diff --git a/spec/std/base64_spec.cr b/spec/std/base64_spec.cr
index 567340d6fa38..300498120bc6 100644
--- a/spec/std/base64_spec.cr
+++ b/spec/std/base64_spec.cr
@@ -169,13 +169,13 @@ describe "Base64" do
         "Tm93IGlzIHRoZSB0aW1lIGZvciBhbGwgZ29vZCBjb2RlcnMKdG8gbGVhcm4gQ3J5c3RhbA=="
     end
     it "with spec symbols" do
-      s = String.build { |b| (160..179).each { |i| b << i.chr } }
+      s = String.build { |buf| (160..179).each { |i| buf << i.chr } }
       se = "wqDCocKiwqPCpMKlwqbCp8KowqnCqsKrwqzCrcKuwq/CsMKxwrLCsw=="
       assert_prints base64_strict_encode(s), se
     end
 
     it "encode to stream returns number of written characters" do
-      s = String.build { |b| (160..179).each { |i| b << i.chr } }
+      s = String.build { |buf| (160..179).each { |i| buf << i.chr } }
       io = IO::Memory.new
       Base64.strict_encode(s, io).should eq(56)
     end
@@ -183,13 +183,13 @@ describe "Base64" do
 
   describe "urlsafe" do
     it "work" do
-      s = String.build { |b| (160..179).each { |i| b << i.chr } }
+      s = String.build { |buf| (160..179).each { |i| buf << i.chr } }
       se = "wqDCocKiwqPCpMKlwqbCp8KowqnCqsKrwqzCrcKuwq_CsMKxwrLCsw=="
       assert_prints base64_urlsafe_encode(s), se
     end
 
     it "encode to stream returns number of written characters" do
-      s = String.build { |b| (160..179).each { |i| b << i.chr } }
+      s = String.build { |buf| (160..179).each { |i| buf << i.chr } }
       io = IO::Memory.new
       Base64.urlsafe_encode(s, io).should eq(56)
     end
diff --git a/spec/std/io/buffered_spec.cr b/spec/std/io/buffered_spec.cr
index d8ef8ff3b979..1fd7990166ed 100644
--- a/spec/std/io/buffered_spec.cr
+++ b/spec/std/io/buffered_spec.cr
@@ -220,10 +220,10 @@ describe "IO::Buffered" do
   end
 
   it "reads more than the buffer's internal capacity" do
-    s = String.build do |str|
+    s = String.build do |buf|
       900.times do
         10.times do |i|
-          str << ('a' + i)
+          buf << ('a' + i)
         end
       end
     end
@@ -241,10 +241,10 @@ describe "IO::Buffered" do
   end
 
   it "writes more than the buffer's internal capacity" do
-    s = String.build do |str|
+    s = String.build do |buf|
       900.times do
         10.times do |i|
-          str << ('a' + i)
+          buf << ('a' + i)
         end
       end
     end
diff --git a/spec/std/io/io_spec.cr b/spec/std/io/io_spec.cr
index ac51d4bddaea..a569e0be32cf 100644
--- a/spec/std/io/io_spec.cr
+++ b/spec/std/io/io_spec.cr
@@ -708,10 +708,10 @@ describe IO do
         end
 
         it "skips invalid byte sequences" do
-          string = String.build do |str|
-            str.write "好".encode("EUC-JP")
-            str.write_byte 255_u8
-            str.write "是".encode("EUC-JP")
+          string = String.build do |io|
+            io.write "好".encode("EUC-JP")
+            io.write_byte 255_u8
+            io.write "是".encode("EUC-JP")
           end
           io = SimpleIOMemory.new(string)
           io.set_encoding("EUC-JP", invalid: :skip)
diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index 905930463cfc..9db7e3c4ce82 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -1903,8 +1903,8 @@ describe "String" do
   end
 
   it "scans using $~" do
-    str = String.build do |str|
-      "fooxooo".scan(/(o+)/) { str << $1 }
+    str = String.build do |io|
+      "fooxooo".scan(/(o+)/) { io << $1 }
     end
     str.should eq("ooooo")
   end
diff --git a/spec/std/yaml/serialization_spec.cr b/spec/std/yaml/serialization_spec.cr
index b65667d8a504..bccbbd35c298 100644
--- a/spec/std/yaml/serialization_spec.cr
+++ b/spec/std/yaml/serialization_spec.cr
@@ -652,8 +652,8 @@ describe "YAML serialization" do
     end
 
     it "writes to a stream" do
-      string = String.build do |str|
-        %w(a b c).to_yaml(str)
+      string = String.build do |io|
+        %w(a b c).to_yaml(io)
       end
       string.should eq("---\n- a\n- b\n- c\n")
     end
diff --git a/spec/std/yaml/yaml_spec.cr b/spec/std/yaml/yaml_spec.cr
index 996b74f06716..e52496915046 100644
--- a/spec/std/yaml/yaml_spec.cr
+++ b/spec/std/yaml/yaml_spec.cr
@@ -157,8 +157,8 @@ describe "YAML" do
     end
 
     it "writes YAML to a stream" do
-      string = String.build do |str|
-        YAML.dump(%w(1 2 3), str)
+      string = String.build do |io|
+        YAML.dump(%w(1 2 3), io)
       end
       string.should eq(%(---\n- "1"\n- "2"\n- "3"\n))
     end
diff --git a/src/compiler/crystal/codegen/ast.cr b/src/compiler/crystal/codegen/ast.cr
index 816958844dd9..93e5ab32ac04 100644
--- a/src/compiler/crystal/codegen/ast.cr
+++ b/src/compiler/crystal/codegen/ast.cr
@@ -11,53 +11,53 @@ module Crystal
     property? abi_info = false
 
     def mangled_name(program, self_type)
-      name = String.build do |str|
-        str << '*'
+      name = String.build do |io|
+        io << '*'
 
         if owner = @owner
           if owner.metaclass?
-            self_type.instance_type.llvm_name(str)
+            self_type.instance_type.llvm_name(io)
             if original_owner != self_type
-              str << '@'
-              original_owner.instance_type.llvm_name(str)
+              io << '@'
+              original_owner.instance_type.llvm_name(io)
             end
-            str << "::"
+            io << "::"
           elsif !owner.is_a?(Crystal::Program)
-            self_type.llvm_name(str)
+            self_type.llvm_name(io)
             if original_owner != self_type
-              str << '@'
-              original_owner.llvm_name(str)
+              io << '@'
+              original_owner.llvm_name(io)
             end
-            str << '#'
+            io << '#'
           end
         end
 
-        str << self.name.gsub('@', '.')
+        io << self.name.gsub('@', '.')
 
         next_def = self.next
         while next_def
-          str << '\''
+          io << '\''
           next_def = next_def.next
         end
 
         if args.size > 0 || uses_block_arg?
-          str << '<'
+          io << '<'
           if args.size > 0
             args.each_with_index do |arg, i|
-              str << ", " if i > 0
-              arg.type.llvm_name(str)
+              io << ", " if i > 0
+              arg.type.llvm_name(io)
             end
           end
           if uses_block_arg?
-            str << ", " if args.size > 0
-            str << '&'
-            block_arg.not_nil!.type.llvm_name(str)
+            io << ", " if args.size > 0
+            io << '&'
+            block_arg.not_nil!.type.llvm_name(io)
           end
-          str << '>'
+          io << '>'
         end
         if return_type = @type
-          str << ':'
-          return_type.llvm_name(str)
+          io << ':'
+          return_type.llvm_name(io)
         end
       end
 
diff --git a/src/compiler/crystal/codegen/codegen.cr b/src/compiler/crystal/codegen/codegen.cr
index 2c357183ffa5..d7ac8e434dcb 100644
--- a/src/compiler/crystal/codegen/codegen.cr
+++ b/src/compiler/crystal/codegen/codegen.cr
@@ -2341,14 +2341,14 @@ module Crystal
 
   def self.safe_mangling(program, name)
     if program.has_flag?("windows")
-      String.build do |str|
+      String.build do |io|
         name.each_char do |char|
           if char.ascii_alphanumeric? || char == '_'
-            str << char
+            io << char
           else
-            str << '.'
-            char.ord.to_s(str, 16, upcase: true)
-            str << '.'
+            io << '.'
+            char.ord.to_s(io, 16, upcase: true)
+            io << '.'
           end
         end
       end
diff --git a/src/compiler/crystal/compiler.cr b/src/compiler/crystal/compiler.cr
index f32ecd344be1..551b469c7d7c 100644
--- a/src/compiler/crystal/compiler.cr
+++ b/src/compiler/crystal/compiler.cr
@@ -691,17 +691,17 @@ module Crystal
                      @llvm_mod : LLVM::Module, @output_dir : String, @bc_flags_changed : Bool)
         @name = "_main" if @name == ""
         @original_name = @name
-        @name = String.build do |str|
+        @name = String.build do |io|
           @name.each_char do |char|
             case char
             when 'a'..'z', '0'..'9', '_'
-              str << char
+              io << char
             when 'A'..'Z'
               # Because OSX has case insensitive filenames, try to avoid
               # clash of 'a' and 'A' by using 'A-' for 'A'.
-              str << char << '-'
+              io << char << '-'
             else
-              str << char.ord
+              io << char.ord
             end
           end
         end
diff --git a/src/compiler/crystal/macros/interpreter.cr b/src/compiler/crystal/macros/interpreter.cr
index af76e087ced3..0d45369c83f2 100644
--- a/src/compiler/crystal/macros/interpreter.cr
+++ b/src/compiler/crystal/macros/interpreter.cr
@@ -157,13 +157,13 @@ module Crystal
     end
 
     def visit(node : StringInterpolation)
-      @last = StringLiteral.new(String.build do |str|
+      @last = StringLiteral.new(String.build do |io|
         node.expressions.each do |exp|
           if exp.is_a?(StringLiteral)
-            str << exp.value
+            io << exp.value
           else
             exp.accept self
-            @last.to_s(str)
+            @last.to_s(io)
           end
         end
       end)
diff --git a/src/compiler/crystal/semantic/call_error.cr b/src/compiler/crystal/semantic/call_error.cr
index 5d3061850e2e..2c97a9855313 100644
--- a/src/compiler/crystal/semantic/call_error.cr
+++ b/src/compiler/crystal/semantic/call_error.cr
@@ -380,13 +380,13 @@ class Crystal::Call
   end
 
   private def raise_no_overload_matches(node, defs, arg_types, inner_exception, &)
-    error_message = String.build do |str|
-      yield str
+    error_message = String.build do |io|
+      yield io
 
-      str.puts
-      str.puts
-      str << "Overloads are:"
-      append_matches(defs, arg_types, str)
+      io.puts
+      io.puts
+      io << "Overloads are:"
+      append_matches(defs, arg_types, io)
     end
 
     node.raise(error_message, inner_exception)
@@ -675,31 +675,31 @@ class Crystal::Call
     end
     all_arguments_sizes.uniq!.sort!
 
-    raise(String.build do |str|
+    raise(String.build do |io|
       if single_message = single_def_error_message(defs, named_args_types)
-        str << single_message
-        str << '\n'
+        io << single_message
+        io << '\n'
       else
-        str << "wrong number of arguments for '"
-        str << full_name(owner, def_name)
-        str << "' (given "
-        str << arg_types.size
-        str << ", expected "
+        io << "wrong number of arguments for '"
+        io << full_name(owner, def_name)
+        io << "' (given "
+        io << arg_types.size
+        io << ", expected "
 
         # If we have 2, 3, 4, show it as 2..4
         if all_arguments_sizes.size > 1 && all_arguments_sizes.last - all_arguments_sizes.first == all_arguments_sizes.size - 1
-          str << all_arguments_sizes.first
-          str << ".."
-          str << all_arguments_sizes.last
+          io << all_arguments_sizes.first
+          io << ".."
+          io << all_arguments_sizes.last
         else
-          all_arguments_sizes.join str, ", "
+          all_arguments_sizes.join io, ", "
         end
 
-        str << '+' if min_splat != Int32::MAX
-        str << ")\n"
+        io << '+' if min_splat != Int32::MAX
+        io << ")\n"
       end
-      str << "Overloads are:"
-      append_matches(defs, arg_types, str)
+      io << "Overloads are:"
+      append_matches(defs, arg_types, io)
     end, inner: inner_exception)
   end
 
diff --git a/src/compiler/crystal/semantic/cleanup_transformer.cr b/src/compiler/crystal/semantic/cleanup_transformer.cr
index 70a8ffe63590..f0b010cd31b9 100644
--- a/src/compiler/crystal/semantic/cleanup_transformer.cr
+++ b/src/compiler/crystal/semantic/cleanup_transformer.cr
@@ -754,11 +754,11 @@ module Crystal
     end
 
     def untyped_expression(node, msg = nil)
-      ex_msg = String.build do |str|
-        str << "can't execute `" << node << "` at " << node.location
+      ex_msg = String.build do |io|
+        io << "can't execute `" << node << "` at " << node.location
         if msg
-          str << ": "
-          str << msg
+          io << ": "
+          io << msg
         end
       end
 
diff --git a/src/compiler/crystal/semantic/default_arguments.cr b/src/compiler/crystal/semantic/default_arguments.cr
index 013caab3c3aa..2618bdc32488 100644
--- a/src/compiler/crystal/semantic/default_arguments.cr
+++ b/src/compiler/crystal/semantic/default_arguments.cr
@@ -72,11 +72,11 @@ class Crystal::Def
       # names that won't clash with local variables defined in the method.
       named_args_temp_names = Array(String).new(named_args.size)
 
-      new_name = String.build do |str|
-        str << name
+      new_name = String.build do |io|
+        io << name
         named_args.each do |named_arg|
-          str << ':'
-          str << named_arg
+          io << ':'
+          io << named_arg
 
           temp_name = program.new_temp_var_name
           named_args_temp_names << temp_name
diff --git a/src/compiler/crystal/semantic/exception.cr b/src/compiler/crystal/semantic/exception.cr
index 359f12ffb882..eea17483e7a0 100644
--- a/src/compiler/crystal/semantic/exception.cr
+++ b/src/compiler/crystal/semantic/exception.cr
@@ -308,35 +308,35 @@ module Crystal
 
   class Program
     def undefined_class_variable(node, owner, similar_name)
-      common = String.build do |str|
-        str << "can't infer the type of class variable '#{node.name}' of #{owner.devirtualize}"
+      common = String.build do |io|
+        io << "can't infer the type of class variable '#{node.name}' of #{owner.devirtualize}"
         if similar_name
-          str << '\n'
-          str << "Did you mean '#{similar_name}'?"
+          io << '\n'
+          io << "Did you mean '#{similar_name}'?"
         end
       end
 
-      msg = String.build do |str|
-        str << common
-        str << "\n\n"
-        str << undefined_variable_message("class variable", node.name, owner.devirtualize)
+      msg = String.build do |io|
+        io << common
+        io << "\n\n"
+        io << undefined_variable_message("class variable", node.name, owner.devirtualize)
       end
       node.raise msg
     end
 
     def undefined_instance_variable(node, owner, similar_name)
-      common = String.build do |str|
-        str << "can't infer the type of instance variable '#{node.name}' of #{owner.devirtualize}"
+      common = String.build do |io|
+        io << "can't infer the type of instance variable '#{node.name}' of #{owner.devirtualize}"
         if similar_name
-          str << '\n'
-          str << "Did you mean '#{similar_name}'?"
+          io << '\n'
+          io << "Did you mean '#{similar_name}'?"
         end
       end
 
-      msg = String.build do |str|
-        str << common
-        str << "\n\n"
-        str << undefined_variable_message("instance variable", node.name, owner.devirtualize)
+      msg = String.build do |io|
+        io << common
+        io << "\n\n"
+        io << undefined_variable_message("instance variable", node.name, owner.devirtualize)
       end
       node.raise msg
     end
diff --git a/src/compiler/crystal/semantic/new.cr b/src/compiler/crystal/semantic/new.cr
index de8ae55312a0..63c077fa7b88 100644
--- a/src/compiler/crystal/semantic/new.cr
+++ b/src/compiler/crystal/semantic/new.cr
@@ -271,11 +271,11 @@ module Crystal
         splat_index = i
         i += 1
 
-        name = String.build do |str|
-          str << "new"
+        name = String.build do |io|
+          io << "new"
           named_args.each do |named_arg|
-            str << ':'
-            str << named_arg
+            io << ':'
+            io << named_arg
 
             # When **opts is expanded for named arguments, we must use internal
             # names that won't clash with local variables defined in the method.
diff --git a/src/compiler/crystal/semantic/semantic_visitor.cr b/src/compiler/crystal/semantic/semantic_visitor.cr
index 0b686db9dc40..a2a937e643b4 100644
--- a/src/compiler/crystal/semantic/semantic_visitor.cr
+++ b/src/compiler/crystal/semantic/semantic_visitor.cr
@@ -534,10 +534,10 @@ abstract class Crystal::SemanticVisitor < Crystal::Visitor
 
   def check_allowed_in_lib(node, type = node.type.instance_type)
     unless type.allowed_in_lib?
-      msg = String.build do |msg|
-        msg << "only primitive types, pointers, structs, unions, enums and tuples are allowed in lib declarations, not #{type}"
-        msg << " (did you mean LibC::Int?)" if type == @program.int
-        msg << " (did you mean LibC::Float?)" if type == @program.float
+      msg = String.build do |io|
+        io << "only primitive types, pointers, structs, unions, enums and tuples are allowed in lib declarations, not #{type}"
+        io << " (did you mean LibC::Int?)" if type == @program.int
+        io << " (did you mean LibC::Float?)" if type == @program.float
       end
       node.raise msg
     end
diff --git a/src/compiler/crystal/syntax/lexer.cr b/src/compiler/crystal/syntax/lexer.cr
index 33f991c5fc0f..f5fab3c7c154 100644
--- a/src/compiler/crystal/syntax/lexer.cr
+++ b/src/compiler/crystal/syntax/lexer.cr
@@ -2183,9 +2183,9 @@ module Crystal
     end
 
     def consume_string_unicode_brace_escape
-      String.build do |str|
+      String.build do |io|
         while true
-          str << consume_braced_unicode_escape(allow_spaces: true).chr
+          io << consume_braced_unicode_escape(allow_spaces: true).chr
           break unless current_char == ' '
         end
       end
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index f2c6bd8ed867..52cd371bf1c0 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -1934,16 +1934,16 @@ module Crystal
         location = @token.location
         next_token_skip_space
 
-        msg = String.build do |msg|
-          msg << %[unexpected token: "|", proc literals specify their parameters like this: ->(]
+        msg = String.build do |io|
+          io << %[unexpected token: "|", proc literals specify their parameters like this: ->(]
           if @token.type.ident?
-            msg << @token.value.to_s << " : Type"
+            io << @token.value.to_s << " : Type"
             next_token_skip_space_or_newline
-            msg << ", ..." if @token.type.op_comma?
+            io << ", ..." if @token.type.op_comma?
           else
-            msg << "param : Type"
+            io << "param : Type"
           end
-          msg << ") { ... }"
+          io << ") { ... }"
         end
 
         raise msg, location
diff --git a/src/compiler/crystal/tools/formatter.cr b/src/compiler/crystal/tools/formatter.cr
index 32168af580e9..edba68b1cca9 100644
--- a/src/compiler/crystal/tools/formatter.cr
+++ b/src/compiler/crystal/tools/formatter.cr
@@ -5005,10 +5005,10 @@ module Crystal
 
       before = line[0...middle]
       after = line[middle..-1]
-      result = String.build do |str|
-        str << before
-        gap.times { str << ' ' }
-        str << after
+      result = String.build do |io|
+        io << before
+        gap.times { io << ' ' }
+        io << after
       end
 
       lines[info.line] = result
@@ -5059,10 +5059,10 @@ module Crystal
       comment_line = line[comment_column..-1]
       gap = max_column - comment_column
 
-      result = String.build do |str|
-        str << source_line
-        gap.times { str << ' ' }
-        str << comment_line
+      result = String.build do |io|
+        io << source_line
+        gap.times { io << ' ' }
+        io << comment_line
       end
       result
     end
@@ -5074,12 +5074,12 @@ module Crystal
         first_line = lines[doc_comment.start_line]
         sharp_index = first_line.index('#').not_nil!
 
-        comment = String.build do |str|
+        comment = String.build do |io|
           (doc_comment.start_line..doc_comment.end_line).each do |i|
             line = lines[i].strip[1..-1]
             line = line[1..-1] if line[0]? == ' '
-            str << line
-            str << '\n'
+            io << line
+            io << '\n'
           end
         end
 
@@ -5087,10 +5087,10 @@ module Crystal
           formatted_comment = Formatter.format(comment)
           formatted_lines = formatted_comment.lines
           formatted_lines.map! do |line|
-            String.build do |str|
-              sharp_index.times { str << ' ' }
-              str << "# "
-              str << line
+            String.build do |io|
+              sharp_index.times { io << ' ' }
+              io << "# "
+              io << line
             end
           end
           lines[doc_comment.start_line..doc_comment.end_line] = formatted_lines
diff --git a/src/crypto/bcrypt/base64.cr b/src/crypto/bcrypt/base64.cr
index 01c59f237d62..2db7f44d85fe 100644
--- a/src/crypto/bcrypt/base64.cr
+++ b/src/crypto/bcrypt/base64.cr
@@ -23,34 +23,34 @@ module Crypto::Bcrypt::Base64
   def self.encode(d, len) : String
     off = 0
 
-    String.build do |str|
+    String.build do |io|
       loop do
         c1 = d[off] & 0xff
         off += 1
-        str << ALPHABET[(c1 >> 2) & 0x3f]
+        io << ALPHABET[(c1 >> 2) & 0x3f]
         c1 = (c1 & 0x03) << 4
 
         if off >= len
-          str << ALPHABET[c1 & 0x3f]
+          io << ALPHABET[c1 & 0x3f]
           break
         end
 
         c2 = d[off] & 0xff
         off += 1
         c1 |= (c2 >> 4) & 0x0f
-        str << ALPHABET[c1 & 0x3f]
+        io << ALPHABET[c1 & 0x3f]
         c1 = (c2 & 0x0f) << 2
 
         if off >= len
-          str << ALPHABET[c1 & 0x3f]
+          io << ALPHABET[c1 & 0x3f]
           break
         end
 
         c2 = d[off] & 0xff
         off += 1
         c1 |= (c2 >> 6) & 0x03
-        str << ALPHABET[c1 & 0x3f]
-        str << ALPHABET[c2 & 0x3f]
+        io << ALPHABET[c1 & 0x3f]
+        io << ALPHABET[c2 & 0x3f]
 
         break if off >= len
       end
diff --git a/src/crystal/elf.cr b/src/crystal/elf.cr
index 365d88fb59b0..a4445b42532f 100644
--- a/src/crystal/elf.cr
+++ b/src/crystal/elf.cr
@@ -105,13 +105,13 @@ module Crystal
         MASKPROC         = 0xf0000000
 
         def short
-          String.build do |str|
-            str << 'W' if write?
-            str << 'A' if alloc?
-            str << 'X' if execinstr?
-            str << 'M' if merge?
-            str << 'S' if strings?
-            str << 'T' if tls?
+          String.build do |io|
+            io << 'W' if write?
+            io << 'A' if alloc?
+            io << 'X' if execinstr?
+            io << 'M' if merge?
+            io << 'S' if strings?
+            io << 'T' if tls?
           end
         end
       end
diff --git a/src/ecr/processor.cr b/src/ecr/processor.cr
index 786879e94273..4bd81cadc9ce 100644
--- a/src/ecr/processor.cr
+++ b/src/ecr/processor.cr
@@ -15,7 +15,7 @@ module ECR
     lexer = Lexer.new string
     token = lexer.next_token
 
-    String.build do |str|
+    String.build do |io|
       while true
         case token.type
         when .string?
@@ -24,10 +24,10 @@ module ECR
 
           string = suppress_leading_indentation(token, string)
 
-          str << buffer_name
-          str << " << "
-          string.inspect(str)
-          str << '\n'
+          io << buffer_name
+          io << " << "
+          string.inspect(io)
+          io << '\n'
         when .output?
           string = token.value
           line_number = token.line_number
@@ -37,12 +37,12 @@ module ECR
 
           suppress_trailing_whitespace(token, suppress_trailing)
 
-          str << "#<loc:push>("
-          append_loc(str, filename, line_number, column_number)
-          str << string
-          str << ")#<loc:pop>.to_s "
-          str << buffer_name
-          str << '\n'
+          io << "#<loc:push>("
+          append_loc(io, filename, line_number, column_number)
+          io << string
+          io << ")#<loc:pop>.to_s "
+          io << buffer_name
+          io << '\n'
         when .control?
           string = token.value
           line_number = token.line_number
@@ -52,12 +52,12 @@ module ECR
 
           suppress_trailing_whitespace(token, suppress_trailing)
 
-          str << "#<loc:push>"
-          append_loc(str, filename, line_number, column_number)
-          str << ' ' unless string.starts_with?(' ')
-          str << string
-          str << "#<loc:pop>"
-          str << '\n'
+          io << "#<loc:push>"
+          append_loc(io, filename, line_number, column_number)
+          io << ' ' unless string.starts_with?(' ')
+          io << string
+          io << "#<loc:pop>"
+          io << '\n'
         when .eof?
           break
         end
diff --git a/src/http/formdata/builder.cr b/src/http/formdata/builder.cr
index 8011b0e98920..6d019da3ba84 100644
--- a/src/http/formdata/builder.cr
+++ b/src/http/formdata/builder.cr
@@ -36,10 +36,10 @@ module HTTP::FormData
     # builder.content_type # => "multipart/form-data; boundary=\"a4VF\""
     # ```
     def content_type : String
-      String.build do |str|
-        str << "multipart/form-data; boundary=\""
-        HTTP.quote_string(@boundary, str)
-        str << '"'
+      String.build do |io|
+        io << "multipart/form-data; boundary=\""
+        HTTP.quote_string(@boundary, io)
+        io << '"'
       end
     end
 
diff --git a/src/ini.cr b/src/ini.cr
index d9cea1c82e82..ce46745e3cbd 100644
--- a/src/ini.cr
+++ b/src/ini.cr
@@ -67,7 +67,7 @@ module INI
   # INI.build({"foo" => {"a" => "1"}}, true) # => "[foo]\na = 1\n\n"
   # ```
   def self.build(ini, space : Bool = false) : String
-    String.build { |str| build str, ini, space }
+    String.build { |io| build io, ini, space }
   end
 
   # Appends INI data to the given IO.
diff --git a/src/io.cr b/src/io.cr
index bce6497db606..b734b107a2a1 100644
--- a/src/io.cr
+++ b/src/io.cr
@@ -552,18 +552,18 @@ abstract class IO
   # io.gets_to_end # => ""
   # ```
   def gets_to_end : String
-    String.build do |str|
+    String.build do |io|
       if decoder = decoder()
         while true
           decoder.read(self)
           break if decoder.out_slice.empty?
 
-          decoder.write(str)
+          decoder.write(io)
         end
       else
         buffer = uninitialized UInt8[DEFAULT_BUFFER_SIZE]
         while (read_bytes = read(buffer.to_slice)) > 0
-          str.write buffer.to_slice[0, read_bytes]
+          io.write buffer.to_slice[0, read_bytes]
         end
       end
     end
diff --git a/src/io/encoding.cr b/src/io/encoding.cr
index 733e6b199168..b8141e8bbde5 100644
--- a/src/io/encoding.cr
+++ b/src/io/encoding.cr
@@ -184,10 +184,10 @@ class IO
 
       # We need to read from the out_slice into a String until we find that byte,
       # or until we consumed limit bytes
-      String.build do |str|
+      String.build do |io1|
         loop do
           limit -= @out_slice.size
-          write str
+          write io1
 
           read(io)
 
@@ -200,16 +200,16 @@ class IO
             else
               index += 1
             end
-            write str, index
+            write io1, index
             break
           else
             if limit < @out_slice.size
-              write(str, limit)
+              write(io1, limit)
               break
             end
           end
         end
-        str.chomp!(delimiter) if chomp
+        io1.chomp!(delimiter) if chomp
       end
     end
 
diff --git a/src/json/builder.cr b/src/json/builder.cr
index 893970e31f1c..cb1daadb0715 100644
--- a/src/json/builder.cr
+++ b/src/json/builder.cr
@@ -401,8 +401,8 @@ module JSON
   # string # => %<{"name":"foo","values":[1,2,3]}>
   # ```
   def self.build(indent = nil, &)
-    String.build do |str|
-      build(str, indent) do |json|
+    String.build do |io|
+      build(io, indent) do |json|
         yield json
       end
     end
diff --git a/src/json/to_json.cr b/src/json/to_json.cr
index 18d4df75ec2f..0c682f184543 100644
--- a/src/json/to_json.cr
+++ b/src/json/to_json.cr
@@ -1,7 +1,7 @@
 class Object
   def to_json : String
-    String.build do |str|
-      to_json str
+    String.build do |io|
+      to_json io
     end
   end
 
@@ -12,8 +12,8 @@ class Object
   end
 
   def to_pretty_json(indent : String = "  ") : String
-    String.build do |str|
-      to_pretty_json str, indent: indent
+    String.build do |io|
+      to_pretty_json io, indent: indent
     end
   end
 
diff --git a/src/kernel.cr b/src/kernel.cr
index 7bca29cef605..c8eeb20de549 100644
--- a/src/kernel.cr
+++ b/src/kernel.cr
@@ -405,8 +405,8 @@ end
 
 # :ditto:
 def sprintf(format_string, args : Array | Tuple) : String
-  String.build(format_string.bytesize) do |str|
-    String::Formatter(typeof(args)).new(format_string, args, str).format
+  String.build(format_string.bytesize) do |io|
+    String::Formatter(typeof(args)).new(format_string, args, io).format
   end
 end
 
diff --git a/src/oauth/signature.cr b/src/oauth/signature.cr
index 2038243f25fa..48f78ec6a32b 100644
--- a/src/oauth/signature.cr
+++ b/src/oauth/signature.cr
@@ -8,11 +8,11 @@ struct OAuth::Signature
   end
 
   def key : String
-    String.build do |str|
-      URI.encode_www_form @client_shared_secret, str, space_to_plus: false
-      str << '&'
+    String.build do |io|
+      URI.encode_www_form @client_shared_secret, io, space_to_plus: false
+      io << '&'
       if token_shared_secret = @token_shared_secret
-        URI.encode_www_form token_shared_secret, str, space_to_plus: false
+        URI.encode_www_form token_shared_secret, io, space_to_plus: false
       end
     end
   end
@@ -42,20 +42,20 @@ struct OAuth::Signature
   private def base_string(request, tls, params)
     host, port = host_and_port(request, tls)
 
-    String.build do |str|
-      str << request.method
-      str << '&'
-      str << (tls ? "https" : "http")
-      str << "%3A%2F%2F"
-      URI.encode_www_form host, str, space_to_plus: false
+    String.build do |io|
+      io << request.method
+      io << '&'
+      io << (tls ? "https" : "http")
+      io << "%3A%2F%2F"
+      URI.encode_www_form host, io, space_to_plus: false
       if port
-        str << "%3A"
-        str << port
+        io << "%3A"
+        io << port
       end
       uri_path = request.path.presence || "/"
-      URI.encode_www_form(uri_path, str, space_to_plus: false)
-      str << '&'
-      str << params
+      URI.encode_www_form(uri_path, io, space_to_plus: false)
+      io << '&'
+      io << params
     end
   end
 
diff --git a/src/openssl/x509/extension.cr b/src/openssl/x509/extension.cr
index b326a35f8309..c668fbae07d3 100644
--- a/src/openssl/x509/extension.cr
+++ b/src/openssl/x509/extension.cr
@@ -12,9 +12,9 @@ module OpenSSL::X509
     end
 
     def initialize(nid : Int32, value : String, critical = false)
-      valstr = String.build do |str|
-        str << "critical," if critical
-        str << value
+      valstr = String.build do |io|
+        io << "critical," if critical
+        io << value
       end
       @ext = LibCrypto.x509v3_ext_nconf_nid(nil, nil, nid, valstr)
       raise Error.new("X509V3_EXT_nconf_nid") if @ext.null?
diff --git a/src/path.cr b/src/path.cr
index 4ec62b39d95d..db721fd5df5f 100644
--- a/src/path.cr
+++ b/src/path.cr
@@ -419,17 +419,17 @@ struct Path
     separators = self.separators
     add_separator_at_end = !remove_final_separator && ends_with_separator?
 
-    new_name = String.build do |str|
+    new_name = String.build do |io|
       if drive
-        str << drive.gsub('/', '\\')
+        io << drive.gsub('/', '\\')
         reader.pos += drive.bytesize
       end
       if root
-        str << separators[0]
+        io << separators[0]
         reader.next_char
-        dotdot = str.bytesize
+        dotdot = io.bytesize
       end
-      anchor_pos = str.bytesize
+      anchor_pos = io.bytesize
 
       while (char = reader.current_char) != Char::ZERO
         curr_pos = reader.pos
@@ -442,43 +442,43 @@ struct Path
         elsif char == '.' && reader.next_char == '.' && (reader.pos + 1 == @name.bytesize || separators.includes?(reader.peek_next_char))
           # .. element: remove to last /
           reader.next_char
-          if str.bytesize > dotdot
-            str.back 1
-            while str.bytesize > dotdot && !separators.includes?((str.buffer + str.bytesize).value.unsafe_chr)
-              str.back 1
+          if io.bytesize > dotdot
+            io.back 1
+            while io.bytesize > dotdot && !separators.includes?((io.buffer + io.bytesize).value.unsafe_chr)
+              io.back 1
             end
           elsif !root
-            if str.bytesize > 0
-              str << separators[0]
+            if io.bytesize > 0
+              io << separators[0]
             end
-            str << ".."
-            dotdot = str.bytesize
+            io << ".."
+            dotdot = io.bytesize
           end
         else
           reader.pos = curr_pos # make sure to reset lookahead used in previous condition
 
           # real path element
           # add slash if needed
-          if str.bytesize > anchor_pos && !separators.includes?((str.buffer + str.bytesize - 1).value.unsafe_chr)
-            str << separators[0]
+          if io.bytesize > anchor_pos && !separators.includes?((io.buffer + io.bytesize - 1).value.unsafe_chr)
+            io << separators[0]
           end
 
           loop do
-            str << char
+            io << char
             char = reader.next_char
             break if separators.includes?(char) || char == Char::ZERO
           end
         end
       end
 
-      if str.empty?
-        str << '.'
+      if io.empty?
+        io << '.'
       end
 
-      last_char = (str.buffer + str.bytesize - 1).value.unsafe_chr
+      last_char = (io.buffer + io.bytesize - 1).value.unsafe_chr
 
       if add_separator_at_end && !separators.includes?(last_char)
-        str << separators[0]
+        io << separators[0]
       end
     end
 
diff --git a/src/process/shell.cr b/src/process/shell.cr
index e206c6c19a5a..bfde3a107687 100644
--- a/src/process/shell.cr
+++ b/src/process/shell.cr
@@ -140,7 +140,7 @@ class Process
       end
       break unless reader.has_next?
 
-      token = String.build do |str|
+      token = String.build do |io|
         while reader.has_next? && !reader.current_char.ascii_whitespace?
           quote = nil
           if reader.current_char.in?('\'', '"')
@@ -152,7 +152,7 @@ class Process
             break unless reader.has_next?
             reader.next_char
             if char == '\\' && quote != '\''
-              str << char if quote == '"'
+              io << char if quote == '"'
               char = reader.current_char
               if reader.has_next?
                 reader.next_char
@@ -161,7 +161,7 @@ class Process
                 char = '\\'
               end
             end
-            str << char
+            io << char
           end
 
           if quote
@@ -203,7 +203,7 @@ class Process
       end
       break unless reader.has_next?
 
-      token = String.build do |str|
+      token = String.build do |io|
         while true
           backslash_count = 0
           while reader.current_char == '\\'
@@ -212,19 +212,19 @@ class Process
           end
 
           if reader.current_char == '"'
-            (backslash_count // 2).times { str << '\\' }
+            (backslash_count // 2).times { io << '\\' }
             if backslash_count.odd?
-              str << '"'
+              io << '"'
             else
               quote = !quote
             end
           else
-            backslash_count.times { str << '\\' }
+            backslash_count.times { io << '\\' }
             break unless reader.has_next?
             # `current_char` is neither '\\' nor '"'
             char = reader.current_char
             break if char.in?(' ', '\t') && !quote
-            str << char
+            io << char
           end
 
           reader.next_char
diff --git a/src/regex.cr b/src/regex.cr
index c0f0ad56b4da..52b62239559e 100644
--- a/src/regex.cr
+++ b/src/regex.cr
@@ -399,15 +399,15 @@ class Regex
   # /#{string}/                    # => /\*\?\{\}\./
   # ```
   def self.escape(str) : String
-    String.build do |result|
+    String.build do |io|
       str.each_byte do |byte|
         {% begin %}
           case byte.unsafe_chr
           when {{*SPECIAL_CHARACTERS}}
-            result << '\\'
-            result.write_byte byte
+            io << '\\'
+            io.write_byte byte
           else
-            result.write_byte byte
+            io.write_byte byte
           end
         {% end %}
       end
diff --git a/src/string.cr b/src/string.cr
index 7c9eed3dd186..ef0306b2ee69 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -180,8 +180,8 @@ class String
   # String.new(slice, "GB2312") # => "好"
   # ```
   def self.new(bytes : Bytes, encoding : String, invalid : Symbol? = nil) : String
-    String.build do |str|
-      String.encode(bytes, encoding, "UTF-8", str, invalid)
+    String.build do |io|
+      String.encode(bytes, encoding, "UTF-8", io, invalid)
     end
   end
 
@@ -285,9 +285,9 @@ class String
   # resizes as needed.
   #
   # ```
-  # str = String.build do |str|
-  #   str << "hello "
-  #   str << 1
+  # str = String.build do |io|
+  #   io << "hello "
+  #   io << 1
   # end
   # str # => "hello 1"
   # ```
@@ -4598,9 +4598,9 @@ class String
       chars.insert(last_alnum, carry)
     end
 
-    String.build(chars.size) do |str|
+    String.build(chars.size) do |io|
       chars.each do |char|
-        str << char
+        io << char
       end
     end
   end
diff --git a/src/time/format.cr b/src/time/format.cr
index 6f09a1c90229..3e2c23f0d90e 100644
--- a/src/time/format.cr
+++ b/src/time/format.cr
@@ -88,8 +88,8 @@ struct Time::Format
 
   # Turns a `Time` into a `String`.
   def format(time : Time) : String
-    String.build do |str|
-      format time, str
+    String.build do |io|
+      format time, io
     end
   end
 
diff --git a/src/uri.cr b/src/uri.cr
index dfdf26f9f419..033226459dfb 100644
--- a/src/uri.cr
+++ b/src/uri.cr
@@ -230,15 +230,15 @@ class URI
       string_size += query.bytesize + 1
     end
 
-    String.build(string_size) do |str|
+    String.build(string_size) do |io|
       if @path.empty?
-        str << "/" unless opaque?
+        io << "/" unless opaque?
       else
-        str << @path
+        io << @path
       end
 
       if (query = @query) && !query.empty?
-        str << '?' << query
+        io << '?' << query
       end
     end
   end
diff --git a/src/uuid.cr b/src/uuid.cr
index 7c8d44478f84..e908abf9895e 100644
--- a/src/uuid.cr
+++ b/src/uuid.cr
@@ -284,9 +284,9 @@ struct UUID
   # uuid2.urn # => "urn:uuid:c49fc136-9362-4414-81a5-9a7e0fcca0f1"
   # ```
   def urn : String
-    String.build(45) do |str|
-      str << "urn:uuid:"
-      to_s(str)
+    String.build(45) do |io|
+      io << "urn:uuid:"
+      to_s(io)
     end
   end
 
diff --git a/src/xml/builder.cr b/src/xml/builder.cr
index 1ebe23efcd47..3f5933644eff 100644
--- a/src/xml/builder.cr
+++ b/src/xml/builder.cr
@@ -323,8 +323,8 @@ module XML
   # string # => "<?xml version=\"1.0\"?>\n<person id=\"1\">\n  <firstname>Jane</firstname>\n  <lastname>Doe</lastname>\n</person>\n"
   # ```
   def self.build(version : String? = nil, encoding : String? = nil, indent = nil, quote_char = nil, &)
-    String.build do |str|
-      build(str, version, encoding, indent, quote_char) do |xml|
+    String.build do |io|
+      build(io, version, encoding, indent, quote_char) do |xml|
         yield xml
       end
     end
@@ -347,8 +347,8 @@ module XML
   # string # => "<person id=\"1\">\n  <firstname>Jane</firstname>\n  <lastname>Doe</lastname>\n</person>\n"
   # ```
   def self.build_fragment(*, indent = nil, quote_char = nil, &)
-    String.build do |str|
-      build_fragment(str, indent: indent, quote_char: quote_char) do |xml|
+    String.build do |io|
+      build_fragment(io, indent: indent, quote_char: quote_char) do |xml|
         yield xml
       end
     end
diff --git a/src/xml/node.cr b/src/xml/node.cr
index 45878e762b0f..24b614165867 100644
--- a/src/xml/node.cr
+++ b/src/xml/node.cr
@@ -426,8 +426,8 @@ class XML::Node
   #
   # See `XML::SaveOptions.xml_default` for default options.
   def to_xml(indent : Int = 2, indent_text = " ", options : SaveOptions = SaveOptions.xml_default) : String
-    String.build do |str|
-      to_xml str, indent, indent_text, options
+    String.build do |io|
+      to_xml io, indent, indent_text, options
     end
   end
 
diff --git a/src/yaml/builder.cr b/src/yaml/builder.cr
index 2764580dde13..12b9878ba660 100644
--- a/src/yaml/builder.cr
+++ b/src/yaml/builder.cr
@@ -240,8 +240,8 @@ module YAML
   # string # => "---\nfoo:\n- 1\n- 2\n"
   # ```
   def self.build(&)
-    String.build do |str|
-      build(str) do |yaml|
+    String.build do |io|
+      build(io) do |yaml|
         yield yaml
       end
     end