[librsvg: 4/5] RELEASING.md - Note about the Minimum Supported Rust Version (MSRV)

[Federico Mena Quintero <federico src gnome org>]

commit 97cd4f908d5b780c27fff125f2cf56807e5f6d08
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Mar 18 11:02:30 2021 -0600

    RELEASING.md - Note about the Minimum Supported Rust Version (MSRV)

 RELEASING.md | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 configure.ac |  2 +-
 2 files changed, 50 insertions(+), 1 deletion(-)
---
diff --git a/RELEASING.md b/RELEASING.md
index 50cd2913..fc77e798 100644
--- a/RELEASING.md
+++ b/RELEASING.md
@@ -43,6 +43,55 @@ tooling for the stable or development GNOME releases.  File an issue
 in their [repository][release-team] to indicate whether the new
 `librsvg-x.y.0` is a stable or development series.
 
+## Minimum supported Rust version (MSRV)
+
+While it may seem desirable to always require the latest released
+version of the Rust toolchain, to get new language features and such,
+this is really inconvenient for distributors of librsvg which do not
+update Rust all the time.  So, we make a compromise.
+
+The `configure.ac` script defines `MININUM_RUST_MAJOR` and
+`MINIMUM_RUST_MINOR` variables with librsvg's minimum supported Rust
+version (MSRV).  These ensure that distros will get an early failure during a
+build, at the `configure` step, if they have a version of Rust that is
+too old — instead of getting an obscure error message from `rustc` in
+the middle of the build when it finds an unsupported language
+construct.
+
+As of March 2021, Cargo does not allow setting a minimum supported
+Rust version; you may want to keep an eye on [the MSRV RFC][msrv-rfc].
+
+Sometimes librsvg's dependencies update their MSRV and librsvg may
+need to increase it as well.  Please consider the following before
+doing this:
+
+* Absolutely do not require a nightly snapshot of the compiler, or
+  crates that only build on nightly.
+
+* Distributions with rolling releases usually keep their Rust
+  toolchains fairly well updated, maybe not always at the latest, but
+  within two or three releases earlier than the latest.  If the MSRV
+  you want is within about six months of the latest, things are
+  probably safe.
+  
+* Enterprise distributions update more slowly.  It is useful to watch
+  for the MSRV that Firefox requires, although sometimes Firefox
+  updates Rust very slowly as well.  Now that distributions are
+  shipping packages other than Firefox that require Rust, they will
+  probably start updating more frequently.
+  
+Generally — two or three releases earlier than the latest stable Rust
+is OK for rolling distros, probably perilous for enterprise distros.
+Releases within a year of an enterprise distro's shipping date are
+probably OK.
+
+If you are not sure, ask on the [forum for GNOME
+distributors][distributors] about their plans!  (That is, posts on
+`discourse.gnome.org` with the `distributor` tag.)
+
+[msrv-rfc]: https://github.com/rust-lang/rfcs/pull/2495
+[distributors]: https://discourse.gnome.org/tag/distributor
+
 ## Format for release notes in NEWS
 
 The `NEWS` file contains the release notes.  Please use something
diff --git a/configure.ac b/configure.ac
index b15171f6..cdffaf68 100644
--- a/configure.ac
+++ b/configure.ac
@@ -102,7 +102,7 @@ AS_IF(test x$RUSTC = xno,
     AC_MSG_ERROR([rustc is required.  Please install the Rust toolchain from https://www.rust-lang.org/])
 )
 
-dnl Minimum version of rustc that we support
+dnl MSRV - Minimum Supported Rust Version
 dnl If you change this, please update COMPILING.md
 MINIMUM_RUST_MAJOR=1
 MINIMUM_RUST_MINOR=48