Tweak wording

compiler/rustc_parse/src/lib.rs

@@ -23,7 +23,7 @@ use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::{AttrItem, Attribute, MetaItemInner, token};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{Diag, EmissionGuarantee, FatalError, PResult};
+use rustc_errors::{Diag, EmissionGuarantee, FatalError, PResult, pluralize};
 use rustc_session::parse::ParseSess;
 use rustc_span::source_map::SourceMap;
 use rustc_span::{FileName, SourceFile, Span};
@@ -112,7 +112,16 @@ pub fn utf8_error<E: EmissionGuarantee>(
     let start = utf8err.valid_up_to();
     let note = format!("invalid utf-8 at byte `{start}`");
     let msg = if let Some(len) = utf8err.error_len() {
-        format!("`{:?}` is not valid utf-8", &contents[start..start + len])
+        format!(
+            "byte{s} `{bytes}` {are} not valid utf-8",
+            bytes = if len == 1 {
+                format!("{:?}", contents[start])
+            } else {
+                format!("{:?}", &contents[start..start + len])
+            },
+            s = pluralize!(len),
+            are = if len == 1 { "is" } else { "are" },
+        )
     } else {
         note.clone()
     };

tests/ui/macros/not-utf8.stderr

@@ -4,7 +4,7 @@ error: couldn't read `$DIR/not-utf8.bin`: stream did not contain valid UTF-8
 LL |     include!("not-utf8.bin");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: `[193]` is not valid utf-8
+note: byte `193` is not valid utf-8
   --> $DIR/not-utf8.bin:1:1
    |
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