gopls/internal/lsp/source: put context first in extracted functions

Put the context first when extracting functions/methods

Fixes golang/go#60738

gopls/internal/lsp/source/extract.go

@@ -363,6 +363,8 @@ func extractFunctionMethod(fset *token.FileSet, start, end token.Pos, src []byte
 		}
 	}
 
+	reorderParams(params, paramTypes)
+
 	// Find the function literal that encloses the selection. The enclosing function literal
 	// may not be the enclosing function declaration (i.e. 'outer'). For example, in the
 	// following block:
@@ -631,6 +633,31 @@ func extractFunctionMethod(fset *token.FileSet, start, end token.Pos, src []byte
 	}, nil
 }
 
+// isSelector checks if e is the selector expr <x>, <sel>.
+func isSelector(e ast.Expr, x, sel string) bool {
+	selectorExpr, ok := e.(*ast.SelectorExpr)
+	if !ok {
+		return false
+	}
+	ident, ok := selectorExpr.X.(*ast.Ident)
+	if !ok {
+		return false
+	}
+	return ident.Name == x && selectorExpr.Sel.Name == sel
+}
+
+// reorderParams reorders the given parameters in-place to follow common Go conventions.
+func reorderParams(params []ast.Expr, paramTypes []*ast.Field) {
+	// If a context is found in the parameters, put it first.
+	for i, t := range paramTypes {
+		if isSelector(t.Type, "context", "Context") {
+			params[0], params[i] = params[i], params[0]
+			paramTypes[0], paramTypes[i] = paramTypes[i], paramTypes[0]
+			break
+		}
+	}
+}
+
 // adjustRangeForCommentsAndWhiteSpace adjusts the given range to exclude unnecessary leading or
 // trailing whitespace characters from selection as well as leading or trailing comments.
 // In the following example, each line of the if statement is indented once. There are also two

gopls/internal/lsp/testdata/extract/extract_method/extract_context.go

@@ -0,0 +1,13 @@
+package extract
+
+import "context"
+
+type A struct {
+	x int
+	y int
+}
+
+func (a *A) AddP(ctx context.Context) (int, error) {
+	sum := a.x + a.y
+	return sum, ctx.Err() //@extractmethod("return", "ctx.Err()"),extractfunc("return", "ctx.Err()")
+}

gopls/internal/lsp/testdata/extract/extract_method/extract_context.go.golden

@@ -0,0 +1,19 @@
+-- methodextraction_extract_context_12_2 --
+package extract
+
+import "context"
+
+type A struct {
+	x int
+	y int
+}
+
+func (a *A) AddP(ctx context.Context) (int, error) {
+	sum := a.x + a.y
+	return a.newMethod(ctx, sum) //@extractmethod("return", "ctx.Err()"),extractfunc("return", "ctx.Err()")
+}
+
+func (*A) newMethod(ctx context.Context, sum int) (int, error) {
+	return sum, ctx.Err()
+}
+

gopls/internal/lsp/testdata/summary.txt.golden

@@ -12,7 +12,7 @@ DiagnosticsCount = 23
 FoldingRangesCount = 2
 SemanticTokenCount = 3
 SuggestedFixCount = 73
-MethodExtractionCount = 6
+MethodExtractionCount = 7
 DefinitionsCount = 46
 TypeDefinitionsCount = 18
 HighlightsCount = 70

gopls/internal/lsp/testdata/summary_go1.18.txt.golden

@@ -12,7 +12,7 @@ DiagnosticsCount = 23
 FoldingRangesCount = 2
 SemanticTokenCount = 3
 SuggestedFixCount = 79
-MethodExtractionCount = 6
+MethodExtractionCount = 7
 DefinitionsCount = 46
 TypeDefinitionsCount = 18
 HighlightsCount = 70

gopls/internal/lsp/testdata/summary_go1.21.txt.golden

@@ -12,7 +12,7 @@ DiagnosticsCount = 24
 FoldingRangesCount = 2
 SemanticTokenCount = 3
 SuggestedFixCount = 79
-MethodExtractionCount = 6
+MethodExtractionCount = 7
 DefinitionsCount = 46
 TypeDefinitionsCount = 18
 HighlightsCount = 70