compiler, runtime: Support Formatted Error Messages

Make it possible to output formatted error messages using a
flexible API that includes an ability to specify styles and
formatters.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Tests/p4rseTests/ErrorTests/Formatting.swift

@@ -18,8 +18,8 @@
 import Common
 import Foundation
 import Macros
-import P4Runtime
 import P4Lang
+import P4Runtime
 import SwiftTreeSitter
 import Testing
 import TreeSitter
@@ -28,7 +28,31 @@ import TreeSitterP4
 @testable import P4Compiler
 
 @Test func test_error_with_location_formatting() async throws {
-    let e = ErrorWithLocation(sourceLocation: SourceLocation(1, 5), withError: "There was an error")
-
-    print(e.format())
+  let formatter = FormatterAnsi()
+  let e = ErrorWithLocation(sourceLocation: SourceLocation(1, 5), withError: "There was an error")
+  let formatted = e.format(formatter)
+  #expect(formatted == "\u{1B}[31;1m{1, 5}\u{1B}[0m: There was an error")
+}
+
+@Test func test_errors_with_location_no_formatting() async throws {
+  let e = ErrorWithLocation(sourceLocation: SourceLocation(1, 5), withError: "There was an error")
+  let e1 = ErrorWithLocation(
+    sourceLocation: SourceLocation(10, 5), withError: "There was another error")
+
+  let formatted = e.append(error: e1).format(FormatterPlain())
+
+  #expect(formatted == "{1, 5}: There was an error\n{10, 5}: There was another error")
+}
+
+@Test func test_errors_with_location_ansi_formatting() async throws {
+  let e = ErrorWithLocation(sourceLocation: SourceLocation(1, 5), withError: "There was an error")
+  let e1 = ErrorWithLocation(
+    sourceLocation: SourceLocation(10, 5), withError: "There was another error")
+
+  let formatted = e.append(error: e1).format(FormatterAnsi())
+
+  #expect(
+    formatted
+      == "\u{1B}[31;1m{1, 5}\u{1B}[0m: There was an error\n\u{1B}[31;1m{10, 5}\u{1B}[0m: There was another error"
+  )
 }

Tests/p4rseTests/StyleTests/Style.swift

@@ -0,0 +1,57 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Common
+import Foundation
+import Macros
+import P4Runtime
+import P4Lang
+import SwiftTreeSitter
+import Testing
+import TreeSitter
+import TreeSitterP4
+
+@testable import P4Compiler
+
+@Test func test_style_add_format() async throws {
+    let red = Style(StyleColor.Red)
+    let bold_red = red.update(addFormat: StyleFormat.Bold)
+
+    #expect(bold_red == Style(StyleColor.Red, [StyleFormat.Bold]))
+}
+
+@Test func test_style_add_format2() async throws {
+    let bold_red = Style(StyleColor.Red, [StyleFormat.Bold])
+    let bold_underline_red = bold_red.update(addFormat: StyleFormat.Underline)
+
+    #expect(bold_underline_red == Style(StyleColor.Red, [StyleFormat.Bold, StyleFormat.Underline]))
+}
+
+
+@Test func test_style_remove_format() async throws {
+    let bold_red = Style(StyleColor.Red, [StyleFormat.Bold])
+    let red = bold_red.update(removeFormat: StyleFormat.Bold)
+
+    #expect(red == Style(StyleColor.Red))
+}
+
+@Test func test_style_remove_format2() async throws {
+    let bold_underline_red = Style(StyleColor.Red, [StyleFormat.Bold, StyleFormat.Underline])
+    let underline_red = bold_underline_red.update(removeFormat: StyleFormat.Bold)
+
+    #expect(underline_red == Style(StyleColor.Red, [StyleFormat.Underline]))
+}

Tests/p4rseTests/ValueTypeParserTests.swift

@@ -42,7 +42,7 @@ import TreeSitterP4
     guard case Result.Error(let e) = err else {
       assert(false, "Expected an error, but had success")
     }
-    #expect(e.format().contains("Failed to parse a statement element: Could not parse a P4 type from \(invalid_type_name)"))
+    #expect(e.msg().contains("Failed to parse a statement element: Could not parse a P4 type from \(invalid_type_name)"))
   }
 }