diff --git a/Cargo.toml b/Cargo.toml
index cd88aa4c..6a1fae5a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -32,6 +32,7 @@ smallvec = "0.4.3"
 [build-dependencies]
 syn = "0.11"
 quote = "0.3"
+rustc_version = "0.2"
 
 [features]
 bench = []
diff --git a/build.rs b/build.rs
index 322c1790..5a38e896 100644
--- a/build.rs
+++ b/build.rs
@@ -4,6 +4,7 @@
 
 extern crate quote;
 extern crate syn;
+extern crate rustc_version;
 
 use std::env;
 use std::path::Path;
@@ -33,6 +34,18 @@ mod codegen {
 }
 
 fn main() {
+    let rustc = rustc_version::version_meta().unwrap();
+    let commit_date = match rustc.commit_date {
+        Some(ref s) => &**s,
+        None => "unknown",  // Sorts after a date
+    };
+    if (rustc.semver.major, rustc.semver.minor, rustc.semver.patch, commit_date) >=
+       (1, 23, 0, "2017-11-20")
+    {
+        // https://github.com/rust-lang/rust/pull/45225
+        println!("cargo:rustc-cfg=rustc_has_pr45225")
+    }
+
     let manifest_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
     let tokenizer_rs = Path::new(&manifest_dir).join("src/tokenizer.rs");
     codegen::main(&tokenizer_rs);
diff --git a/macros/lib.rs b/macros/lib.rs
index 80d3e545..d530dd47 100644
--- a/macros/lib.rs
+++ b/macros/lib.rs
@@ -8,7 +8,7 @@ extern crate proc_macro;
 #[macro_use] extern crate quote;
 extern crate syn;
 
-use std::ascii::AsciiExt;
+#[allow(unused_imports)] use std::ascii::AsciiExt;
 
 define_proc_macros! {
     /// Input: the arms of a `match` expression.
diff --git a/src/nth.rs b/src/nth.rs
index ea0b9e54..b3a6a600 100644
--- a/src/nth.rs
+++ b/src/nth.rs
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use std::ascii::AsciiExt;
+#[allow(unused_imports)] use std::ascii::AsciiExt;
 
 use super::{Token, Parser, ParserInput, BasicParseError};
 
diff --git a/src/parser.rs b/src/parser.rs
index faaa773b..76736a85 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -5,7 +5,7 @@
 use cow_rc_str::CowRcStr;
 use smallvec::SmallVec;
 use std::ops::Range;
-use std::ascii::AsciiExt;
+#[allow(unused_imports)] use std::ascii::AsciiExt;
 use std::ops::BitOr;
 use tokenizer::{Token, Tokenizer, SourcePosition, SourceLocation};
 
diff --git a/src/rules_and_declarations.rs b/src/rules_and_declarations.rs
index 32072094..f12330de 100644
--- a/src/rules_and_declarations.rs
+++ b/src/rules_and_declarations.rs
@@ -6,7 +6,7 @@
 
 use cow_rc_str::CowRcStr;
 use parser::{parse_until_before, parse_until_after, parse_nested_block, ParserState};
-use std::ascii::AsciiExt;
+#[allow(unused_imports)] use std::ascii::AsciiExt;
 use super::{Token, Parser, Delimiter, ParseError, BasicParseError, BasicParseErrorKind};
 
 /// Parse `!important`.
diff --git a/src/serializer.rs b/src/serializer.rs
index ecab8928..1a737946 100644
--- a/src/serializer.rs
+++ b/src/serializer.rs
@@ -4,7 +4,7 @@
 
 use dtoa_short::{self, Notation};
 use itoa;
-use std::ascii::AsciiExt;
+#[allow(unused_imports)] use std::ascii::AsciiExt;
 use std::fmt::{self, Write};
 use std::io;
 use std::str;
diff --git a/src/size_of_tests.rs b/src/size_of_tests.rs
index de81172d..b07abc07 100644
--- a/src/size_of_tests.rs
+++ b/src/size_of_tests.rs
@@ -37,10 +37,10 @@ size_of_test!(std_cow_str, Cow<'static, str>, 32);
 size_of_test!(cow_rc_str, CowRcStr, 16);
 
 size_of_test!(tokenizer, ::tokenizer::Tokenizer, 72);
-size_of_test!(parser_input, ::parser::ParserInput, 144);
+size_of_test!(parser_input, ::parser::ParserInput, if cfg!(rustc_has_pr45225) { 136 } else { 144 });
 size_of_test!(parser, ::parser::Parser, 16);
 size_of_test!(source_position, ::SourcePosition, 8);
 size_of_test!(parser_state, ::ParserState, 24);
 
 size_of_test!(basic_parse_error, ::BasicParseError, 48);
-size_of_test!(parse_error_lower_bound, ::ParseError<()>, 56);
+size_of_test!(parse_error_lower_bound, ::ParseError<()>, if cfg!(rustc_has_pr45225) { 48 } else { 56 });
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index e5fb9b0a..a74d7c2f 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -6,7 +6,7 @@
 
 use std::ops::Range;
 use std::char;
-use std::ascii::AsciiExt;
+#[allow(unused_imports)] use std::ascii::AsciiExt;
 use std::i32;
 
 use parser::ParserState;