Work around prelude redundant import warnings

rust-lang/rust#117772

    warning: the item `String` is imported redundantly
       --> src/ensure.rs:2:5
        |
    2   | use alloc::string::String;
        |     ^^^^^^^^^^^^^^^^^^^^^
        |
       ::: nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:115:13
        |
    115 |     pub use super::v1::*;
        |             --------- the item `String` is already defined here
        |
        = note: `#[warn(unused_imports)]` on by default

    warning: the item `Box` is imported redundantly
       --> src/error.rs:7:5
        |
    7   | use alloc::boxed::Box;
        |     ^^^^^^^^^^^^^^^^^
        |
       ::: nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:115:13
        |
    115 |     pub use super::v1::*;
        |             --------- the item `Box` is already defined here

    warning: the item `Box` is imported redundantly
       --> src/ptr.rs:1:5
        |
    1   | use alloc::boxed::Box;
        |     ^^^^^^^^^^^^^^^^^
        |
       ::: nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:115:13
        |
    115 |     pub use super::v1::*;
        |             --------- the item `Box` is already defined here

src/chain.rs

@@ -2,7 +2,7 @@ use self::ChainState::*;
 use crate::StdError;
 
 #[cfg(feature = "std")]
-use alloc::vec;
+use alloc::vec::{self, Vec};
 
 #[cfg(feature = "std")]
 pub(crate) use crate::Chain;

src/fmt.rs

@@ -43,6 +43,7 @@ impl ErrorImpl {
         #[cfg(any(std_backtrace, feature = "backtrace"))]
         {
             use crate::backtrace::BacktraceStatus;
+            use alloc::string::ToString;
 
             let backtrace = unsafe { Self::backtrace(this) };
             if let BacktraceStatus::Captured = backtrace.status() {
@@ -102,6 +103,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::*;
+    use alloc::string::String;
 
     #[test]
     fn one_digit() {

src/kind.rs

@@ -49,6 +49,8 @@ use core::fmt::{Debug, Display};
 
 #[cfg(feature = "std")]
 use crate::StdError;
+#[cfg(feature = "std")]
+use alloc::boxed::Box;
 
 pub struct Adhoc;
 

src/lib.rs

@@ -209,7 +209,7 @@
 #![doc(html_root_url = "https://docs.rs/anyhow/1.0.79")]
 #![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]
 #![cfg_attr(doc_cfg, feature(doc_cfg))]
-#![cfg_attr(not(feature = "std"), no_std)]
+#![no_std]
 #![deny(dead_code, unused_imports, unused_mut)]
 #![cfg_attr(
     not(anyhow_no_unsafe_op_in_unsafe_fn_lint),
@@ -247,6 +247,9 @@ compile_error!("Build script probe failed to compile.");
 
 extern crate alloc;
 
+#[cfg(feature = "std")]
+extern crate std;
+
 #[macro_use]
 mod backtrace;
 mod chain;

src/wrapper.rs

@@ -1,6 +1,9 @@
 use crate::StdError;
 use core::fmt::{self, Debug, Display};
 
+#[cfg(feature = "std")]
+use alloc::boxed::Box;
+
 #[cfg(error_generic_member_access)]
 use std::error::Request;
 

tests/test_repr.rs

@@ -4,7 +4,6 @@ mod drop;
 
 use self::drop::{DetectDrop, Flag};
 use anyhow::Error;
-use std::marker::Unpin;
 use std::mem;
 
 #[test]