feat: no main error page (#2709)

* feat: no main error page

Signed-off-by: Carlos A Becker <caarlos0@gmail.com>

* docs: improve seo

Signed-off-by: Carlos A Becker <caarlos0@gmail.com>

internal/builders/golang/build.go

@@ -250,7 +250,7 @@ func checkMain(build config.Build) error {
 				}
 			}
 		}
-		return fmt.Errorf("build for %s does not contain a main function", build.Binary)
+		return errNoMain{build.Binary}
 	}
 	file, err := parser.ParseFile(token.NewFileSet(), main, nil, 0)
 	if err != nil {
@@ -259,7 +259,15 @@ func checkMain(build config.Build) error {
 	if hasMain(file) {
 		return nil
 	}
-	return fmt.Errorf("build for %s does not contain a main function", build.Binary)
+	return errNoMain{build.Binary}
+}
+
+type errNoMain struct {
+	bin string
+}
+
+func (e errNoMain) Error() string {
+	return fmt.Sprintf("build for %s does not contain a main function\nLearn more at https://goreleaser.com/errors/no-main\n", e.bin)
 }
 
 func hasMain(file *ast.File) bool {

internal/builders/golang/build_test.go

@@ -581,7 +581,7 @@ func TestRunPipeWithoutMainFunc(t *testing.T) {
 		ctx.Config.Builds[0].Main = ""
 		require.EqualError(t, Default.Build(ctx, ctx.Config.Builds[0], api.Options{
 			Target: runtimeTarget,
-		}), `build for no-main does not contain a main function`)
+		}), errNoMain{"no-main"}.Error())
 	})
 	t.Run("not main.go", func(t *testing.T) {
 		ctx := newCtx(t)
@@ -595,14 +595,14 @@ func TestRunPipeWithoutMainFunc(t *testing.T) {
 		ctx.Config.Builds[0].Main = "."
 		require.EqualError(t, Default.Build(ctx, ctx.Config.Builds[0], api.Options{
 			Target: runtimeTarget,
-		}), `build for no-main does not contain a main function`)
+		}), errNoMain{"no-main"}.Error())
 	})
 	t.Run("fixed main.go", func(t *testing.T) {
 		ctx := newCtx(t)
 		ctx.Config.Builds[0].Main = "main.go"
 		require.EqualError(t, Default.Build(ctx, ctx.Config.Builds[0], api.Options{
 			Target: runtimeTarget,
-		}), `build for no-main does not contain a main function`)
+		}), errNoMain{"no-main"}.Error())
 	})
 	t.Run("using gomod.proxy", func(t *testing.T) {
 		ctx := newCtx(t)
@@ -613,7 +613,7 @@ func TestRunPipeWithoutMainFunc(t *testing.T) {
 		ctx.Config.Builds[0].UnproxiedMain = "."
 		require.EqualError(t, Default.Build(ctx, ctx.Config.Builds[0], api.Options{
 			Target: runtimeTarget,
-		}), `build for no-main does not contain a main function`)
+		}), errNoMain{"no-main"}.Error())
 	})
 	t.Run("using gomod.proxy and template", func(t *testing.T) {
 		ctx := newCtx(t)
@@ -625,7 +625,7 @@ func TestRunPipeWithoutMainFunc(t *testing.T) {
 		ctx.Config.Builds[0].UnproxiedMain = "{{ .Env.Main }}"
 		require.EqualError(t, Default.Build(ctx, ctx.Config.Builds[0], api.Options{
 			Target: runtimeTarget,
-		}), `build for no-main does not contain a main function`)
+		}), errNoMain{"no-main"}.Error())
 	})
 	t.Run("using gomod.proxy and invalid template", func(t *testing.T) {
 		ctx := newCtx(t)

www/docs/errors/no-main.md

@@ -0,0 +1,33 @@
+# Build does not contain a main function
+
+This usually happens if you're trying to build a library or if you didn't setup the `builds.main` section in your `.goreleaser.yml` and you `main.go` is not in the root folder.
+
+Here's an example error:
+
+```sh
+   ⨯ build failed after 0.11s error=build for foo does not contain a main function
+
+Learn more at https://goreleaser.com/errors/no-main
+```
+
+## If you are building a library
+
+Add something like this to your config:
+
+```yaml
+# .goreleaser.yml
+builds:
+- skip: true
+```
+
+## If your `main.go` is not in the root folder
+
+Add something like this to your config:
+
+```yaml
+# .goreleaser.yml
+builds:
+- main: ./path/to/your/main/pkg/
+```
+
+For more info, check the [builds documentation](/customization/build/).

www/mkdocs.yml

@@ -131,6 +131,7 @@ nav:
     - goreleaser jsonschema: cmd/goreleaser_jsonschema.md
 - Common errors:
   - errors/dirty.md
+  - errors/no-main.md
 - deprecations.md
 - Cookbooks:
   - About: cookbooks/index.md