This PR adds a `@[deprecated_arg]` attribute that marks individual
function parameters as deprecated. When a caller uses the old parameter
name, the elaborator emits a deprecation warning with a code action hint
to rename or delete the argument, and silently forwards the value to the
correct binder.
Supported forms:
- `@[deprecated_arg old new (since := "...")]` — renamed parameter,
warns and forwards
- `@[deprecated_arg old new "reason" (since := "...")]` — with custom
message
- `@[deprecated_arg removed (since := "...")]` — removed parameter,
errors with delete hint
- `@[deprecated_arg removed "reason" (since := "...")]` — removed with
custom message
A warning is emitted if `(since := "...")` is omitted.
When a parameter is deprecated without a replacement, the elaborator
treats it as no longer present: using it as a named argument produces an
error. Note that positional uses of deprecated arguments are not checked
— if a function's arity changed, the caller will simply get a "function
expected" error.
The `linter.deprecated.arg` option (default `true`) controls behavior:
when enabled, renamed args produce warnings and removed args produce
specific deprecation errors with code action hints; when disabled, both
fall through to the standard "invalid argument name" error. This lets
library authors phase out old parameter names without breaking
downstream code immediately.
Example (renamed parameter):
```lean
@[deprecated_arg old new (since := "2026-03-18")]
def f (new : Nat) : Nat := new
/--
warning: parameter `old` of `f` has been deprecated, use `new` instead
Hint: Rename this argument:
o̵l̵d̵n̲e̲w̲
---
info: f 42 : Nat
-/
#guard_msgs in
#check f (old := 42)
```
Example (removed parameter):
```lean
@[deprecated_arg removed (since := "2026-03-18")]
def g (x : Nat) : Nat := x
/--
error: parameter `removed` of `g` has been deprecated
Hint: Delete this argument:
(̵r̵e̵m̵o̵v̵e̵d̵ ̵:̵=̵ ̵4̵2̵)̵
-/
#guard_msgs in
#check g (removed := 42)
```
