rust.eclass: add ERUST_{SLOT,TYPE}_OVERRIDE user variables

These variables enable users (though most likely Gentoo developers)
to override the selection of the Rust implementation by the eclass.

This means that _only_ the specified SLOT and/or TYPE ('source'/'binary')
will be checked for, with the eclass `die`ing with
"No Rust slot satisfying the package's dependencies..." if the
selected impl is not available.

These variables are intended to enable reproducing bugs and testing
packages against specific dev-lang/rust{,-bin} packages; they must
not be set in ebuilds.

Closes: https://bugs.gentoo.org/945752
Signed-off-by: Matt Jolly <kangie@gentoo.org>

1 files changed, 44 insertions, 5 deletions

diff --git a/eclass/rust.eclass b/eclass/rust.eclass
index 2bf0b27b3f40..eb14ca2329b5 100644
--- a/eclass/rust.eclass
+++ b/eclass/rust.eclass
@@ -102,6 +102,26 @@ declare -a -g -r _RUST_SLOTS_ORDERED=(
 	"1.54.0"
 )
 
+# == user control knobs ==
+
+# @ECLASS_VARIABLE: ERUST_SLOT_OVERRIDE
+# @USER_VARIABLE
+# @DESCRIPTION:
+# Specify the version (slot) of Rust to be used by the package. This is
+# useful for troubleshooting and debugging purposes; If unset, the newest
+# acceptable Rust version will be used. May be combined with ERUST_TYPE_OVERRIDE.
+# This variable must not be set in ebuilds.
+
+# @ECLASS_VARIABLE: ERUST_TYPE_OVERRIDE
+# @USER_VARIABLE
+# @DESCRIPTION:
+# Specify the type of Rust to be used by the package from options:
+# 'source' or 'binary' (-bin). This is useful for troubleshooting and
+# debugging purposes. If unset, the standard eclass logic will be used
+# to determine the type of Rust to use (i.e. prefer source if binary
+# is also available). May be combined with ERUST_SLOT_OVERRIDE.
+# This variable must not be set in ebuilds.
+
 # == control variables ==
 
 # @ECLASS_VARIABLE: RUST_MAX_VER
@@ -336,6 +356,10 @@ _get_rust_slot() {
 			fi
 		fi
 
+		if [[ -n "${ERUST_SLOT_OVERRIDE}" && "${slot}" != "${ERUST_SLOT_OVERRIDE}" ]]; then
+			continue
+		fi
+
 		# If we're in LLVM mode we can skip any slots that don't match the selected USE
 		if [[ -n "${RUST_NEEDS_LLVM}" ]]; then
 			if [[ "${llvm_slot}" != "${llvm_r1_slot}" ]]; then
@@ -349,12 +373,27 @@ _get_rust_slot() {
 			rust_check_deps && return
 		else
 			local usedep="${RUST_REQ_USE+[${RUST_REQ_USE}]}"
-			# When checking for installed packages prefer the non `-bin` package
+			# When checking for installed packages prefer the source package;
 			# if effort was put into building it we should use it.
-			local rust_pkgs=(
-				"dev-lang/rust:${slot}${usedep}"
-				"dev-lang/rust-bin:${slot}${usedep}"
-			)
+			local rust_pkgs
+			case "${ERUST_TYPE_OVERRIDE}" in
+				source)
+					rust_pkgs=(
+						"dev-lang/rust:${slot}${usedep}"
+					)
+					;;
+				binary)
+					rust_pkgs=(
+						"dev-lang/rust-bin:${slot}${usedep}"
+					)
+					;;
+				*)
+					rust_pkgs=(
+						"dev-lang/rust:${slot}${usedep}"
+						"dev-lang/rust-bin:${slot}${usedep}"
+					)
+					;;
+			esac
 			local _pkg
 			for _pkg in "${rust_pkgs[@]}"; do
 				if has_version "${hv_switch}" "${_pkg}"; then