Disable std prelude

This makes visible the dependency on std prelude macros such as
`error::Error`. Helpful for planning how to do no-std.

Tested with `cargo hack clippy --feature-powerset -- --deny warnings`
(all 2112 combinations).

Author: tamird

codegen/src/debug.rs

@@ -198,7 +198,7 @@ pub fn generate(defs: &Definitions) -> Result<()> {
         quote! {
             #![allow(unknown_lints, non_local_definitions)]
 
-            use std::fmt::{self, Debug};
+            use core::fmt::{self, Debug};
 
             #impls
         },

codegen/src/eq.rs

@@ -158,6 +158,8 @@ pub fn generate(defs: &Definitions) -> Result<()> {
         quote! {
             #[cfg(any(feature = "derive", feature = "full"))]
             use crate::tt::TokenStreamHelper;
+            #[cfg(feature = "extra-traits")]
+            use alloc::string::ToString;
 
             #impls
         },

codegen/src/fold.rs

@@ -255,6 +255,11 @@ pub fn generate(defs: &Definitions) -> Result<()> {
                 clippy::needless_pass_by_ref_mut,
             )]
 
+            #[cfg(any(feature = "derive", feature = "full"))]
+            use alloc::boxed::Box;
+            #[cfg(any(feature = "derive", feature = "full"))]
+            use alloc::vec::Vec;
+
             #full_macro
 
             /// Syntax tree traversal to transform the nodes of an owned syntax tree.

codegen/src/hash.rs

@@ -160,7 +160,9 @@ pub fn generate(defs: &Definitions) -> Result<()> {
         quote! {
             #[cfg(any(feature = "derive", feature = "full"))]
             use crate::tt::TokenStreamHelper;
-            use std::hash::{Hash, Hasher};
+            #[cfg(feature = "extra-traits")]
+            use alloc::string::ToString;
+            use core::hash::{Hash, Hasher};
 
             #impls
         },

codegen/src/snapshot.rs

@@ -363,7 +363,7 @@ pub fn generate(defs: &Definitions) -> Result<()> {
 
             use super::{Lite, Present};
             use ref_cast::RefCast;
-            use std::fmt::{self, Debug, Display};
+            use core::fmt::{self, Debug, Display};
 
             #impls
         },

codegen/src/visit_mut.rs

@@ -233,6 +233,8 @@ pub fn generate(defs: &Definitions) -> Result<()> {
             #![allow(unused_variables)]
             #![allow(clippy::needless_pass_by_ref_mut)]
 
+            #[cfg(any(feature = "derive", feature = "full"))]
+            use alloc::vec::Vec;
             #[cfg(any(feature = "full", feature = "derive"))]
             use crate::punctuated::Punctuated;
 

src/attr.rs

@@ -10,11 +10,15 @@ use crate::meta::{self, ParseNestedMeta};
 use crate::parse::{Parse, ParseStream, Parser};
 use crate::path::Path;
 use crate::token;
-use proc_macro2::TokenStream;
+#[cfg(feature = "parsing")]
+use alloc::format;
+#[cfg(feature = "parsing")]
+use alloc::vec::Vec;
 #[cfg(feature = "printing")]
-use std::iter;
+use core::iter;
 #[cfg(feature = "printing")]
-use std::slice;
+use core::slice;
+use proc_macro2::TokenStream;
 
 ast_struct! {
     /// An attribute, like `#[repr(transparent)]`.
@@ -653,8 +657,9 @@ pub(crate) mod parsing {
     use crate::parse::{Parse, ParseStream};
     use crate::path::Path;
     use crate::{mac, token};
+    use alloc::vec::Vec;
+    use core::fmt::{self, Display};
     use proc_macro2::Ident;
-    use std::fmt::{self, Display};
 
     pub(crate) fn parse_inner(input: ParseStream, attrs: &mut Vec<Attribute>) -> Result<()> {
         while input.peek(Token![#]) && input.peek2(Token![!]) {

src/bigint.rs

@@ -1,4 +1,6 @@
-use std::ops::{AddAssign, MulAssign};
+use alloc::string::String;
+use alloc::vec::Vec;
+use core::ops::{AddAssign, MulAssign};
 
 // For implementing base10_digits() accessor on LitInt.
 pub(crate) struct BigInt {

src/buffer.rs

@@ -7,11 +7,13 @@
 
 use crate::ext::TokenStreamExt as _;
 use crate::Lifetime;
+use alloc::boxed::Box;
+use alloc::vec::Vec;
+use core::cmp::Ordering;
+use core::marker::PhantomData;
+use core::ptr;
 use proc_macro2::extra::DelimSpan;
 use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
-use std::cmp::Ordering;
-use std::marker::PhantomData;
-use std::ptr;
 
 /// Internal type which is used instead of `TokenTree` to represent a token tree
 /// within a `TokenBuffer`.

src/classify.rs

@@ -8,10 +8,10 @@ use crate::path::{Path, PathArguments};
 use crate::punctuated::Punctuated;
 #[cfg(any(feature = "printing", feature = "full"))]
 use crate::ty::{ReturnType, Type};
+#[cfg(any(feature = "printing", feature = "full"))]
+use core::ops::ControlFlow;
 #[cfg(feature = "full")]
 use proc_macro2::{Delimiter, TokenStream, TokenTree};
-#[cfg(any(feature = "printing", feature = "full"))]
-use std::ops::ControlFlow;
 
 #[cfg(feature = "full")]
 pub(crate) fn requires_semi_to_be_stmt(expr: &Expr) -> bool {