fix(toml): Warn when edition is unuset, even when MSRV is unset

This came up in rust-lang#14108.  This got overlooked when we added the MSRV
limits to the warning.

Users can silience this by setting a very old MSRV.
I didn't bother finding a way to word a specialized message for this.
Wording for this case seemed hard and I figure someone in this situation
knows how to resolve it.
Instead, we are targeting the majority of users who aren't setting a
`package.rust-version` in the first place.

src/cargo/util/toml/mod.rs

@@ -1154,8 +1154,8 @@ fn to_real_manifest(
         // so if they can't use a new edition, don't bother to tell them to set it.
         // This also avoids having to worry about whether `package.edition` is compatible with
         // their MSRV.
-        if msrv_edition != default_edition {
-            let tip = if msrv_edition == latest_edition {
+        if msrv_edition != default_edition || rust_version.is_none() {
+            let tip = if msrv_edition == latest_edition || rust_version.is_none() {
                 format!(" while the latest is {latest_edition}")
             } else {
                 format!(" while {msrv_edition} is compatible with `rust-version`")

tests/testsuite/edition.rs

@@ -142,6 +142,7 @@ fn unset_edition_with_unset_rust_version() {
     p.cargo("check -v")
         .with_stderr(
             "\
+[WARNING] no edition set: defaulting to the 2015 edition while the latest is 2021
 [CHECKING] foo [..]
 [RUNNING] `rustc [..] --edition=2015 [..]`
 [FINISHED] [..]