auto-fix slow_vector_initialization in some cases (rust-lang#13947)

changelog: [`slow_vector_initialization`]: auto-fix when appropriate

I made a change for `slow_vector_initialization` lint suggestion to use
`vec!` with size and remove the unneeded `resize` (or similar one) call
in rust-lang#13912, while only the former one was suggested in the previous
implementation. Now, I think this lint can be automatically fixed with
no unnecessary code in some cases. I wrote “in some cases” because if
there are comments between vector declaration and `resize`, Clippy
shouldn't apply auto-fix because the comment may informational.

Author: blyxyas

Diff for: clippy_lints/src/slow_vector_initialization.rs

@@ -3,6 +3,7 @@ use clippy_utils::macros::matching_root_macro_call;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::{
     SpanlessEq, get_enclosing_block, is_integer_literal, is_path_diagnostic_item, path_to_local, path_to_local_id,
+    span_contains_comment,
 };
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::{Visitor, walk_block, walk_expr, walk_stmt};
@@ -206,14 +207,22 @@ impl SlowVectorInit {
         let span_to_replace = slow_fill
             .span
             .with_lo(vec_alloc.allocation_expr.span.source_callsite().lo());
+
+        // If there is no comment in `span_to_replace`, Clippy can automatically fix the code.
+        let app = if span_contains_comment(cx.tcx.sess.source_map(), span_to_replace) {
+            Applicability::Unspecified
+        } else {
+            Applicability::MachineApplicable
+        };
+
         span_lint_and_sugg(
             cx,
             SLOW_VECTOR_INITIALIZATION,
             span_to_replace,
             msg,
             "consider replacing this with",
             format!("vec![0; {len_expr}]"),
-            Applicability::Unspecified,
+            app,
         );
     }
 }

Diff for: tests/ui/slow_vector_initialization.fixed

@@ -0,0 +1,85 @@
+#![allow(clippy::useless_vec)]
+use std::iter::repeat;
+fn main() {
+    resize_vector();
+    extend_vector();
+    mixed_extend_resize_vector();
+    from_empty_vec();
+}
+
+fn extend_vector() {
+    // Extend with constant expression
+    let len = 300;
+    let mut vec1 = vec![0; len];
+
+    // Extend with len expression
+    let mut vec2 = vec![0; len - 10];
+
+    // 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![0; len];
+}
+
+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![0; 30];
+
+    let mut extend_vec = vec![0; 30];
+}
+
+fn resize_vector() {
+    // Resize with constant expression
+    let len = 300;
+    let mut vec1 = vec![0; len];
+
+    // Resize mismatch len
+    let mut vec2 = Vec::with_capacity(200);
+    vec2.resize(10, 0);
+
+    // Resize with len expression
+    let mut vec3 = vec![0; len - 10];
+
+    let mut vec4 = vec![0; len];
+
+    // Reinitialization should be warned
+    vec1 = vec![0; 10];
+}
+
+fn from_empty_vec() {
+    // Resize with constant expression
+    let len = 300;
+    let mut vec1 = vec![0; len];
+
+    // Resize with len expression
+    let mut vec3 = vec![0; len - 10];
+
+    // Reinitialization should be warned
+    vec1 = vec![0; 10];
+
+    vec1 = vec![0; 10];
+
+    macro_rules! x {
+        () => {
+            vec![]
+        };
+    }
+
+    // `vec![]` comes from another macro, don't warn
+    vec1 = x!();
+    vec1.resize(10, 0);
+}
+
+fn do_stuff(vec: &mut [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));
+}

Diff for: tests/ui/slow_vector_initialization.rs

@@ -1,4 +1,4 @@
-//@no-rustfix
+#![allow(clippy::useless_vec)]
 use std::iter::repeat;
 fn main() {
     resize_vector();