Don't format fn keywords when not selected with --file-lines

src/items.rs

@@ -2465,10 +2465,31 @@ fn rewrite_fn_base(
     let where_clause = &fn_sig.generics.where_clause;
 
     let mut result = String::with_capacity(1024);
-    result.push_str(&fn_sig.to_str(context));
 
-    // fn foo
-    result.push_str("fn ");
+    // Everything before `fn`
+    let before_ident_span = mk_sp(span.lo(), ident.span.lo());
+    let fn_lo = context
+        .snippet_provider
+        .span_before_last(before_ident_span, "fn");
+    let span_before_fn = mk_sp(span.lo(), fn_lo);
+
+    if file_lines_contains!(context, span_before_fn) {
+        result.push_str(&fn_sig.to_str(context));
+    } else {
+        result.push_str(context.snippet(mk_sp(span_before_fn.lo(), fn_lo)));
+    }
+
+    result.push_str("fn");
+
+    // If both `fn` and the ident are selected, put them both on the same line.
+    // Otherwise, preserve the snippet between `fn` and the ident.
+    let fn_ident_span = mk_sp(fn_lo, ident.span.hi());
+    if file_lines_contains!(context, fn_ident_span) {
+        result.push(' ');
+    } else {
+        let fn_hi = fn_lo + BytePos(2);
+        result.push_str(context.snippet(mk_sp(fn_hi, ident.span.lo())));
+    }
 
     // Generics.
     let overhead = if let FnBraceStyle::SameLine = fn_brace_style {

src/source_map.rs

@@ -65,7 +65,7 @@ impl SpanUtils for SnippetProvider {
             offset += additional_offset + needle.len();
         }
 
-        original.lo() + BytePos(offset as u32 - 1)
+        original.lo() + BytePos(offset as u32 - needle.len() as u32)
     }
 
     fn opt_span_after(&self, original: Span, needle: &str) -> Option<BytePos> {

src/utils.rs

@@ -381,6 +381,16 @@ macro_rules! out_of_file_lines_range {
     };
 }
 
+/// Returns `true` if the given span is fully contained by the selected file lines.
+macro_rules! file_lines_contains {
+    ($self:ident, $span:expr) => {
+        $self
+            .config
+            .file_lines()
+            .contains(&$self.psess.lookup_line_range($span))
+    };
+}
+
 macro_rules! skip_out_of_file_lines_range_err {
     ($self:ident, $span:expr) => {
         if out_of_file_lines_range!($self, $span) {

tests/source/issue-6868-before-fn/fn_and_ident.rs

@@ -0,0 +1,16 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/fn_and_ident.rs","range":[11,12]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn
+foo
+(
+)
+{
+}

tests/source/issue-6868-before-fn/fully_selected.rs

@@ -0,0 +1,16 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/fully_selected.rs","range":[3,11]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn
+foo
+(
+)
+{
+}

tests/source/issue-6868-before-fn/partial.rs

@@ -0,0 +1,16 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/partial.rs","range":[7,8]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn
+foo
+(
+)
+{
+}

tests/source/issue-6868-before-fn/unselected.rs

@@ -0,0 +1,16 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/unselected.rs","range":[11,14]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn
+foo
+(
+)
+{
+}

tests/target/issue-6868-before-fn/fn_and_ident.rs

@@ -0,0 +1,12 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/fn_and_ident.rs","range":[11,12]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn foo() {
+}

tests/target/issue-6868-before-fn/fully_selected.rs

@@ -0,0 +1,5 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/fully_selected.rs","range":[3,11]}]
+
+pub(crate) const unsafe extern "C" fn
+foo() {
+}

tests/target/issue-6868-before-fn/partial.rs

@@ -0,0 +1,13 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/partial.rs","range":[7,8]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn
+foo() {
+}

tests/target/issue-6868-before-fn/unselected.rs

@@ -0,0 +1,12 @@
+// rustfmt-file_lines: [{"file":"tests/source/issue-6868-before-fn/unselected.rs","range":[11,14]}]
+
+pub
+(
+crate
+)
+const
+unsafe
+extern
+"C"
+fn foo() {
+}