Change std imports to core where possible

Useful for seeing what are the gaps toward being compatible with
no-std. Useful for copying chunks of code into a no-std crate.

Author: dtolnay

src/detection.rs

@@ -1,4 +1,4 @@
-use std::sync::atomic::{AtomicUsize, Ordering};
+use core::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Once;
 
 static WORKS: AtomicUsize = AtomicUsize::new(0);

src/fallback.rs

@@ -2,18 +2,18 @@ use crate::parse::{self, Cursor};
 use crate::rcvec::{RcVec, RcVecBuilder, RcVecIntoIter, RcVecMut};
 use crate::{Delimiter, Spacing, TokenTree};
 #[cfg(span_locations)]
-use std::cell::RefCell;
+use core::cell::RefCell;
 #[cfg(span_locations)]
-use std::cmp;
-use std::fmt::{self, Debug, Display, Write};
-use std::iter::FromIterator;
-use std::mem::ManuallyDrop;
-use std::ops::RangeBounds;
+use core::cmp;
+use core::fmt::{self, Debug, Display, Write};
+use core::iter::FromIterator;
+use core::mem::ManuallyDrop;
+use core::ops::RangeBounds;
+use core::ptr;
+use core::str::FromStr;
 #[cfg(procmacro2_semver_exempt)]
 use std::path::Path;
 use std::path::PathBuf;
-use std::ptr;
-use std::str::FromStr;
 
 /// Force use of proc-macro2's fallback implementation of the API for now, even
 /// if the compiler's implementation is available.

src/lib.rs

@@ -137,15 +137,15 @@ use crate::fallback as imp;
 mod imp;
 
 use crate::marker::Marker;
-use std::cmp::Ordering;
+use core::cmp::Ordering;
+use core::fmt::{self, Debug, Display};
+use core::hash::{Hash, Hasher};
+use core::iter::FromIterator;
+use core::ops::RangeBounds;
+use core::str::FromStr;
 use std::error::Error;
-use std::fmt::{self, Debug, Display};
-use std::hash::{Hash, Hasher};
-use std::iter::FromIterator;
-use std::ops::RangeBounds;
 #[cfg(procmacro2_semver_exempt)]
 use std::path::PathBuf;
-use std::str::FromStr;
 
 /// An abstract stream of tokens, or more concretely a sequence of token trees.
 ///
@@ -1271,7 +1271,7 @@ impl Display for Literal {
 pub mod token_stream {
     use crate::marker::Marker;
     use crate::{imp, TokenTree};
-    use std::fmt::{self, Debug};
+    use core::fmt::{self, Debug};
 
     pub use crate::TokenStream;
 

src/marker.rs

@@ -1,4 +1,4 @@
-use std::marker::PhantomData;
+use core::marker::PhantomData;
 use std::panic::{RefUnwindSafe, UnwindSafe};
 use std::rc::Rc;
 
@@ -9,7 +9,7 @@ pub(crate) type Marker = PhantomData<ProcMacroAutoTraits>;
 pub(crate) use self::value::*;
 
 mod value {
-    pub(crate) use std::marker::PhantomData as Marker;
+    pub(crate) use core::marker::PhantomData as Marker;
 }
 
 pub(crate) struct ProcMacroAutoTraits(Rc<()>);

src/parse.rs

@@ -3,8 +3,8 @@ use crate::fallback::{
     TokenStreamBuilder,
 };
 use crate::{Delimiter, Punct, Spacing, TokenTree};
-use std::char;
-use std::str::{Bytes, CharIndices, Chars};
+use core::char;
+use core::str::{Bytes, CharIndices, Chars};
 
 #[derive(Copy, Clone, Eq, PartialEq)]
 pub(crate) struct Cursor<'a> {

src/rcvec.rs

@@ -1,6 +1,6 @@
-use std::mem;
+use core::mem;
+use core::slice;
 use std::rc::Rc;
-use std::slice;
 use std::vec;
 
 pub(crate) struct RcVec<T> {

src/wrapper.rs

@@ -1,12 +1,12 @@
 use crate::detection::inside_proc_macro;
 use crate::{fallback, Delimiter, Punct, Spacing, TokenTree};
-use std::fmt::{self, Debug, Display};
-use std::iter::FromIterator;
-use std::ops::RangeBounds;
+use core::fmt::{self, Debug, Display};
+use core::iter::FromIterator;
+use core::ops::RangeBounds;
+use core::str::FromStr;
 use std::panic;
 #[cfg(super_unstable)]
 use std::path::PathBuf;
-use std::str::FromStr;
 
 #[derive(Clone)]
 pub(crate) enum TokenStream {