x/tools/gopls: recovered panic in RenderPackageDoc

[adonovan]

Steps:

1. Clone repo and visit this file: https://codeberg.org/pfad.fr/vanitydoc/src/branch/main/example/nested/chan.go
2. Source Actions > View Package Documentation...

Observe panic (recovered by net/http) below.

The offending line is:

buf.WriteString(escape(string(file.Src[start:end])))

panic serving 127.0.0.1:58457: runtime error: slice bounds out of range [:-1]
goroutine 4715 [running]:
net/http.(*conn).serve.func1()
	/Users/adonovan/w/goroot/src/net/http/server.go:1898 +0xb0
panic({0x105945a00?, 0x14007088978?})
	/Users/adonovan/w/goroot/src/runtime/panic.go:759 +0x124
golang.org/x/tools/gopls/internal/golang.RenderPackageDoc.func4.2(0x0)
	/Users/adonovan/w/xtools/gopls/internal/golang/pkgdoc.go:187 +0xfc
golang.org/x/tools/gopls/internal/golang.RenderPackageDoc.func4.3({0x1059aa120?, 0x106001dc0})
	/Users/adonovan/w/xtools/gopls/internal/golang/pkgdoc.go:193 +0x128
go/ast.inspector.Visit(0x1400191b350, {0x1059aa120?, 0x106001dc0?})
	/Users/adonovan/w/goroot/src/go/ast/walk.go:386 +0x38
go/ast.Walk({0x1059a6dc0?, 0x1400191b350?}, {0x1059aa120, 0x106001dc0})
	/Users/adonovan/w/goroot/src/go/ast/walk.go:51 +0x44
go/ast.walkIdentList(...)
	/Users/adonovan/w/goroot/src/go/ast/walk.go:20
go/ast.Walk({0x1059a6dc0?, 0x1400191b350?}, {0x1059aa988, 0x1400210cb90})
	/Users/adonovan/w/goroot/src/go/ast/walk.go:314 +0x661
go/ast.Walk({0x1059a6dc0?, 0x1400191b350?}, {0x1059aa618, 0x140018d5b40})
	/Users/adonovan/w/goroot/src/go/ast/walk.go:344 +0x20e8
go/ast.Inspect(...)
	/Users/adonovan/w/goroot/src/go/ast/walk.go:397
golang.org/x/tools/gopls/internal/golang.RenderPackageDoc.func4({0x1059aa618, 0x140018d5b40})
	/Users/adonovan/w/xtools/gopls/internal/golang/pkgdoc.go:190 +0x1e5
golang.org/x/tools/gopls/internal/golang.RenderPackageDoc.func6({0x140023b2780, 0x16, 0x105530eb5?})
	/Users/adonovan/w/xtools/gopls/internal/golang/pkgdoc.go:300 +0x5c
golang.org/x/tools/gopls/internal/golang.RenderPackageDoc(0x14006893c80, 0x14002949860, 0x140080432f0)
	/Users/adonovan/w/xtools/gopls/internal/golang/pkgdoc.go:316 +0x958

@oliverpool

[adonovan]

This appears to be a consequence of doc.New mutating the AST, even though it is not permitted to do so in PreserveAST mode. The NamePos of the ok Ident becomes zero. Still investigating...

[gopherbot]

Change https://go.dev/cl/573575 mentions this issue: gopls/internal/golang: work around bug in go/doc