diff --git a/src/items.rs b/src/items.rs
index 0e814644304..65b990be18b 100644
--- a/src/items.rs
+++ b/src/items.rs
@@ -2500,6 +2500,7 @@ fn rewrite_fn_base(
         ret_str_len,
         fn_brace_style,
         multi_line_ret_str,
+        where_clause,
     );
 
     debug!(
@@ -2602,10 +2603,20 @@ fn rewrite_fn_base(
                 // the closing parenthesis of the param and the arrow '->' is considered.
                 let mut sig_length = result.len() + indent.width() + ret_str_len + 1;
 
-                // If there is no where-clause, take into account the space after the return type
-                // and the brace.
+                // If there is no where-clause.
                 if where_clause.predicates.is_empty() {
-                    sig_length += 2;
+                    if context.config.style_edition() >= StyleEdition::Edition2027 {
+                        let line_ending_overhead = match fn_brace_style {
+                            FnBraceStyle::NextLine => 0, // No brace to account for
+                            FnBraceStyle::SameLine => 2, // Trailing space and brace, e.g. ` {`
+                            FnBraceStyle::None => 1,     // Trailing `;`
+                        };
+                        sig_length += line_ending_overhead;
+                    } else {
+                        // Take into account the space after the return type and the brace.
+                        // 2 = ' {'
+                        sig_length += 2;
+                    }
                 }
 
                 sig_length > context.config.max_width()
@@ -2889,6 +2900,7 @@ fn compute_budgets_for_params(
     ret_str_len: usize,
     fn_brace_style: FnBraceStyle,
     force_vertical_layout: bool,
+    where_clause: &ast::WhereClause,
 ) -> (usize, usize, Indent) {
     debug!(
         "compute_budgets_for_params {} {:?}, {}, {:?}",
@@ -2903,7 +2915,15 @@ fn compute_budgets_for_params(
         let overhead = if ret_str_len == 0 { 2 } else { 3 };
         let mut used_space = indent.width() + result.len() + ret_str_len + overhead;
         match fn_brace_style {
-            FnBraceStyle::None => used_space += 1,     // 1 = `;`
+            _ if context.config.style_edition() >= StyleEdition::Edition2027
+                && where_clause.predicates.len() > 0 =>
+            {
+                // Don't add anything to `used_space` if we have a where clause.
+                // For all `FnBraceStyle` values if we have a where cluase that can't fit
+                // on the current line it'll be written to the next line.
+                // Therefore, we don't need to account for a trailing `;` or `{}`
+            }
+            FnBraceStyle::None => used_space += 1, // 1 = `;`
             FnBraceStyle::SameLine => used_space += 2, // 2 = `{}`
             FnBraceStyle::NextLine => (),
         }
diff --git a/tests/source/issue-6539/brace_next_line.rs b/tests/source/issue-6539/brace_next_line.rs
new file mode 100644
index 00000000000..04dbc84f4e4
--- /dev/null
+++ b/tests/source/issue-6539/brace_next_line.rs
@@ -0,0 +1,16 @@
+// rustfmt-style_edition: 2027
+// rustfmt-brace_style: AlwaysNextLine
+// rustfmt-where_single_line: false
+
+pub trait Trait
+{
+    fn a_one_hundred_column_fn_decl_no_body(&self, aaa: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause<T>(&self, a: T, bb: f64) -> f64 where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause2<T>(&self, aaa: T, bbb: f64) -> f64 where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_with_body(&self, aaaaa: f64, bbbbb: f64, ccc: f64) -> f64 {}
+
+    fn an_over_one_hundred_column_fn_decl_with_body_and_where_clause<T>(&self, aaaaa: f64) -> f64 where T: Debug {}
+}
diff --git a/tests/source/issue-6539/brace_next_line_where_single_line.rs b/tests/source/issue-6539/brace_next_line_where_single_line.rs
new file mode 100644
index 00000000000..feb75359939
--- /dev/null
+++ b/tests/source/issue-6539/brace_next_line_where_single_line.rs
@@ -0,0 +1,16 @@
+// rustfmt-style_edition: 2027
+// rustfmt-brace_style: AlwaysNextLine
+// rustfmt-where_single_line: true
+
+pub trait Trait
+{
+    fn a_one_hundred_column_fn_decl_no_body(&self, aaa: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause<T>(&self, a: T, bb: f64) -> f64 where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause2<T>(&self, aaa: T, bbb: f64) -> f64 where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_with_body(&self, aaaaa: f64, bbbbb: f64, ccc: f64) -> f64 {}
+
+    fn an_over_one_hundred_column_fn_decl_with_body_and_where_clause<T>(&self, aaaaa: f64) -> f64 where T: Debug {}
+}
diff --git a/tests/source/issue-6539/default.rs b/tests/source/issue-6539/default.rs
new file mode 100644
index 00000000000..568ced6a2fd
--- /dev/null
+++ b/tests/source/issue-6539/default.rs
@@ -0,0 +1,15 @@
+// rustfmt-style_edition: 2027
+
+pub trait Trait {
+    fn a_one_hundred_column_fn_decl_no_body(&self, aaa: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn a_one_hundred_one_column_fn_decl_no_body(self, a: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause<T>(&self, a: T, bb: f64) -> f64 where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause2<T>(&self, aaa: T, bbb: f64) -> f64 where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_with_body(&self, aaaaa: f64, bbbbb: f64, ccc: f64) -> f64 {}
+
+    fn an_over_one_hundred_column_fn_decl_with_body_and_where_clause<T>(&self, aaaaa: f64) -> f64 where T: Debug {}
+}
diff --git a/tests/target/issue-6539/brace_next_line.rs b/tests/target/issue-6539/brace_next_line.rs
new file mode 100644
index 00000000000..35cb57fa6ca
--- /dev/null
+++ b/tests/target/issue-6539/brace_next_line.rs
@@ -0,0 +1,30 @@
+// rustfmt-style_edition: 2027
+// rustfmt-brace_style: AlwaysNextLine
+// rustfmt-where_single_line: false
+
+pub trait Trait
+{
+    fn a_one_hundred_column_fn_decl_no_body(&self, aaa: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause<T>(&self, a: T, bb: f64) -> f64
+    where
+        T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause2<T>(
+        &self,
+        aaa: T,
+        bbb: f64,
+    ) -> f64
+    where
+        T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_with_body(&self, aaaaa: f64, bbbbb: f64, ccc: f64) -> f64
+    {
+    }
+
+    fn an_over_one_hundred_column_fn_decl_with_body_and_where_clause<T>(&self, aaaaa: f64) -> f64
+    where
+        T: Debug,
+    {
+    }
+}
diff --git a/tests/target/issue-6539/brace_next_line_where_single_line.rs b/tests/target/issue-6539/brace_next_line_where_single_line.rs
new file mode 100644
index 00000000000..bdf7cdd9c02
--- /dev/null
+++ b/tests/target/issue-6539/brace_next_line_where_single_line.rs
@@ -0,0 +1,28 @@
+// rustfmt-style_edition: 2027
+// rustfmt-brace_style: AlwaysNextLine
+// rustfmt-where_single_line: true
+
+pub trait Trait
+{
+    fn a_one_hundred_column_fn_decl_no_body(&self, aaa: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause<T>(&self, a: T, bb: f64) -> f64
+    where T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause2<T>(
+        &self,
+        aaa: T,
+        bbb: f64,
+    ) -> f64
+    where
+        T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_with_body(&self, aaaaa: f64, bbbbb: f64, ccc: f64) -> f64
+    {
+    }
+
+    fn an_over_one_hundred_column_fn_decl_with_body_and_where_clause<T>(&self, aaaaa: f64) -> f64
+    where T: Debug
+    {
+    }
+}
diff --git a/tests/target/issue-6539/default.rs b/tests/target/issue-6539/default.rs
new file mode 100644
index 00000000000..2392a52cf08
--- /dev/null
+++ b/tests/target/issue-6539/default.rs
@@ -0,0 +1,40 @@
+// rustfmt-style_edition: 2027
+
+pub trait Trait {
+    fn a_one_hundred_column_fn_decl_no_body(&self, aaa: f64, b: f64, c: f64, d: f64, e: f64) -> f64;
+
+    fn a_one_hundred_one_column_fn_decl_no_body(
+        self,
+        a: f64,
+        b: f64,
+        c: f64,
+        d: f64,
+        e: f64,
+    ) -> f64;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause<T>(&self, a: T, bb: f64) -> f64
+    where
+        T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_no_body_and_where_clause2<T>(
+        &self,
+        aaa: T,
+        bbb: f64,
+    ) -> f64
+    where
+        T: Debug;
+
+    fn an_over_one_hundred_column_fn_decl_with_body(
+        &self,
+        aaaaa: f64,
+        bbbbb: f64,
+        ccc: f64,
+    ) -> f64 {
+    }
+
+    fn an_over_one_hundred_column_fn_decl_with_body_and_where_clause<T>(&self, aaaaa: f64) -> f64
+    where
+        T: Debug,
+    {
+    }
+}