Downgrade unsafe_vector_initialization to restriction

This lint looks for:

    let mut vec = Vec::with_capacity(len);
    vec.set_len(len);

The suggested replacement is `vec![0; len]`.

This is far too opinionated to be a deny-by-default lint because the performance
characteristics of the suggested replacement are totally different.

I am not convinced that this lint has value beyond what deny(unsafe_code) gives
you. Unsafe code is unsafe but please don't deny-by-default lint it if that's
the only reason.

Author: dtolnay

clippy_lints/src/lib.rs

@@ -495,6 +495,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         panic_unimplemented::UNIMPLEMENTED,
         shadow::SHADOW_REUSE,
         shadow::SHADOW_SAME,
+        slow_vector_initialization::UNSAFE_VECTOR_INITIALIZATION,
         strings::STRING_ADD,
         write::PRINT_STDOUT,
         write::USE_DEBUG,
@@ -727,7 +728,6 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         returns::UNUSED_UNIT,
         serde_api::SERDE_API_MISUSE,
         slow_vector_initialization::SLOW_VECTOR_INITIALIZATION,
-        slow_vector_initialization::UNSAFE_VECTOR_INITIALIZATION,
         strings::STRING_LIT_AS_BYTES,
         suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL,
         suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL,
@@ -974,7 +974,6 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         ranges::ITERATOR_STEP_BY_ZERO,
         regex::INVALID_REGEX,
         serde_api::SERDE_API_MISUSE,
-        slow_vector_initialization::UNSAFE_VECTOR_INITIALIZATION,
         suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL,
         suspicious_trait_impl::SUSPICIOUS_OP_ASSIGN_IMPL,
         swap::ALMOST_SWAPPED,

clippy_lints/src/slow_vector_initialization.rs

@@ -55,7 +55,7 @@ declare_clippy_lint! {
 /// ```
 declare_clippy_lint! {
     pub UNSAFE_VECTOR_INITIALIZATION,
-    correctness,
+    restriction,
     "unsafe vector initialization"
 }