instead of collecting newly formatted Strings into one String, only create a single String and write!() to it (clippy::format_collect)

Author: matthiaskrgr

compiler/rustc_hir_typeck/src/method/prelude2021.rs

@@ -14,6 +14,7 @@ use rustc_span::symbol::kw::{Empty, Underscore};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
 use rustc_trait_selection::infer::InferCtxtExt;
+use std::fmt::Write;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(super) fn lint_dot_call_from_2018(
@@ -143,16 +144,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
                     let (self_adjusted, precise) = self.adjust_expr(pick, self_expr, sp);
                     if precise {
-                        let args = args
-                            .iter()
-                            .map(|arg| {
-                                let span = arg.span.find_ancestor_inside(sp).unwrap_or_default();
-                                format!(
-                                    ", {}",
-                                    self.sess().source_map().span_to_snippet(span).unwrap()
-                                )
-                            })
-                            .collect::<String>();
+                        let args = args.iter().fold(String::new(), |mut string, arg| {
+                            let span = arg.span.find_ancestor_inside(sp).unwrap_or_default();
+                            write!(
+                                string,
+                                ", {}",
+                                self.sess().source_map().span_to_snippet(span).unwrap()
+                            )
+                            .unwrap();
+                            string
+                        });
 
                         lint.span_suggestion(
                             sp,

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -1,6 +1,7 @@
 //! Diagnostics related methods for `Ty`.
 
 use std::borrow::Cow;
+use std::fmt::Write;
 use std::ops::ControlFlow;
 
 use crate::ty::{
@@ -335,10 +336,10 @@ pub fn suggest_constraining_type_params<'a>(
             //                                           - insert: `, X: Bar`
             suggestions.push((
                 generics.tail_span_for_predicate_suggestion(),
-                constraints
-                    .iter()
-                    .map(|&(constraint, _)| format!(", {param_name}: {constraint}"))
-                    .collect::<String>(),
+                constraints.iter().fold(String::new(), |mut string, &(constraint, _)| {
+                    write!(string, ", {param_name}: {constraint}").unwrap();
+                    string
+                }),
                 SuggestChangingConstraintsMessage::RestrictTypeFurther { ty: param_name },
             ));
             continue;