diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 9da582a6..a12a2523 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -36,7 +36,7 @@ jobs:
           override: true
 
       - name: Downgrade phf to a version compatible with the MSRV
-        run: cargo update --package phf:0.11.0 --precise 0.10.1
+        run: cargo update --package phf --precise 0.10.1
         if: matrix.toolchain == '1.40.0'
 
       - name: Cargo build
diff --git a/build.rs b/build.rs
index 07a72ffe..89407f6f 100644
--- a/build.rs
+++ b/build.rs
@@ -42,5 +42,9 @@ fn main() {
         println!("cargo:rustc-cfg=rustc_has_pr45225")
     }
 
+    if std::mem::size_of::<std::borrow::Cow<'static, str>>() == 24 {
+        println!("cargo:rustc-cfg=rustc_has_better_cow_layout")
+    }
+
     codegen::main();
 }
diff --git a/src/size_of_tests.rs b/src/size_of_tests.rs
index 7e39e4c6..f5a63673 100644
--- a/src/size_of_tests.rs
+++ b/src/size_of_tests.rs
@@ -38,7 +38,7 @@ macro_rules! size_of_test {
 
 // Some of these assume 64-bit
 size_of_test!(token, Token, 32);
-size_of_test!(std_cow_str, Cow<'static, str>, 32);
+size_of_test!(std_cow_str, Cow<'static, str>, if cfg!(rustc_has_better_cow_layout) { 24 } else { 32 });
 size_of_test!(cow_rc_str, CowRcStr, 16);
 
 size_of_test!(tokenizer, crate::tokenizer::Tokenizer, 72);
@@ -51,9 +51,15 @@ size_of_test!(parser, crate::parser::Parser, 16);
 size_of_test!(source_position, crate::SourcePosition, 8);
 size_of_test!(parser_state, crate::ParserState, 24);
 
-size_of_test!(basic_parse_error, crate::BasicParseError, 48);
+size_of_test!(basic_parse_error, crate::BasicParseError, if cfg!(rustc_has_better_cow_layout) { 40 } else { 48 });
 size_of_test!(
     parse_error_lower_bound,
     crate::ParseError<()>,
-    if cfg!(rustc_has_pr45225) { 48 } else { 56 }
+    if cfg!(rustc_has_better_cow_layout) {
+        40
+    } else if cfg!(rustc_has_pr45225) {
+        48
+    } else {
+        56
+    }
 );