Add the enum keyword to the c_enum macro

Our `style.sh` script can't handle these easily, and it seems like
`ctest` may struggle with this macro. Add the `enum` keyword so the
expanded code is valid Rust.

(backport <#4494>)
(cherry picked from commit 19e04c8)

Author: tgross35

ci/style.sh

@@ -26,7 +26,7 @@ while IFS= read -r file; do
 
     # Turn all braced macro `foo! { /* ... */ }` invocations into
     # `fn foo_fmt_tmp() { /* ... */ }`.
-    perl -pi -e 's/(?!macro_rules|c_enum)\b(\w+)!\s*\{/fn $1_fmt_tmp() {/g' "$file"
+    perl -pi -e 's/(?!macro_rules)\b(\w+)!\s*\{/fn $1_fmt_tmp() {/g' "$file"
 
     # Replace `if #[cfg(...)]` within `cfg_if` with `if cfg_tmp!([...])` which
     # `rustfmt` will format. We put brackets within the parens so it is easy to

src/macros.rs

@@ -223,7 +223,7 @@ macro_rules! e {
 macro_rules! c_enum {
     (
         $(#[repr($repr:ty)])?
-        $ty_name:ident {
+        enum $ty_name:ident {
             $($variant:ident $(= $value:literal)?,)+
         }
     ) => {
@@ -411,7 +411,7 @@ mod tests {
     fn c_enumbasic() {
         // By default, variants get sequential values.
         c_enum! {
-            e {
+            enum e {
                 VAR0,
                 VAR1,
                 VAR2,
@@ -428,7 +428,7 @@ mod tests {
         // By default, variants get sequential values.
         c_enum! {
             #[repr(u16)]
-            e {
+            enum e {
                 VAR0,
             }
         }
@@ -440,7 +440,7 @@ mod tests {
     fn c_enumset_value() {
         // Setting an explicit value resets the count.
         c_enum! {
-            e {
+            enum e {
                 VAR2 = 2,
                 VAR3,
                 VAR4,
@@ -457,7 +457,7 @@ mod tests {
         // C enums always take one more than the previous value, unless set to a specific
         // value. Duplicates are allowed.
         c_enum! {
-            e {
+            enum e {
                 VAR0,
                 VAR2_0 = 2,
                 VAR3_0,

src/unix/linux_like/linux/mod.rs

@@ -95,7 +95,7 @@ e! {
 }
 
 c_enum! {
-    pid_type {
+    enum pid_type {
         PIDTYPE_PID,
         PIDTYPE_TGID,
         PIDTYPE_PGID,
@@ -4559,14 +4559,14 @@ pub const RTNLGRP_STATS: c_uint = 0x24;
 
 // linux/cn_proc.h
 c_enum! {
-    proc_cn_mcast_op {
+    enum proc_cn_mcast_op {
         PROC_CN_MCAST_LISTEN = 1,
         PROC_CN_MCAST_IGNORE = 2,
     }
 }
 
 c_enum! {
-    proc_cn_event {
+    enum proc_cn_event {
         PROC_EVENT_NONE = 0x00000000,
         PROC_EVENT_FORK = 0x00000001,
         PROC_EVENT_EXEC = 0x00000002,