fix(toml): Give MSRV-aware guidance

src/cargo/core/features.rs

@@ -180,9 +180,12 @@ pub type AllowFeatures = BTreeSet<String>;
 /// [`is_stable`]: Edition::is_stable
 /// [`toml::to_real_manifest`]: crate::util::toml::to_real_manifest
 /// [`features!`]: macro.features.html
-#[derive(Clone, Copy, Debug, Hash, PartialOrd, Ord, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(
+    Default, Clone, Copy, Debug, Hash, PartialOrd, Ord, Eq, PartialEq, Serialize, Deserialize,
+)]
 pub enum Edition {
     /// The 2015 edition
+    #[default]
     Edition2015,
     /// The 2018 edition
     Edition2018,
@@ -199,6 +202,12 @@ impl Edition {
     pub const LATEST_UNSTABLE: Option<Edition> = Some(Edition::Edition2024);
     /// The latest stable edition.
     pub const LATEST_STABLE: Edition = Edition::Edition2021;
+    pub const ALL: &'static [Edition] = &[
+        Self::Edition2015,
+        Self::Edition2018,
+        Self::Edition2021,
+        Self::Edition2024,
+    ];
     /// Possible values allowed for the `--edition` CLI flag.
     ///
     /// This requires a static value due to the way clap works, otherwise I

src/cargo/util/toml/mod.rs

@@ -9,6 +9,7 @@ use crate::AlreadyPrintedError;
 use anyhow::{anyhow, bail, Context as _};
 use cargo_platform::Platform;
 use cargo_util::paths;
+use cargo_util_schemas::core::PartialVersion;
 use cargo_util_schemas::manifest;
 use cargo_util_schemas::manifest::RustVersion;
 use itertools::Itertools;
@@ -595,12 +596,37 @@ pub fn to_real_manifest(
         }
         edition
     } else {
+        let msrv_edition = if let Some(rust_version) = &rust_version {
+            Edition::ALL
+                .iter()
+                .filter(|e| {
+                    e.first_version()
+                        .map(|e| {
+                            let e = PartialVersion::from(e);
+                            e <= **rust_version
+                        })
+                        .unwrap_or_default()
+                })
+                .max()
+                .copied()
+        } else {
+            None
+        }
+        .unwrap_or_default();
+        let default_edition = Edition::default();
+        let latest_edition = Edition::LATEST_STABLE;
+
+        let tip = if msrv_edition == default_edition {
+            String::new()
+        } else if msrv_edition == latest_edition {
+            format!(" while the latest is {latest_edition}")
+        } else {
+            format!(" while {msrv_edition} is compatible with `rust-version`")
+        };
         warnings.push(format!(
-            "no edition set: defaulting to the {} edition while the latest is {}",
-            Edition::Edition2015,
-            Edition::LATEST_STABLE
+            "no edition set: defaulting to the {default_edition} edition{tip}",
         ));
-        Edition::Edition2015
+        default_edition
     };
     // Add these lines if start a new unstable edition.
     // ```

tests/testsuite/edition.rs

@@ -123,6 +123,33 @@ fn edition_unstable() {
         .run();
 }
 
+#[cargo_test]
+fn unset_edition_works_with_no_newer_compatible_edition() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = 'foo'
+                version = '0.1.0'
+                rust-version = "1.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -v")
+        .with_stderr(
+            "\
+[WARNING] no edition set: defaulting to the 2015 edition
+[CHECKING] foo [..]
+[RUNNING] `rustc [..] --edition=2015 [..]`
+[FINISHED] [..]
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn unset_edition_works_on_old_msrv() {
     let p = project()
@@ -141,7 +168,7 @@ fn unset_edition_works_on_old_msrv() {
     p.cargo("check -v")
         .with_stderr(
             "\
-[WARNING] no edition set: defaulting to the 2015 edition while the latest is 2021
+[WARNING] no edition set: defaulting to the 2015 edition while 2018 is compatible with `rust-version`
 [CHECKING] foo [..]
 [RUNNING] `rustc [..] --edition=2015 [..]`
 [FINISHED] [..]