_vector_initialization.rs

@@ -26,6 +26,9 @@ declare_clippy_lint! {
     /// let mut vec1 = Vec::with_capacity(len);
     /// vec1.resize(len, 0);
     ///
+    /// let mut vec1 = Vec::with_capacity(len);
+    /// vec1.resize(vec1.capacity(), 0);
+    ///
     /// let mut vec2 = Vec::with_capacity(len);
     /// vec2.extend(repeat(0).take(len));
     /// ```
@@ -211,23 +214,20 @@ impl<'a, 'tcx> VectorInitializationVisitor<'a, 'tcx> {
 
     /// Checks if the given expression is resizing a vector with 0
     fn search_slow_resize_filling(&mut self, expr: &'tcx Expr<'_>) {
-        if_chain! {
-            if self.initialization_found;
-            if let ExprKind::MethodCall(path, [self_arg, len_arg, fill_arg], _) = expr.kind;
-            if path_to_local_id(self_arg, self.vec_alloc.local_id);
-            if path.ident.name == sym!(resize);
-
+        if self.initialization_found
+            && let ExprKind::MethodCall(path, [self_arg, len_arg, fill_arg], _) = expr.kind
+            && path_to_local_id(self_arg, self.vec_alloc.local_id)
+            && path.ident.name == sym!(resize)
             // Check that is filled with 0
-            if let ExprKind::Lit(ref lit) = fill_arg.kind;
-            if let LitKind::Int(0, _) = lit.node;
-
-            // Check that len expression is equals to `with_capacity` expression
-            if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr);
-
-            then {
-                self.slow_expression = Some(InitializationType::Resize(expr));
+            && let ExprKind::Lit(ref lit) = fill_arg.kind
+            && let LitKind::Int(0, _) = lit.node {
+                // Check that len expression is equals to `with_capacity` expression
+                if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr) {
+                    self.slow_expression = Some(InitializationType::Resize(expr));
+                } else if let ExprKind::MethodCall(path, _, _) = len_arg.kind && path.ident.as_str() == "capacity" {
+                    self.slow_expression = Some(InitializationType::Resize(expr));
+                }
             }
-        }
     }
 
     /// Returns `true` if give expression is `repeat(0).take(...)`
@@ -240,12 +240,15 @@ impl<'a, 'tcx> VectorInitializationVisitor<'a, 'tcx> {
             if let Some(repeat_expr) = take_args.get(0);
             if self.is_repeat_zero(repeat_expr);
 
-            // Check that len expression is equals to `with_capacity` expression
             if let Some(len_arg) = take_args.get(1);
-            if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr);
 
             then {
-                return true;
+                // Check that len expression is equals to `with_capacity` expression
+                if SpanlessEq::new(self.cx).eq_expr(len_arg, self.vec_alloc.len_expr) {
+                    return true;
+                } else if let ExprKind::MethodCall(path, _, _) = len_arg.kind && path.ident.as_str() == "capacity" {
+                    return true;
+                }
             }
         }
 

tests/ui/slow_vector_initialization.rs

@@ -19,6 +19,9 @@ fn extend_vector() {
     // Extend with mismatching expression should not be warned
     let mut vec3 = Vec::with_capacity(24322);
     vec3.extend(repeat(0).take(2));
+
+    let mut vec4 = Vec::with_capacity(len);
+    vec4.extend(repeat(0).take(vec4.capacity()));
 }
 
 fn mixed_extend_resize_vector() {
@@ -48,6 +51,9 @@ fn resize_vector() {
     let mut vec3 = Vec::with_capacity(len - 10);
     vec3.resize(len - 10, 0);
 
+    let mut vec4 = Vec::with_capacity(len);
+    vec4.resize(vec4.capacity(), 0);
+
     // Reinitialization should be warned
     vec1 = Vec::with_capacity(10);
     vec1.resize(10, 0);

tests/ui/slow_vector_initialization.stderr

@@ -17,44 +17,60 @@ LL |     vec2.extend(repeat(0).take(len - 10));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:31:5
+  --> $DIR/slow_vector_initialization.rs:24:5
+   |
+LL |     let mut vec4 = Vec::with_capacity(len);
+   |                    ----------------------- help: consider replace allocation with: `vec![0; len]`
+LL |     vec4.extend(repeat(0).take(vec4.capacity()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:34:5
    |
 LL |     let mut resized_vec = Vec::with_capacity(30);
    |                           ---------------------- help: consider replace allocation with: `vec![0; 30]`
 LL |     resized_vec.resize(30, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:34:5
+  --> $DIR/slow_vector_initialization.rs:37:5
    |
 LL |     let mut extend_vec = Vec::with_capacity(30);
    |                          ---------------------- help: consider replace allocation with: `vec![0; 30]`
 LL |     extend_vec.extend(repeat(0).take(30));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:41:5
+  --> $DIR/slow_vector_initialization.rs:44:5
    |
 LL |     let mut vec1 = Vec::with_capacity(len);
    |                    ----------------------- help: consider replace allocation with: `vec![0; len]`
 LL |     vec1.resize(len, 0);
    |     ^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:49:5
+  --> $DIR/slow_vector_initialization.rs:52:5
    |
 LL |     let mut vec3 = Vec::with_capacity(len - 10);
    |                    ---------------------------- help: consider replace allocation with: `vec![0; len - 10]`
 LL |     vec3.resize(len - 10, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:53:5
+  --> $DIR/slow_vector_initialization.rs:55:5
+   |
+LL |     let mut vec4 = Vec::with_capacity(len);
+   |                    ----------------------- help: consider replace allocation with: `vec![0; len]`
+LL |     vec4.resize(vec4.capacity(), 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: slow zero-filling initialization
+  --> $DIR/slow_vector_initialization.rs:59:5
    |
 LL |     vec1 = Vec::with_capacity(10);
    |            ---------------------- help: consider replace allocation with: `vec![0; 10]`
 LL |     vec1.resize(10, 0);
    |     ^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 7 previous errors
+error: aborting due to 9 previous errors