Use `try_reserve()` instead of `with_capacity()`

`String::with_capacity()` panics if the requested memory could not be
allocated. `Template::render()` is fallible method, and the fact that it
can panic is not documented.

This commit uses `String::try_reserve()` instead, so even for an
exceedingly large `SIZE_HINT` the method should not panic. In the
generated code `write!()` calls will fail instead with
`Err(std::fmt::Error)`.

I do not test if `try_reserve()` returned an error, because the
rendering might succeed anyway, if less bytes are written than estimated.

1 files changed, 2 insertions, 1 deletions

diff --git a/askama/src/lib.rs b/askama/src/lib.rs
index 906c79a..8806490 100644
--- a/askama/src/lib.rs
+++ b/askama/src/lib.rs
@@ -82,7 +82,8 @@ pub use crate::error::{Error, Result};
 pub trait Template: fmt::Display {
     /// Helper method which allocates a new `String` and renders into it
     fn render(&self) -> Result<String> {
-        let mut buf = String::with_capacity(Self::SIZE_HINT);
+        let mut buf = String::new();
+        let _ = buf.try_reserve(Self::SIZE_HINT);
         self.render_into(&mut buf)?;
         Ok(buf)
     }