parser: warn on @[deprecated_after] used without a corresponding @[deprecated] attribute (#25712)

Author: spytheman

vlib/v/parser/attribute.v

@@ -158,6 +158,8 @@ fn (mut p Parser) attributes() {
 	if p.attrs.len == 0 {
 		p.error_with_pos('attributes cannot be empty', start_pos.extend(p.tok.pos()))
 		return
+	} else {
+		p.check_deprecated_attributes()
 	}
 	// TODO: remove when old attr syntax is removed
 	if p.inside_struct_attr_decl && p.tok.kind == .lsbr {
@@ -170,3 +172,25 @@ fn (mut p Parser) attributes() {
 		return
 	}
 }
+
+fn (mut p Parser) check_deprecated_attributes() {
+	mut deprecated := false
+	mut deprecated_after := false
+	mut deprecated_after_pos := token.Pos{}
+	for attr in p.attrs {
+		match attr.name {
+			'deprecated' {
+				deprecated = true
+			}
+			'deprecated_after' {
+				deprecated_after = true
+				deprecated_after_pos = attr.pos
+			}
+			else {}
+		}
+	}
+	if deprecated_after && !deprecated {
+		p.warn_with_pos('@[deprecated_after] is only valid, in the presence of a `@[deprecated]` attribute',
+			deprecated_after_pos)
+	}
+}

vlib/v/parser/tests/deprecated_after_without_deprecated.out

@@ -0,0 +1,26 @@
+vlib/v/parser/tests/deprecated_after_without_deprecated.vv:1:1: warning: @[deprecated_after] is only valid, in the presence of a `@[deprecated]` attribute
+    1 | @[deprecated_after: '2025-10-10']
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    2 | fn my_fn() {
+    3 | }
+vlib/v/parser/tests/deprecated_after_without_deprecated.vv:5:1: warning: @[deprecated_after] is only valid, in the presence of a `@[deprecated]` attribute
+    3 | }
+    4 | 
+    5 | @[deprecated_after: '2025-10-10']
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    6 | const my_const = 123
+    7 |
+vlib/v/parser/tests/deprecated_after_without_deprecated.vv:8:1: warning: @[deprecated_after] is only valid, in the presence of a `@[deprecated]` attribute
+    6 | const my_const = 123
+    7 | 
+    8 | @[deprecated_after: '2025-10-10']
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    9 | type MyType = u8
+   10 |
+vlib/v/parser/tests/deprecated_after_without_deprecated.vv:11:1: warning: @[deprecated_after] is only valid, in the presence of a `@[deprecated]` attribute
+    9 | type MyType = u8
+   10 | 
+   11 | @[deprecated_after: '2025-10-10']
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   12 | interface MyInterface {
+   13 |     x int

vlib/v/parser/tests/deprecated_after_without_deprecated.vv

@@ -0,0 +1,14 @@
+@[deprecated_after: '2025-10-10']
+fn my_fn() {
+}
+
+@[deprecated_after: '2025-10-10']
+const my_const = 123
+
+@[deprecated_after: '2025-10-10']
+type MyType = u8
+
+@[deprecated_after: '2025-10-10']
+interface MyInterface {
+	x int
+}

vlib/v/util/diff/diff.v

@@ -43,30 +43,32 @@ pub enum DiffTool {
 @[params]
 pub struct CompareOptions {
 pub:
-	tool DiffTool @[deprecated_after: '2025-12-31']
+	tool DiffTool @[deprecated: 'use compare_files or compare_text'; deprecated_after: '2025-12-31']
 	// Custom args used with the diff command.
-	args string @[deprecated_after: '2025-12-31']
+	args string @[deprecated: 'use compare_files or compare_text'; deprecated_after: '2025-12-31']
 	// Sets the environment variable whose value can overwrite a diff command passed to a compare function.
 	// It also enables the use of commands that are not in the list of known diff tools.
 	// Set it to `none` to disable it.
-	env_overwrite_var ?string = 'VDIFF_CMD' @[deprecated_after: '2025-12-31']
+	env_overwrite_var ?string = 'VDIFF_CMD' @[deprecated: 'use compare_files or compare_text'; deprecated_after: '2025-12-31']
 }
 
 @[params]
 pub struct CompareTextOptions {
 	CompareOptions
 pub:
-	base_name   string = 'base' @[deprecated_after: '2025-12-31']
-	target_name string = 'target' @[deprecated_after: '2025-12-31']
+	base_name   string = 'base' @[deprecated: 'use compare_files or compare_text'; deprecated_after: '2025-12-31']
+	target_name string = 'target' @[deprecated: 'use compare_files or compare_text'; deprecated_after: '2025-12-31']
 }
 
 // Allows public checking for the available tools and prevents repeated searches
 // when using compare functions with automatic diff tool detection.
+@[deprecated: 'use compare_files or compare_text']
 @[deprecated_after: '2025-12-31']
 pub fn available_tools() []DiffTool {
 	return []
 }
 
+@[deprecated: 'use compare_files or compare_text']
 @[deprecated_after: '2025-12-31']
 pub fn find_working_diff_command() !string {
 	return error('deprecated')

vlib/x/crypto/chacha20/chacha.v

@@ -189,6 +189,7 @@ pub fn (mut c Cipher) free() {
 
 // reset quickly sets all Cipher's fields to default value.
 // This method will be deprecated.
+@[deprecated: 'do not use .reset() at all, create a new Cipher instead']
 @[deprecated_after: '2025-11-30']
 @[unsafe]
 pub fn (mut c Cipher) reset() {