Lint only the first statment/expression after alloc

Instead of searching for all the successive expressions after a vector
allocation, check only the first expression.

This is done to minimize the amount of false positives of the lint.

clippy_lints/src/slow_vector_initialization.rs

@@ -7,7 +7,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use crate::rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use crate::rustc::hir::intravisit::{walk_expr, walk_stmt, walk_block, NestedVisitorMap, Visitor};
 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass, Lint};
 use crate::rustc::{declare_tool_lint, lint_array};
 use crate::rustc::hir::*;
@@ -353,21 +353,42 @@ impl<'a, 'tcx> VectorInitializationVisitor<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for VectorInitializationVisitor<'a, 'tcx> {
-    fn visit_expr(&mut self, expr: &'tcx Expr) {
-        // Stop the search if we already found a slow zero-filling initialization
-        if self.slow_expression.is_some() {
-            return
-        }
+    fn visit_stmt(&mut self, stmt: &'tcx Stmt) {
+        if self.initialization_found {
+            match stmt.node {
+                StmtKind::Expr(ref expr, _) |
+                StmtKind::Semi(ref expr, _) => {
+                    self.search_slow_extend_filling(expr);
+                    self.search_slow_resize_filling(expr);
+                    self.search_unsafe_set_len(expr);
+                },
+                _ => (),
+            }
 
+            self.initialization_found = false;
+        } else {
+            walk_stmt(self, stmt);
+        }
+    }
+
+    fn visit_block(&mut self, block: &'tcx Block) {
+        if self.initialization_found {
+            if let Some(ref s) = block.stmts.get(0) {
+                self.visit_stmt( s)
+            }
+
+            self.initialization_found = false;
+        } else {
+            walk_block(self, block);
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr) {
         // Skip all the expressions previous to the vector initialization
         if self.vec_alloc.allocation_expr.id == expr.id {
             self.initialization_found = true;
         }
 
-        self.search_slow_extend_filling(expr);
-        self.search_slow_resize_filling(expr);
-        self.search_unsafe_set_len(expr);
-
         walk_expr(self, expr);
     }
 

tests/ui/slow_vector_initialization.rs

@@ -34,13 +34,13 @@ fn extend_vector() {
 fn mixed_extend_resize_vector() {
     // Mismatching len
     let mut mismatching_len = Vec::with_capacity(30);
+    mismatching_len.extend(repeat(0).take(40));
 
     // Slow initialization
     let mut resized_vec = Vec::with_capacity(30);
-    let mut extend_vec = Vec::with_capacity(30);
-
     resized_vec.resize(30, 0);
-    mismatching_len.extend(repeat(0).take(40));
+
+    let mut extend_vec = Vec::with_capacity(30);
     extend_vec.extend(repeat(0).take(30));
 }
 
@@ -70,3 +70,14 @@ fn unsafe_vector() {
         unsafe_vec.set_len(200);
     }
 }
+
+fn do_stuff(vec: &mut Vec<u8>) {
+
+}
+
+fn extend_vector_with_manipulations_between() {
+    let len = 300;
+    let mut vec1:Vec<u8> = Vec::with_capacity(len);
+    do_stuff(&mut vec1);
+    vec1.extend(repeat(0).take(len));
+}

tests/ui/slow_vector_initialization.stderr

@@ -17,20 +17,18 @@ error: slow zero-filling initialization
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
-  --> $DIR/slow_vector_initialization.rs:42:5
+  --> $DIR/slow_vector_initialization.rs:41:5
    |
-39 |     let mut resized_vec = Vec::with_capacity(30);
+40 |     let mut resized_vec = Vec::with_capacity(30);
    |                           ---------------------- help: consider replace allocation with: `vec![0; 30]`
-...
-42 |     resized_vec.resize(30, 0);
+41 |     resized_vec.resize(30, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: slow zero-filling initialization
   --> $DIR/slow_vector_initialization.rs:44:5
    |
-40 |     let mut extend_vec = Vec::with_capacity(30);
+43 |     let mut extend_vec = Vec::with_capacity(30);
    |                          ---------------------- help: consider replace allocation with: `vec![0; 30]`
-...
 44 |     extend_vec.extend(repeat(0).take(30));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^