diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 7e88b58c0e2..00f2f37146d 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -939,6 +939,7 @@ pub fn expand_preparsed_format_args(
 
     let msg = "format argument must be a string literal";
     let fmt_sp = efmt.span;
+    let efmt_kind_is_lit: bool = matches!(efmt.kind, ast::ExprKind::Lit(_));
     let (fmt_str, fmt_style, fmt_span) = match expr_to_spanned_string(ecx, efmt, msg) {
         Ok(mut fmt) if append_newline => {
             fmt.0 = Symbol::intern(&format!("{}\n", fmt.0));
@@ -989,7 +990,19 @@ pub fn expand_preparsed_format_args(
 
     if !parser.errors.is_empty() {
         let err = parser.errors.remove(0);
-        let sp = fmt_span.from_inner(err.span);
+        let sp = if efmt_kind_is_lit {
+            fmt_span.from_inner(err.span)
+        } else {
+            // The format string could be another macro invocation, e.g.:
+            //     format!(concat!("abc", "{}"), 4);
+            // However, `err.span` is an inner span relative to the *result* of
+            // the macro invocation, which is why we would get a nonsensical
+            // result calling `fmt_span.from_inner(err.span)` as above, and
+            // might even end up inside a multibyte character (issue #86085).
+            // Therefore, we conservatively report the error for the entire
+            // argument span here.
+            fmt_span
+        };
         let mut e = ecx.struct_span_err(sp, &format!("invalid format string: {}", err.description));
         e.span_label(sp, err.label + " in format string");
         if let Some(note) = err.note {
diff --git a/src/test/ui/fmt/format-concat-span.rs b/src/test/ui/fmt/format-concat-span.rs
new file mode 100644
index 00000000000..ce92df0ad92
--- /dev/null
+++ b/src/test/ui/fmt/format-concat-span.rs
@@ -0,0 +1,15 @@
+// If the format string is another macro invocation, rustc would previously
+// compute nonsensical spans, such as:
+//
+//   error: invalid format string: unmatched `}` found
+//    --> test.rs:2:17
+//     |
+//   2 |     format!(concat!("abc}"));
+//     |                 ^ unmatched `}` in format string
+//
+// This test checks that this behavior has been fixed.
+
+fn main() {
+    format!(concat!("abc}"));
+    //~^ ERROR: invalid format string: unmatched `}` found
+}
diff --git a/src/test/ui/fmt/format-concat-span.stderr b/src/test/ui/fmt/format-concat-span.stderr
new file mode 100644
index 00000000000..da46f40abcb
--- /dev/null
+++ b/src/test/ui/fmt/format-concat-span.stderr
@@ -0,0 +1,11 @@
+error: invalid format string: unmatched `}` found
+  --> $DIR/format-concat-span.rs:13:13
+   |
+LL |     format!(concat!("abc}"));
+   |             ^^^^^^^^^^^^^^^ unmatched `}` in format string
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/fmt/issue-86085.rs b/src/test/ui/fmt/issue-86085.rs
new file mode 100644
index 00000000000..63d42b76969
--- /dev/null
+++ b/src/test/ui/fmt/issue-86085.rs
@@ -0,0 +1,6 @@
+// Tests for an ICE with the fuzzed input below.
+
+fn main ( ) {
+format ! ( concat ! ( r#"lJ𐏿Æ�.𐏿�"# , "r} {}" )     ) ;
+//~^ ERROR: invalid format string: unmatched `}` found
+}
diff --git a/src/test/ui/fmt/issue-86085.stderr b/src/test/ui/fmt/issue-86085.stderr
new file mode 100644
index 00000000000..ee7d8a5cc23
--- /dev/null
+++ b/src/test/ui/fmt/issue-86085.stderr
@@ -0,0 +1,11 @@
+error: invalid format string: unmatched `}` found
+  --> $DIR/issue-86085.rs:4:12
+   |
+LL | format ! ( concat ! ( r#"lJ𐏿Æ�.𐏿�"# , "r} {}" )     ) ;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unmatched `}` in format string
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+