From 76ab5a7a43fa93348193a4945f958d264594ebd8 Mon Sep 17 00:00:00 2001
From: Julien Cretin <cretin@google.com>
Date: Wed, 15 May 2024 18:33:05 +0200
Subject: [PATCH] Add lifetime container attribute for user-provided lifetime

This PR is for discussion and related to #2190. As discussed there, the benefit is not obvious (except for providing more control to the user).

With this PR, a user can use `#[serde(lifetime = 'a)]` to control the lifetime of the implementation of `Deserialize`. In particular, the user is now able to control all the implementation parameters when using `#[serde(bound = "", lifetime = 'a)]`.

See https://github.com/ia0/wasefire/blob/ccf7bc30ac8440b30981b2f39b734de10d1a037c/crates/protocol/src/lib.rs#L65 for an example in real code.
---
 serde_derive/src/de.rs               |  9 +++++++--
 serde_derive/src/internals/attr.rs   | 11 +++++++++++
 serde_derive/src/internals/symbol.rs |  1 +
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/serde_derive/src/de.rs b/serde_derive/src/de.rs
index e3b737c61..7e1c6565d 100644
--- a/serde_derive/src/de.rs
+++ b/serde_derive/src/de.rs
@@ -225,13 +225,15 @@ fn requires_default(field: &attr::Field, _variant: Option<&attr::Variant>) -> bo
 enum BorrowedLifetimes {
     Borrowed(BTreeSet<syn::Lifetime>),
     Static,
+    User(syn::Lifetime),
 }
 
 impl BorrowedLifetimes {
     fn de_lifetime(&self) -> syn::Lifetime {
-        match *self {
+        match self {
             BorrowedLifetimes::Borrowed(_) => syn::Lifetime::new("'de", Span::call_site()),
             BorrowedLifetimes::Static => syn::Lifetime::new("'static", Span::call_site()),
+            BorrowedLifetimes::User(a) => a.clone(),
         }
     }
 
@@ -244,6 +246,7 @@ impl BorrowedLifetimes {
                 bounds: bounds.iter().cloned().collect(),
             }),
             BorrowedLifetimes::Static => None,
+            BorrowedLifetimes::User(_) => None,
         }
     }
 }
@@ -264,7 +267,9 @@ fn borrowed_lifetimes(cont: &Container) -> BorrowedLifetimes {
             lifetimes.extend(field.attrs.borrowed_lifetimes().iter().cloned());
         }
     }
-    if lifetimes.iter().any(|b| b.to_string() == "'static") {
+    if let Some(a) = cont.attrs.lifetime() {
+        BorrowedLifetimes::User(a.clone())
+    } else if lifetimes.iter().any(|b| b.to_string() == "'static") {
         BorrowedLifetimes::Static
     } else {
         BorrowedLifetimes::Borrowed(lifetimes)
diff --git a/serde_derive/src/internals/attr.rs b/serde_derive/src/internals/attr.rs
index 0cfb23bf1..d3b079d84 100644
--- a/serde_derive/src/internals/attr.rs
+++ b/serde_derive/src/internals/attr.rs
@@ -208,6 +208,7 @@ pub struct Container {
     default: Default,
     rename_all_rules: RenameAllRules,
     rename_all_fields_rules: RenameAllRules,
+    lifetime: Option<syn::Lifetime>,
     ser_bound: Option<Vec<syn::WherePredicate>>,
     de_bound: Option<Vec<syn::WherePredicate>>,
     tag: TagType,
@@ -294,6 +295,7 @@ impl Container {
         let mut rename_all_de_rule = Attr::none(cx, RENAME_ALL);
         let mut rename_all_fields_ser_rule = Attr::none(cx, RENAME_ALL_FIELDS);
         let mut rename_all_fields_de_rule = Attr::none(cx, RENAME_ALL_FIELDS);
+        let mut lifetime = Attr::none(cx, LIFETIME);
         let mut ser_bound = Attr::none(cx, BOUND);
         let mut de_bound = Attr::none(cx, BOUND);
         let mut untagged = BoolAttr::none(cx, UNTAGGED);
@@ -440,6 +442,10 @@ impl Container {
                             }
                         }
                     }
+                } else if meta.path == LIFETIME {
+                    // #[serde(lifetime = 'a)]
+                    let a: syn::Lifetime = meta.value()?.parse()?;
+                    lifetime.set(&meta.path, a);
                 } else if meta.path == BOUND {
                     // #[serde(bound = "T: SomeBound")]
                     // #[serde(bound(serialize = "...", deserialize = "..."))]
@@ -579,6 +585,7 @@ impl Container {
                 serialize: rename_all_fields_ser_rule.get().unwrap_or(RenameRule::None),
                 deserialize: rename_all_fields_de_rule.get().unwrap_or(RenameRule::None),
             },
+            lifetime: lifetime.get(),
             ser_bound: ser_bound.get(),
             de_bound: de_bound.get(),
             tag: decide_tag(cx, item, untagged, internal_tag, content),
@@ -619,6 +626,10 @@ impl Container {
         &self.default
     }
 
+    pub fn lifetime(&self) -> Option<&syn::Lifetime> {
+        self.lifetime.as_ref()
+    }
+
     pub fn ser_bound(&self) -> Option<&[syn::WherePredicate]> {
         self.ser_bound.as_ref().map(|vec| &vec[..])
     }
diff --git a/serde_derive/src/internals/symbol.rs b/serde_derive/src/internals/symbol.rs
index 572391a80..b0d7095f0 100644
--- a/serde_derive/src/internals/symbol.rs
+++ b/serde_derive/src/internals/symbol.rs
@@ -19,6 +19,7 @@ pub const FLATTEN: Symbol = Symbol("flatten");
 pub const FROM: Symbol = Symbol("from");
 pub const GETTER: Symbol = Symbol("getter");
 pub const INTO: Symbol = Symbol("into");
+pub const LIFETIME: Symbol = Symbol("lifetime");
 pub const NON_EXHAUSTIVE: Symbol = Symbol("non_exhaustive");
 pub const OTHER: Symbol = Symbol("other");
 pub const REMOTE: Symbol = Symbol("remote");