gopls/internal: CodeAction: quickfix to generate missing method

gopls/doc/features/diagnostics.md

@@ -127,7 +127,7 @@ Client support:
 <!-- Below we list any quick fixes (by their internal fix name)
      that aren't analyzers. -->
 
-### `stubMethods`: Declare missing methods of type
+### `stubMissingInterfaceMethods`: Declare missing methods of I
 
 When a value of a concrete type is assigned to a variable of an
 interface type, but the concrete type does not possess all the
@@ -169,6 +169,34 @@ position.
 client there, or a progress notification indicating that something
 happened.)
 
+### `StubMissingCalledFunction`: Declare missing method T.f
+
+When you attempt to call a method on a type that does not have that method,
+the compiler will report an error such as "type X has no field or method Y".
+In this scenario, gopls now offers a quick fix to generate a stub declaration of
+the missing method, inferring its type from the call.
+
+Consider the following code where `Foo` does not have a method `bar`:
+
+```go
+type Foo struct{}
+
+func main() {
+  var s string
+  f := Foo{}
+  s = f.bar("str", 42) // error: f.bar undefined (type Foo has no field or method bar)
+}
+```
+
+Gopls will offer a quick fix, "Declare missing method Foo.bar".
+When invoked, it creates the following declaration:
+
+```go
+func (f Foo) bar(s string, i int) string {
+	panic("unimplemented")
+}
+```
+
 <!--
 
 dorky details and deletia:

gopls/doc/release/v0.17.0.md

@@ -63,3 +63,12 @@ function's Go `func` declaration. If the function is implemented in C
 or assembly, the function has no body. Executing a second Definition
 query (while already at the Go declaration) will navigate you to the
 assembly implementation.
+
+## Generate missing method from function call
+
+When you attempt to call a method on a type that does not have that method,
+the compiler will report an error like “type X has no field or method Y”.
+Gopls now offers a new code action, “Declare missing method of T.f”,
+where T is the concrete type and f is the undefined method.
+The stub method's signature is inferred
+from the context of the call.

gopls/internal/golang/codeaction.go

@@ -301,20 +301,32 @@ func quickFix(ctx context.Context, req *codeActionsRequest) error {
 			continue
 		}
 
+		msg := typeError.Error()
+		switch {
 		// "Missing method" error? (stubmethods)
 		// Offer a "Declare missing methods of INTERFACE" code action.
-		// See [stubMethodsFixer] for command implementation.
-		msg := typeError.Error()
-		if strings.Contains(msg, "missing method") ||
-			strings.HasPrefix(msg, "cannot convert") ||
-			strings.Contains(msg, "not implement") {
+		// See [stubMissingInterfaceMethodsFixer] for command implementation.
+		case strings.Contains(msg, "missing method"),
+			strings.HasPrefix(msg, "cannot convert"),
+			strings.Contains(msg, "not implement"):
 			path, _ := astutil.PathEnclosingInterval(req.pgf.File, start, end)
-			si := stubmethods.GetStubInfo(req.pkg.FileSet(), info, path, start)
+			si := stubmethods.GetIfaceStubInfo(req.pkg.FileSet(), info, path, start)
 			if si != nil {
 				qf := typesutil.FileQualifier(req.pgf.File, si.Concrete.Obj().Pkg(), info)
 				iface := types.TypeString(si.Interface.Type(), qf)
 				msg := fmt.Sprintf("Declare missing methods of %s", iface)
-				req.addApplyFixAction(msg, fixStubMethods, req.loc)
+				req.addApplyFixAction(msg, fixMissingInterfaceMethods, req.loc)
+			}
+
+		// "type X has no field or method Y" compiler error.
+		// Offer a "Declare missing method T.f" code action.
+		// See [stubMissingCalledFunctionFixer] for command implementation.
+		case strings.Contains(msg, "has no field or method"):
+			path, _ := astutil.PathEnclosingInterval(req.pgf.File, start, end)
+			si := stubmethods.GetCallStubInfo(req.pkg.FileSet(), info, path, start)
+			if si != nil {
+				msg := fmt.Sprintf("Declare missing method %s.%s", si.Receiver.Obj().Name(), si.MethodName)
+				req.addApplyFixAction(msg, fixMissingCalledFunction, req.loc)
 			}
 		}
 	}

gopls/internal/golang/completion/format.go

@@ -17,6 +17,7 @@ import (
 	"golang.org/x/tools/gopls/internal/golang/completion/snippet"
 	"golang.org/x/tools/gopls/internal/protocol"
 	"golang.org/x/tools/gopls/internal/util/safetoken"
+	"golang.org/x/tools/gopls/internal/util/typesutil"
 	"golang.org/x/tools/internal/event"
 	"golang.org/x/tools/internal/imports"
 	"golang.org/x/tools/internal/typesinternal"
@@ -62,7 +63,7 @@ func (c *completer) item(ctx context.Context, cand candidate) (CompletionItem, e
 	if isTypeName(obj) && c.wantTypeParams() {
 		// obj is a *types.TypeName, so its type must be Alias|Named.
 		tparams := typesinternal.TypeParams(obj.Type().(typesinternal.NamedOrAlias))
-		label += golang.FormatTypeParams(tparams)
+		label += typesutil.FormatTypeParams(tparams)
 		insert = label // maintain invariant above (label == insert)
 	}
 

gopls/internal/golang/fix.go

@@ -58,14 +58,15 @@ func singleFile(fixer1 singleFileFixer) fixer {
 
 // Names of ApplyFix.Fix created directly by the CodeAction handler.
 const (
-	fixExtractVariable   = "extract_variable"
-	fixExtractFunction   = "extract_function"
-	fixExtractMethod     = "extract_method"
-	fixInlineCall        = "inline_call"
-	fixInvertIfCondition = "invert_if_condition"
-	fixSplitLines        = "split_lines"
-	fixJoinLines         = "join_lines"
-	fixStubMethods       = "stub_methods"
+	fixExtractVariable         = "extract_variable"
+	fixExtractFunction         = "extract_function"
+	fixExtractMethod           = "extract_method"
+	fixInlineCall              = "inline_call"
+	fixInvertIfCondition       = "invert_if_condition"
+	fixSplitLines              = "split_lines"
+	fixJoinLines               = "join_lines"
+	fixMissingInterfaceMethods = "stub_missing_interface_method"
+	fixMissingCalledFunction   = "stub_missing_called_function"
 )
 
 // ApplyFix applies the specified kind of suggested fix to the given
@@ -102,14 +103,15 @@ func ApplyFix(ctx context.Context, fix string, snapshot *cache.Snapshot, fh file
 
 		// Ad-hoc fixers: these are used when the command is
 		// constructed directly by logic in server/code_action.
-		fixExtractFunction:   singleFile(extractFunction),
-		fixExtractMethod:     singleFile(extractMethod),
-		fixExtractVariable:   singleFile(extractVariable),
-		fixInlineCall:        inlineCall,
-		fixInvertIfCondition: singleFile(invertIfCondition),
-		fixSplitLines:        singleFile(splitLines),
-		fixJoinLines:         singleFile(joinLines),
-		fixStubMethods:       stubMethodsFixer,
+		fixExtractFunction:         singleFile(extractFunction),
+		fixExtractMethod:           singleFile(extractMethod),
+		fixExtractVariable:         singleFile(extractVariable),
+		fixInlineCall:              inlineCall,
+		fixInvertIfCondition:       singleFile(invertIfCondition),
+		fixSplitLines:              singleFile(splitLines),
+		fixJoinLines:               singleFile(joinLines),
+		fixMissingInterfaceMethods: stubMissingInterfaceMethodsFixer,
+		fixMissingCalledFunction:   stubMissingCalledFunctionFixer,
 	}
 	fixer, ok := fixers[fix]
 	if !ok {