x/tools/gopls: panic in deslice when completing unknown type in append

[chlunde]

What did you do?

Type t in struct in an append variadic arg for a slice with a type now yet defined.

package bad

func _() {
       a := unknown
       a = append(a, struct) //@complete(")")
}

What did you expect to see?

Not much

What did you see instead?

Crash

Stack trace

panic: runtime error: invalid memory address or nil pointer dereference                                                                                                                                                                        
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x79e7db]                                                                                                                                                                        
goroutine 10025 [running]:

golang.org/x/tools/internal/lsp/source.deslice(...)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/source/util.go:429

golang.org/x/tools/internal/lsp/source.(*completer).builtinArgType(0xc0086f0600, 0xe25040, 0xc000100cd0, 0xc0071bc240, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/source/completion_builtin.go:70 +0x3bb

golang.org/x/tools/internal/lsp/source.expectedCandidate.func1(0xc0086f0600, 0xe25040, 0xc000100cd0, 0xc0071bc240, 0xc01032c160)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/source/completion.go:1481 +0xd1

golang.org/x/tools/internal/lsp/source.expectedCandidate(0xc0086f0600, 0xc00025b4b0, 0xc010ae66c0, 0xc0000c5300, 0x116f, 0x1300, 0xb9e2e2, 0x52, 0xc0101eb990, 0x6, ...)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/source/completion.go:1565 +0x3c7

golang.org/x/tools/internal/lsp/source.Completion(0xe12e60, 0xc010ae66c0, 0xe24f00, 0xc012cc2120, 0xe11560, 0xc012cc20c0, 0x4065a00000000000, 0x4045000000000000, 0x0, 0x0, ...)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/source/completion.go:517 +0x9eb

golang.org/x/tools/internal/lsp.(*Server).completion(0xc0002e6680, 0xe12e60, 0xc010b00450, 0xc010536060, 0x0, 0x0, 0xc01032c000)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/completion.go:27 +0x6b0

golang.org/x/tools/internal/lsp.(*Server).Completion(0xc0002e6680, 0xe12e60, 0xc010b00450, 0xc010536060, 0xc010536060, 0x0, 0x0)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/server_gen.go:24 +0x49

golang.org/x/tools/internal/lsp/protocol.serverHandler.Deliver(0xe2d340, 0xc0002e6680, 0xe12e60, 0xc010b00450, 0xc010a7e1c0, 0xae8c00, 0xe10000)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/lsp/protocol/tsserver.go:345 +0x2064

golang.org/x/tools/internal/jsonrpc2.(*Conn).Run.func1(0xc0075ec4e0, 0xc010a7e1c0, 0xc0002eaba0, 0xe12e60, 0xc010b00450, 0x0, 0x0, 0xc0071500f0)

/home/chlunde/opt/gotoolpath/pkg/mod/golang.org/x/[email protected]/internal/jsonrpc2/jsonrpc2.go:372 +0x160

created by golang.org/x/tools/internal/jsonrpc2.(*Conn).Run

Build info

gopls v0.3.4

golang.org/x/tools/gopls v0.3.4
    golang.org/x/tools/[email protected] h1:4GC7q/pXQ/tsxHBGVdsMdlB4gCxVC06m/7rIXg1Px4E=
    github.com/BurntSushi/[email protected] h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
    github.com/sergi/[email protected] h1:Kpca3qRNrduNnOQeazBd0ysaKrUJiIuISHxogkT9RPQ=
    golang.org/x/[email protected] h1:WG0RUwxtNT4qqaXX3DPA8zHFNm/D9xaBpxzHt1WcA/E=
    golang.org/x/[email protected] h1:8gQV6CLnAEikrhgkHFbMAEhagSSnXWGV915qUMm9mrU=
    golang.org/x/[email protected] h1:hKrQy/q8/Xivoqgw6nGiz1jqpn1WGBLDcWLZwW0983E=
    golang.org/x/[email protected] h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
    honnef.co/go/[email protected] h1:sXmLre5bzIR6ypkjXCDI3jHPssRhc8KD/Ome589sc3U=
    mvdan.cc/xurls/[email protected] h1:KaMb5GLhlcSX+e+qhbRJODnUUBvlw01jt4yrjFIHAuA=

Hack/patch

Not sure if this is the right approach ¯\(ツ)/¯

diff --git a/internal/lsp/testdata/lsp/primarymod/bad/bad2.go b/internal/lsp/testdata/lsp/primarymod/bad/bad2.go
new file mode 100644
index 00000000..acce3e45
--- /dev/null
+++ b/internal/lsp/testdata/lsp/primarymod/bad/bad2.go
@@ -0,0 +1,6 @@
+package bad
+
+func _() {
+       a := unknown
+       a = append(a, struct) //@complete(")")
+}
diff --git a/internal/lsp/testdata/lsp/summary.txt.golden b/internal/lsp/testdata/lsp/summary.txt.golden
index c4e7d68d..ece318e5 100644
--- a/internal/lsp/testdata/lsp/summary.txt.golden
+++ b/internal/lsp/testdata/lsp/summary.txt.golden
@@ -1,6 +1,6 @@
 -- summary --
 CodeLensCount = 2
-CompletionsCount = 237
+CompletionsCount = 238
 CompletionSnippetCount = 75
 UnimportedCompletionsCount = 11
 DeepCompletionsCount = 5
diff --git a/internal/lsp/source/completion_builtin.go b/internal/lsp/source/completion_builtin.go
index d65eb8f4..26c553f0 100644
--- a/internal/lsp/source/completion_builtin.go
+++ b/internal/lsp/source/completion_builtin.go
@@ -64,7 +64,7 @@ func (c *completer) builtinArgType(obj types.Object, call *ast.CallExpr, parentI
                // Check if we are completing the variadic append() param.
                if exprIdx == 1 && len(call.Args) <= 2 {
                        inf.variadicType = deslice(inf.objType)
-               } else if exprIdx > 0 {
+               } else if exprIdx > 0 && inf.objType != nil {
                        // If we are completing an individual element of the variadic
                        // param, "deslice" the expected type.
                        inf.objType = deslice(inf.objType)

[stamblerre]

Thanks for the report! The patch you proposed above sounds reasonable to me - please send us a PR if you're interested in contributing :)

/cc @muirdm

[gopherbot]

Change https://golang.org/cl/227026 mentions this issue: internal/lsp: fix panic in builtin completions