[PATCH proxmox] schema: api_string_type: add from_str method to validate before allocating

[Gabriel Goller <g.goller@proxmox.com>]

The existing from_string takes ownership of a String, meaning callers
with a &str had to heap-allocate first and discard the allocation on
validation failure. The new from_str validates the &str against the
schema's check_constraints before calling to_string(), so no allocation
happens on the error path.

Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
---

Motivation:
This is used quite a lot in proxmox-ve-rs, where we often do e.g.
NetAfi::from_str(afi.to_string()), where from_str is the implementation provided
by the api_string_type macro.

 proxmox-schema/src/api_type_macros.rs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/proxmox-schema/src/api_type_macros.rs b/proxmox-schema/src/api_type_macros.rs
index f3740d14fd35..dd83f684baba 100644
--- a/proxmox-schema/src/api_type_macros.rs
+++ b/proxmox-schema/src/api_type_macros.rs
@@ -27,6 +27,8 @@
 /// * `fn as_str(&self) -> &str`
 /// * `fn from_string(inner: String) -> Result<Self, anyhow::Error>` using
 ///   `StringSchema::check_constraints`.
+/// * `fn from_str(inner: &str) -> Result<Self, anyhow::Error>` using
+///   `StringSchema::check_constraints`.
 /// * `unsafe fn from_string_unchecked(inner: String) -> Self`
 #[macro_export]
 macro_rules! api_string_type {
@@ -101,6 +103,21 @@ macro_rules! api_string_type {
                 }
                 Ok(Self(inner))
             }
+
+            /// Create an instance from a `&str`, validating it using the API schema's
+            /// [`check_constraints`](::proxmox_schema::StringSchema::check_constraints())
+            /// method.
+            ///
+            /// Validates the string *before* allocating, so no heap allocation occurs on
+            /// failure. Only allocates when the value is guaranteed to be valid.
+            pub fn from_str(inner: &str) -> Result<Self, ::anyhow::Error> {
+                use $crate::ApiType;
+                match &Self::API_SCHEMA {
+                    $crate::Schema::String(s) => s.check_constraints(inner)?,
+                    _ => unreachable!(),
+                }
+                Ok(Self(inner.to_string()))
+            }
         }
 
         impl ::std::fmt::Display for $name {
-- 
2.47.3