internal/diff: abolish errors

Computing the difference between two strings is logically an
infallible operation. This change makes the code reflect that. The
actual failures were unreachable given consistent inputs, but that was
hard to see from the complexity of the logic surrounding span.Span.
(The problem only occurs when converting offsets beyond the end of the
file to Spans, but the code preserves the integrity of offsets.)

gopls' "old" hooks.ComputeEdits impl (based on sergi/go-diff) now
reports a bug and returns a single diff for the entire file if it

Also, first steps towards simpler API and a
reusable diff package in x/tools:

- add TODO for new API. In particular, the diff package shouldn't care
  about filenames, spans, and URIs. These are gopls concerns.
- diff.String is the main diff function.
- diff.Unified prints edits in unified form;
  all its internals are now hidden.
- the ComputeEdits func type is moved to gopls (source.DiffFunction)
- remove all non-critical uses of myers.ComputeEdits. The only
  remaining one is in gopls' defaults, but perhaps that gets
  overridden by the default GoDiff setting in hooks, to BothDiffs
  (sergi + pjw impls), so maybe it's now actually unused in practice?

Change-Id: I6ceb5c670897abbf285b243530a7372dfa41edf6
Run-TryBot: Alan Donovan <>
Reviewed-by: Robert Findley <>
TryBot-Result: Gopher Robot <>
gopls-CI: kokoro <>
25 files changed
tree: 8b3f67e531966d25fece1280e96958d88a97e028
  1. benchmark/
  2. blog/
  3. cmd/
  4. container/
  5. copyright/
  6. cover/
  7. go/
  8. godoc/
  9. gopls/
  10. imports/
  11. internal/
  12. playground/
  13. present/
  14. refactor/
  15. txtar/
  16. .gitattributes
  17. .gitignore
  18. .prettierrc
  19. codereview.cfg
  21. go.mod
  22. go.sum

Go Tools


This repository provides the module, comprising various tools and packages mostly for static analysis of Go programs, some of which are listed below. Use the “Go reference” link above for more information about any package.

It also contains the module, whose root package is a language-server protocol (LSP) server for Go. An LSP server analyses the source code of a project and responds to requests from a wide range of editors such as VSCode and Vim, allowing them to support IDE-like functionality.

Selected commands:

  • cmd/goimports formats a Go program like go fmt and additionally inserts import statements for any packages required by the file after it is edited.
  • cmd/callgraph prints the call graph of a Go program.
  • cmd/digraph is a utility for manipulating directed graphs in textual notation.
  • cmd/stringer generates declarations (including a String method) for “enum” types.
  • cmd/toolstash is a utility to simplify working with multiple versions of the Go toolchain.

These commands may be fetched with a command such as go install

Selected packages:

  • go/ssa provides a static single-assignment form (SSA) intermediate representation (IR) for Go programs, similar to a typical compiler, for use by analysis tools.

  • go/packages provides a simple interface for loading, parsing, and type checking a complete Go program from source code.

  • go/analysis provides a framework for modular static analysis of Go programs.

  • go/callgraph provides call graphs of Go programs using a variety of algorithms with different trade-offs.

  • go/ast/inspector provides an optimized means of traversing a Go parse tree for use in analysis tools.

  • go/cfg provides a simple control-flow graph (CFG) for a Go function.

  • go/expect reads Go source files used as test inputs and interprets special comments within them as queries or assertions for testing.

  • go/gcexportdata and go/gccgoexportdata read and write the binary files containing type information used by the standard and gccgo compilers.

  • go/types/objectpath provides a stable naming scheme for named entities (“objects”) in the go/types API.

Numerous other packages provide more esoteric functionality.


This repository uses Gerrit for code changes. To learn how to submit changes, see

The main issue tracker for the tools repository is located at Prefix your issue with “x/tools/(your subdir):” in the subject line, so it is easy to find.

JavaScript and CSS Formatting

This repository uses prettier to format JS and CSS files.

The version of prettier used is 1.18.2.

It is encouraged that all JS and CSS code be run through this before submitting a change. However, it is not a strict requirement enforced by CI.