Replace deprecated proc-macro-error2 with proc-macro2-diagnostics (#4613)

proc-macro-error2 is unmaintained (RUSTSEC-2026-0173). Migrate the
kani_macros crate to proc-macro2-diagnostics, one of the alternatives
recommended by the advisory.

The abort!/abort_call_site! macros (which panicked and were caught by
the #[proc_macro_error] wrapper) are replaced by threading
Result<TokenStream, Diagnostic> through the macro helpers and emitting
the diagnostic at each entry point via
Diagnostic::emit_as_item_tokens(). The structured note/help
sub-diagnostics and spans are preserved, and proc-macro2-diagnostics
auto-enables native nightly emission, so the compiler output is
unchanged.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

---------

Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>

Author: tautschnig

Cargo.lock

@@ -119,9 +119,9 @@ dependencies = [
 
 [[package]]
 name = "anyhow"
-version = "1.0.102"
+version = "1.0.103"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
+checksum = "2a4385e2e34eb35d6b3efe798b9eb88096925d87726c0798709bf56d9ed84af3"
 
 [[package]]
 name = "ar_archive_writer"
@@ -1110,8 +1110,8 @@ dependencies = [
 name = "kani_macros"
 version = "0.67.0"
 dependencies = [
- "proc-macro-error2",
  "proc-macro2",
+ "proc-macro2-diagnostics",
  "quote",
  "strum",
  "strum_macros",
@@ -1675,34 +1675,24 @@ dependencies = [
 ]
 
 [[package]]
-name = "proc-macro-error-attr2"
-version = "2.0.0"
+name = "proc-macro2"
+version = "1.0.106"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "96de42df36bb9bba5542fe9f1a054b8cc87e172759a1868aa05c1f3acc89dfc5"
+checksum = "8fd00f0bb2e90d81d1044c2b32617f68fcb9fa3bb7640c23e9c748e53fb30934"
 dependencies = [
- "proc-macro2",
- "quote",
+ "unicode-ident",
 ]
 
 [[package]]
-name = "proc-macro-error2"
-version = "2.0.1"
+name = "proc-macro2-diagnostics"
+version = "0.10.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "11ec05c52be0a07b08061f7dd003e7d7092e0472bc731b4af7bb1ef876109802"
+checksum = "af066a9c399a26e020ada66a034357a868728e72cd426f3adcd35f80d88d88c8"
 dependencies = [
- "proc-macro-error-attr2",
  "proc-macro2",
  "quote",
  "syn",
-]
-
-[[package]]
-name = "proc-macro2"
-version = "1.0.106"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8fd00f0bb2e90d81d1044c2b32617f68fcb9fa3bb7640c23e9c748e53fb30934"
-dependencies = [
- "unicode-ident",
+ "version_check",
 ]
 
 [[package]]

library/kani_macros/Cargo.toml

@@ -13,7 +13,7 @@ proc-macro = true
 
 [dependencies]
 proc-macro2 = "1.0"
-proc-macro-error2 = { version = "2.0.0", features = ["nightly"] }
+proc-macro2-diagnostics = { version = "0.10", default-features = false }
 quote = "1.0.20"
 syn = { version = "2.0.18", features = ["full", "visit-mut", "visit", "extra-traits"] }
 strum = "0.27.1"