Adding 'toolchain' parameter to control rust builds (#20)

* 'toolchain' parameter

* Documentation

* Tooltip clarification

Author: Ilia-Kosenkov

R/source.R

@@ -11,6 +11,9 @@
 #'   be added to the `Cargo.toml` file.
 #' @param profile Rust profile. Can be either `"dev"` or `"release"`. The default,
 #'   `"dev"`, compiles faster but produces slower code.
+#' @param toolchain Rust toolchain. The default, `NULL`, compiles with the
+#'  system default toolchain. Accepts valid Rust toolchain qualifiers,
+#'  such as `"nightly"`, or (on Windows) `"stable-msvc"`.
 #' @param extendr_version Version of the extendr-api crate, provided as a Rust
 #'   version string. `"*"` will use the latest available version on crates.io.
 #' @param extendr_macros_version Version of the extendr-macros crate, if different
@@ -77,7 +80,9 @@ rust_source <- function(file, code = NULL, dependencies = NULL,
                           'extendr-api = { git = "https://github.com/extendr/extendr" }',
                           'extendr-macros = { git = "https://github.com/extendr/extendr" }'
                         ),
-                        profile = c("dev", "release"), extendr_version = "*",
+                        profile = c("dev", "release"),
+                        toolchain = NULL,
+                        extendr_version = "*",
                         extendr_macros_version = extendr_version,
                         env = parent.frame(),
                         use_extendr_api = TRUE,
@@ -124,6 +129,7 @@ rust_source <- function(file, code = NULL, dependencies = NULL,
   status <- system2(
     command = "cargo",
     args = c(
+      sprintf("+%s", toolchain),
       "build",
       "--lib",
       if (!is.null(specific_target)) sprintf("--target %s", specific_target) else NULL,