gopls/internal/analysis/undeclaredname: merge into CodeAction

This change removes the undeclaredname analyzer,
and instead computes it directly from the
protocol.QuickFix code action producer,
follow the same structure in CL 617035.

Also:
- include return types for undeclared function
- add basic return type test in missingfunction.txt,
the major functionality should be the same as
fromcall_returns.txt
- add more cases in TypesFromContext, and
corresponding tests.

Fixes golang/go#47558

Change-Id: Ic79cf2f07a3baee60631bf0b8f5c2b9f4f4393d0
GitHub-Last-Rev: 33b78d7
GitHub-Pull-Request: #538
Reviewed-on: https://go-review.googlesource.com/c/tools/+/623156
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: xzbdmw

gopls/doc/analyzers.md

@@ -842,26 +842,6 @@ Default: on.
 
 Package documentation: [timeformat](https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/timeformat)
 
-<a id='undeclaredname'></a>
-## `undeclaredname`: suggested fixes for "undeclared name: <>"
-
-
-This checker provides suggested fixes for type errors of the
-type "undeclared name: <>". It will either insert a new statement,
-such as:
-
-	<> :=
-
-or a new function declaration, such as:
-
-	func <>(inferred parameters) {
-		panic("implement me!")
-	}
-
-Default: on.
-
-Package documentation: [undeclaredname](https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/undeclaredname)
-
 <a id='unmarshal'></a>
 ## `unmarshal`: report passing non-pointer or non-interface values to unmarshal
 

gopls/doc/features/diagnostics.md

@@ -197,6 +197,56 @@ func (f Foo) bar(s string, i int) string {
 }
 ```
 
+### `CreateUndeclared`: Create missing declaration for "undeclared name: X"
+
+A Go compiler error "undeclared name: X" indicates that a variable or function is being used before
+it has been declared in the current scope. In this scenario, gopls offers a quick fix to create the declaration.
+
+#### Declare a new variable
+
+When you reference a variable that hasn't been declared:
+
+```go
+func main() {
+  x := 42
+  min(x, y) // error: undefined: y
+}
+```
+
+The quick fix would be:
+
+```go
+func main() {
+  x := 42
+  y :=
+  min(x, y)
+}
+```
+
+#### Declare a new function
+
+Similarly, if you call a function that hasn't been declared:
+
+```go
+func main() {
+  var s string
+  s = doSomething(42) // error: undefined: doSomething
+}
+```
+
+Gopls will insert a new function declaration below,
+inferring its type from the call:
+
+```go
+func main() {
+  var s string
+  s = doSomething(42)
+}
+
+func doSomething(i int) string {
+  panic("unimplemented")
+}
+```
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