diff --git a/Makefile b/Makefile
index 2ebc3e1aec..11d1e0ea3a 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@ TARGET?=$(shell rustc -Z unstable-options --print target-spec-json | grep llvm-t
 
 CARGO?=cargo
 CARGO_TEST?=$(CARGO)
-CARGOFLAGS?=
+CARGOFLAGS?=-Z build-std=core,alloc,compiler_builtins
 RUSTCFLAGS?=
 export OBJCOPY?=objcopy
 
diff --git a/rust-toolchain b/rust-toolchain
index 40973dae63..3e2f5f2cac 100644
--- a/rust-toolchain
+++ b/rust-toolchain
@@ -1 +1 @@
-nightly-2020-07-27
+nightly-2021-03-15
diff --git a/src/header/wchar/utf8.rs b/src/header/wchar/utf8.rs
index e13c0ba5d7..d3fa8c65fe 100644
--- a/src/header/wchar/utf8.rs
+++ b/src/header/wchar/utf8.rs
@@ -10,9 +10,39 @@ use crate::{
 
 use super::mbstate_t;
 
+// Based on
+// https://github.com/rust-lang/rust/blob/f24ce9b/library/core/src/str/validations.rs#L232-L257,
+// because apparently somebody removed the `pub use` statement from `core::str`.
+
+// https://tools.ietf.org/html/rfc3629
+static UTF8_CHAR_WIDTH: [u8; 256] = [
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+    1, // 0x1F
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+    1, // 0x3F
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+    1, // 0x5F
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+    1, // 0x7F
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, // 0x9F
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, // 0xBF
+    0, 0, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+    2, // 0xDF
+    3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // 0xEF
+    4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xFF
+];
+
+// Given a first byte, determines how many bytes are in this UTF-8 character.
+#[inline]
+fn utf8_char_width(b: u8) -> usize {
+    UTF8_CHAR_WIDTH[usize::from(b)].into()
+}
+
 //It's guaranteed that we don't have any nullpointers here
 pub unsafe fn mbrtowc(pwc: *mut wchar_t, s: *const c_char, n: usize, ps: *mut mbstate_t) -> usize {
-    let size = str::utf8_char_width(*s as u8);
+    let size = utf8_char_width(*s as u8);
     if size > n {
         platform::errno = errno::EILSEQ;
         return -2isize as usize;