Fix and enable clippy::format_collect

Summary: Just two simple fixes.

Reviewed By: JakobDegen

Differential Revision: D52508143

fbshipit-source-id: ddec9e6bbc659549b1b54a2e763bbc9c5263c979

allocative/allocative/src/golden.rs

@@ -10,6 +10,7 @@
 #![cfg(test)]
 
 use std::env;
+use std::fmt::Write;
 use std::fs;
 
 use crate::Allocative;
@@ -50,8 +51,10 @@ fn make_golden<T: Allocative>(value: &T) -> (String, String) {
         "{header}{flamegraph}",
         header = golden_header()
             .lines()
-            .map(|line| format!("# {}\n", line))
-            .collect::<String>()
+            .fold(String::new(), |mut output, line| {
+                let _ = writeln!(output, "# {}", line);
+                output
+            })
     );
 
     let flamegraph_svg = flamegraph_svg.replace(

starlark/src/docs/markdown.rs

@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+use std::fmt::Write;
 use std::slice;
 
 use dupe::Dupe;
@@ -143,11 +144,11 @@ fn render_function_parameters(params: &[DocParam]) -> Option<String> {
             DocParam::Args { name, docs, .. } => Some((name, docs)),
             DocParam::Kwargs { name, docs, .. } => Some((name, docs)),
         })
-        .map(|(name, docs)| {
+        .fold(String::new(), |mut output, (name, docs)| {
             let docs = render_doc_string(DSOpts::Combined, docs).unwrap_or_default();
-            format!("* `{name}`: {docs}\n")
-        })
-        .collect();
+            let _ = writeln!(output, "* `{name}`: {docs}");
+            output
+        });
     Some(param_list)
 }