Allow specifying only a tag for images in config

.changes/1491.json

@@ -0,0 +1,5 @@
+{
+    "description": "Allow specifying only a tag for images in config",
+    "issues": [1169],
+    "type": "changed"
+}

docs/config_file.md

@@ -221,6 +221,19 @@ the default one. Normal Docker behavior applies, so:
 - If only `image:latest` is specified, then Docker won't look in Docker Hub.
 - If the tag is omitted, then Docker will use the `latest` tag.
 
+If you specify a tag but no image name, `cross` will use the default image with
+the tag you provided:
+
+```toml
+[target.aarch64-unknown-linux-gnu]
+# Translates to `ghcr.io/cross-rs/aarch64-unknown-linux-gnu:edge`
+image = ":edge"
+
+[target.x86_64-unknown-linux-musl]
+# Translates to `ghcr.io/cross-rs/x86_64-unknown-linux-musl@sha256:77db671d8356a64ae72a3e1415e63f547f26d374fbe3c4762c1cd36c7eac7b99`
+image = "@sha256:77db671d8356a64ae72a3e1415e63f547f26d374fbe3c4762c1cd36c7eac7b99"
+```
+
 The `image` key can also take the toolchains/platforms supported by the image:
 
 ```toml

src/config.rs

@@ -41,10 +41,10 @@ impl<T: PartialEq> PartialEq<(Option<T>, Option<T>)> for ConfVal<T> {
 }
 
 #[derive(Debug)]
-struct Environment(&'static str, Option<HashMap<&'static str, &'static str>>);
+pub(crate) struct Environment(&'static str, Option<HashMap<&'static str, &'static str>>);
 
 impl Environment {
-    fn new(map: Option<HashMap<&'static str, &'static str>>) -> Self {
+    pub(crate) fn new(map: Option<HashMap<&'static str, &'static str>>) -> Self {
         Environment("CROSS", map)
     }
 
@@ -352,7 +352,7 @@ impl Config {
     }
 
     #[cfg(test)]
-    fn new_with(toml: Option<CrossToml>, env: Environment) -> Self {
+    pub(crate) fn new_with(toml: Option<CrossToml>, env: Environment) -> Self {
         Config { toml, env }
     }
 

src/docker/image.rs

@@ -27,6 +27,12 @@ pub struct PossibleImage {
 }
 
 impl PossibleImage {
+    pub fn target_is_specified(&self) -> bool {
+        // Hostnames cannot start with either of these characters,
+        // nor can image names.
+        !(self.name.starts_with(':') || self.name.starts_with('@'))
+    }
+
     pub fn to_definite_with(&self, engine: &Engine, msg_info: &mut MessageInfo) -> Image {
         if self.toolchain.is_empty() {
             Image {

src/docker/shared.rs

@@ -1251,6 +1251,15 @@ fn get_user_image(
         image = config.zig_image(target).map_err(GetImageError::Other)?;
     }
 
+    if let Some(image) = &mut image {
+        if !image.target_is_specified() {
+            // The provided image name is just the tag or hash part,
+            // so add the default registry and image name before it.
+            let target_name = get_target_name(target, uses_zig);
+            image.name = format!("{CROSS_IMAGE}/{target_name}{}", image.name);
+        }
+    }
+
     Ok(image)
 }
 
@@ -1542,8 +1551,10 @@ pub fn path_hash(path: &Path, count: usize) -> Result<String> {
 
 #[cfg(test)]
 mod tests {
+    use std::collections::HashMap;
+
     use super::*;
-    use crate::id;
+    use crate::{config::Environment, id};
 
     #[cfg(not(target_os = "windows"))]
     use crate::file::PathExt;
@@ -1598,6 +1609,48 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_tag_only_image() -> Result<()> {
+        let target: Target = TargetTriple::X86_64UnknownLinuxGnu.into();
+        let test = |map, expected_ver: &str, expected_ver_zig: &str| -> Result<()> {
+            let env = Environment::new(Some(map));
+            let config = Config::new_with(None, env);
+            for (uses_zig, expected_ver) in [(false, expected_ver), (true, expected_ver_zig)] {
+                let expected_image_target = if uses_zig {
+                    "zig"
+                } else {
+                    "x86_64-unknown-linux-gnu"
+                };
+                let expected = format!("ghcr.io/cross-rs/{expected_image_target}{expected_ver}");
+
+                let image = get_image(&config, &target, uses_zig)?;
+                assert_eq!(image.name, expected);
+                let image_name = get_image_name(&config, &target, uses_zig)?;
+                assert_eq!(image_name, expected);
+            }
+            Ok(())
+        };
+
+        let default_ver = format!(":{DEFAULT_IMAGE_VERSION}");
+        let mut map = HashMap::new();
+        test(map.clone(), &default_ver, &default_ver)?;
+
+        map.insert("CROSS_TARGET_X86_64_UNKNOWN_LINUX_GNU_IMAGE", ":edge");
+        test(map.clone(), ":edge", ":edge")?;
+
+        // `image` always takes precedence over `zig.image`, even when `uses_zig` is `true`
+        map.insert(
+            "CROSS_TARGET_X86_64_UNKNOWN_LINUX_GNU_ZIG_IMAGE",
+            "@sha256:foobar",
+        );
+        test(map.clone(), ":edge", ":edge")?;
+
+        map.remove("CROSS_TARGET_X86_64_UNKNOWN_LINUX_GNU_IMAGE");
+        test(map.clone(), &default_ver, "@sha256:foobar")?;
+
+        Ok(())
+    }
+
     mod directories {
         use super::*;
         use crate::cargo::cargo_metadata_with_args;