[thunderkatz]

Summary

Output of go fix ./... on Go 1.27.3 (plus a stray blank line removed from WriteHeader):

• ioutil.ReadAll -> io.ReadAll
• for i := 0; i < n; i++ -> for i := range n
• manual clamp -> min()
• reverse index loops -> slices.Backward
• string += in a loop -> strings.Builder (mail/header.go)

Manual changes:

• the remaining errors.As calls -> errors.AsType (by hand; go fix skips the new(T)/&T{} target forms)
• bytes.IndexByte + slicing -> bytes.Cut (AddRaw, trimAroundNewlines, parseHeaderFieldLine), HasSuffix+TrimSuffix -> bytes.CutSuffix, sort.SliceStable -> slices.SortStableFunc (all skipped by go fix)
• append(s[:i], s[i+1:]...) -> slices.Delete (also zeroes the vacated slot), b.N loops -> b.Loop(), last ioutil.Discard -> io.Discard
• charset/charset.go: %v -> %w, so errors.Is/As see the underlying error. This is the one behavior-visible change.

multipart.go's NextPart: %v is intentionally left as %v: wrapping would make a truncated body satisfy errors.Is(err, io.EOF).

Test plan

• go test ./...
• go fix -diff ./... is empty afterwards

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

The invalid-charset tests do not verify error unwrapping, leaving the intentional %w behavior unprotected against regression to %v.

1 open issue

• Add a regression test for wrapped charset lookup errors · New

What changed in this PR

Modernizes the message, mail, and textproto packages with current Go idioms and standard-library APIs. Charset lookup errors now expose their underlying cause; multipart EOF handling remains unchanged.

Changes:

• Replace legacy I/O helpers, loops, sorting, and slice deletion patterns.
• Use typed error matching and strings.Builder for comment parsing.
• Wrap charset lookup errors and modernize benchmark loops.

File	Description
textproto/​multipart.go	Simplify read limits, replace discard helper, and document EOF handling.
textproto/​multipart_test.go	Modernize I/O helpers and loop syntax.
textproto/​header.go	Modernize sorting, byte splitting, reverse iteration, and deletion.
textproto/​bench_test.go	Switch benchmarks to b.Loop().
mail/​reader_test.go	Replace ioutil.ReadAll with io.ReadAll.
mail/​header.go	Build parsed comments with strings.Builder.
entity_test.go	Modernize I/O helpers and typed error assertions.
encoding.go	Update error matching, loops, and suffix splitting.
encoding_test.go	Replace the legacy read helper.
charset/​charset.go	Preserve underlying charset lookup errors with %w.
charset/​charset_test.go	Replace the legacy read helper.
charset.go	Use typed matching for unknown-charset errors.

_{Review effort: Balanced}

[dlynch-sublime]

lgtm