Allow specifying only a subtarget for images in config

.changes/1491.json

@@ -1,5 +1,5 @@
 {
-    "description": "Allow specifying only a tag for images in config",
+    "description": "Allow specifying only a tag or subtarget for images in config",
     "issues": [1169],
     "type": "changed"
 }

Cargo.toml

@@ -86,6 +86,12 @@ search = "<!-- next-url -->"
 replace = "<!-- next-url -->\n\n[Unreleased]: https://github.com/cross-rs/{{crate_name}}/compare/v{{version}}...HEAD"
 exactly = 1
 
+[[package.metadata.release.pre-release-replacements]]
+file = "docs/config_file.md"
+search = "(# Translates to `.*?:).*?(-centos`)"
+replace = "${1}{{version}}$2"
+exactly = 1
+
 [package.metadata.binstall]
 pkg-url = "{ repo }/releases/download/v{ version }/{ name }-{ target }.tar.gz"
 bin-dir = "{ bin }{ binary-ext }"

docs/config_file.md

@@ -234,6 +234,14 @@ image = ":edge"
 image = "@sha256:77db671d8356a64ae72a3e1415e63f547f26d374fbe3c4762c1cd36c7eac7b99"
 ```
 
+You can also specify a subtarget with no tag nor image name:
+
+```toml
+[target.x86_64-unknown-linux-gnu]
+# Translates to `ghcr.io/cross-rs/x86_64-unknown-linux-gnu:0.3.0-centos`
+image = "-centos"
+```
+
 The `image` key can also take the toolchains/platforms supported by the image:
 
 ```toml

src/docker/image.rs

@@ -2,7 +2,12 @@ use std::str::FromStr;
 
 use serde::{Deserialize, Serialize};
 
-use crate::{errors::*, shell::MessageInfo, TargetTriple};
+use crate::{
+    docker::{CROSS_IMAGE, DEFAULT_IMAGE_VERSION},
+    errors::*,
+    shell::MessageInfo,
+    TargetTriple,
+};
 
 use super::Engine;
 
@@ -114,20 +119,39 @@ pub enum ImageReference {
     Name(String),
     /// Unqualified reference, only a tag or digest
     Identifier(String),
+    /// Unqualified reference, only a subtarget
+    Subtarget(String),
 }
 
 impl ImageReference {
     pub fn get(&self) -> &str {
         match self {
             Self::Name(s) => s,
             Self::Identifier(s) => s,
+            Self::Subtarget(s) => s,
         }
     }
+
+    pub fn ensure_qualified(&mut self, target_name: &str) {
+        let image_name = match self {
+            Self::Name(_) => return,
+            Self::Identifier(id) => {
+                format!("{CROSS_IMAGE}/{target_name}{id}")
+            }
+            Self::Subtarget(sub) => {
+                format!("{CROSS_IMAGE}/{target_name}:{DEFAULT_IMAGE_VERSION}{sub}")
+            }
+        };
+
+        *self = Self::Name(image_name);
+    }
 }
 
 impl From<String> for ImageReference {
     fn from(s: String) -> Self {
-        if s.starts_with(':') || s.starts_with('@') {
+        if s.starts_with('-') {
+            Self::Subtarget(s)
+        } else if s.starts_with(':') || s.starts_with('@') {
             Self::Identifier(s)
         } else {
             Self::Name(s)

src/docker/shared.rs

@@ -5,7 +5,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::{env, fs, time};
 
 use super::custom::{Dockerfile, PreBuild};
-use super::image::{ImageReference, PossibleImage};
+use super::image::PossibleImage;
 use super::Image;
 use super::PROVIDED_IMAGES;
 use super::{engine::*, ProvidedImage};
@@ -1252,10 +1252,8 @@ fn get_user_image(
     }
 
     if let Some(image) = &mut image {
-        if let ImageReference::Identifier(id) = &image.reference {
-            let target_name = get_target_name(target, uses_zig);
-            image.reference = ImageReference::Name(format!("{CROSS_IMAGE}/{target_name}{id}"));
-        }
+        let target_name = get_target_name(target, uses_zig);
+        image.reference.ensure_qualified(target_name);
     }
 
     Ok(image)
@@ -1633,6 +1631,10 @@ mod tests {
         let mut map = HashMap::new();
         test(map.clone(), &default_ver, &default_ver)?;
 
+        map.insert("CROSS_TARGET_X86_64_UNKNOWN_LINUX_GNU_IMAGE", "-centos");
+        let centos_tag = format!("{default_ver}-centos");
+        test(map.clone(), &centos_tag, &centos_tag)?;
+
         map.insert("CROSS_TARGET_X86_64_UNKNOWN_LINUX_GNU_IMAGE", ":edge");
         test(map.clone(), ":edge", ":edge")?;