internal/lsp/completion: fix invalid struct literal field snippet

In cases like:

    type foo struct { a int; b float64 }
    foo{b<>}

We were completing to "foo{int(b: <float64>)}" (the problem being the
nonsensical int() conversion).

The expected type at "<>" is int to allow completions to match "a".
When we pass the *types.Var representing "b" through the candidate
matching machinery, we say "Oh, a float64! I can convert that to my
expected type of int!".

Fix by bailing out of candidate matching early if the candidate is a
composite literal struct field name. Field names aren't really objects
you can do anything to.

Fixes golang/go#43789.

Change-Id: Ie4dab166973dfcdcb519f864532ead1f792d25a3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/289130
Run-TryBot: Muir Manders <muir@mnd.rs>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>

internal/lsp/source/completion/completion.go

@@ -2444,6 +2444,11 @@ func (c *completer) matchingCandidate(cand *candidate) bool {
 		return false
 	}
 
+	// Bail out early if we are completing a field name in a composite literal.
+	if v, ok := cand.obj.(*types.Var); ok && v.IsField() && c.wantStructFieldCompletions() {
+		return true
+	}
+
 	if isTypeName(cand.obj) {
 		return c.matchingTypeName(cand)
 	} else if c.wantTypeName() {

internal/lsp/testdata/snippets/snippets.go.in

@@ -51,3 +51,11 @@ func _() {
 
 	f.Baz()     //@snippet("B", snipMethodBazBaz, "BazBaz(${1:})", "BazBaz(${1:at AliasType})")
 }
+
+func _() {
+	type bar struct {
+		a int
+		b float64 //@item(snipBarB, "b", "float64", "field")
+	}
+	bar{b} //@snippet("}", snipBarB, "b: ${1:}", "b: ${1:float64}")
+}

internal/lsp/testdata/summary.txt.golden

@@ -2,7 +2,7 @@
 CallHierarchyCount = 2
 CodeLensCount = 5
 CompletionsCount = 258
-CompletionSnippetCount = 89
+CompletionSnippetCount = 90
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8