Fix clippy issues related to appending format!-ed strings. (FuelLabs#…

…3231)

This replaces appending the output of format! calls to a string with a
writeln! call, as suggested by clippy.

This started showing up locally, dunno why it does not show up on CI...

sway-ir/src/analysis/dominator.rs

@@ -3,6 +3,7 @@ use crate::{block::Block, BranchToWithArgs, Context, Function};
 /// The algorithms implemented here are from the paper
 // "A Simple, Fast Dominance Algorithm" -- Keith D. Cooper, Timothy J. Harvey, and Ken Kennedy.
 use rustc_hash::{FxHashMap, FxHashSet};
+use std::fmt::Write;
 
 /// Represents a node in the dominator tree.
 pub struct DomTreeNode {
@@ -186,11 +187,12 @@ pub fn print_dot(context: &Context, func_name: &str, dom_tree: &DomTree) -> Stri
     let mut res = format!("digraph {} {{\n", func_name);
     for (b, idom) in dom_tree.iter() {
         if let Some(idom) = idom.parent {
-            res += &(format!(
-                "\t{} -> {}\n",
+            let _ = writeln!(
+                res,
+                "\t{} -> {}",
                 idom.get_label(context),
                 b.get_label(context)
-            ))
+            );
         }
     }
     res += "}\n";

sway-ir/src/function.rs

@@ -7,6 +7,7 @@
 //! existing in the function scope.
 
 use std::collections::{BTreeMap, HashMap};
+use std::fmt::Write;
 
 use rustc_hash::{FxHashMap, FxHashSet};
 
@@ -473,11 +474,12 @@ impl Function {
             let n = worklist.pop().unwrap();
             visited.insert(n);
             for BranchToWithArgs { block: n_succ, .. } in n.successors(context) {
-                res += &(format!(
+                let _ = writeln!(
+                    res,
                     "\t{} -> {}\n",
                     n.get_label(context),
                     n_succ.get_label(context)
-                ));
+                );
                 if !visited.contains(&n_succ) {
                     worklist.push(n_succ);
                 }