Go 1.5 (2015-08-19)

Go 1.5 was released in August 2015, a milestone release. In this version, the Go compiler and runtime were completely rewritten in Go (achieving bootstrapping), and a brand new concurrent garbage collector was introduced.

Major Changes

Language

• Map Literal Simplification: When the key or value type of a Map is known, the type name can be omitted in the literal.

  // Before Go 1.5
  m := map[Point]string{
      Point{29.935523, 52.891566}:   "Persepolis",
  }
  // Go 1.5 and later
  m := map[Point]string{
      {29.935523, 52.891566}:   "Persepolis",
  }

Toolchain

• Vendor Support: The go command introduced experimental support for the vendor directory. Enabled by setting the environment variable GO15VENDOREXPERIMENT=1. When enabled, the go command prioritizes looking for dependencies in the vendor directory.
• Internal Packages: Packages in a directory named internal can only be imported by code in the directory tree rooted at the parent of internal. This allows library authors to hide implementation details within a project.
• Trace Tool: Added the go tool trace command for fine-grained program execution tracing and visualization.
• Go Doc: Added the go doc command (distinct from godoc), designed for command-line use, providing a cleaner documentation viewing experience.
• Build Modes: go build added the -buildmode option, supporting building shared libraries and plugins, among other modes.

Runtime

• Self-hosting: Removed all C code; the compiler, linker, and runtime are now written entirely in Go and a small amount of assembly. Building Go 1.5 requires Go 1.4 (or another Go version) installed as a bootstrap compiler.
• Concurrent Garbage Collection: The new garbage collector aims to reduce latency. Through concurrent execution and better scheduling, STW (Stop-The-World) pause times are typically under 10 milliseconds, often much lower.
• GOMAXPROCS: The default value changed from 1 to the number of available CPU cores. This may change the behavior of some concurrent programs but generally improves performance on multi-core machines.

Standard Library

• flag: Improved the output format of PrintDefaults to make it more readable.
• math/big: Added Float type, supporting arbitrary-precision floating-point arithmetic.
• net: On Unix systems, the DNS resolver no longer relies on cgo and defaults to a pure Go implementation (if configuration allows), reducing thread consumption.

References

For more details, please refer to the official release notes: Go 1.5 Release Notes