Auto merge of #13505 - epage:edition-warn, r=weihanglo

feat(toml): Warn on unset Edition

### What does this PR try to resolve?

On [Internals](https://internals.rust-lang.org/t/idea-rustc-cargo-should-warn-on-unspecified-edition/20309), the idea came up for warning on unset Edition.

Besides helping people who forget to set the Edition, this creates symmetry between `Cargo.toml` and cargo scripts (#12207).  While the default is different in each case, we are making the default obvious and guiding people away from it.

### How should we test and review this PR?

There are separate commits for adding tests (and refactors) so the changes in behavior will be more obvious

### Additional information

This builds on
- #13499
- #13504

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;
@@ -563,14 +564,69 @@ pub fn to_real_manifest(
         source_id,
     );
 
+    let rust_version = if let Some(rust_version) = &package.rust_version {
+        let rust_version = field_inherit_with(rust_version.clone(), "rust_version", || {
+            inherit()?.rust_version()
+        })?;
+        Some(rust_version)
+    } else {
+        None
+    };
+
     let edition = if let Some(edition) = package.edition.clone() {
         let edition: Edition = field_inherit_with(edition, "edition", || inherit()?.edition())?
             .parse()
             .with_context(|| "failed to parse the `edition` key")?;
         package.edition = Some(manifest::InheritableField::Value(edition.to_string()));
+        if let Some(rust_version) = &rust_version {
+            let req = rust_version.to_caret_req();
+            if let Some(first_version) = edition.first_version() {
+                let unsupported =
+                    semver::Version::new(first_version.major, first_version.minor - 1, 9999);
+                if req.matches(&unsupported) {
+                    bail!(
+                        "rust-version {} is older than first version ({}) required by \
+                            the specified edition ({})",
+                        rust_version,
+                        first_version,
+                        edition,
+                    )
+                }
+            }
+        }
         edition
     } else {
-        Edition::Edition2015
+        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 {default_edition} edition{tip}",
+        ));
+        default_edition
     };
     // Add these lines if start a new unstable edition.
     // ```
@@ -588,29 +644,6 @@ pub fn to_real_manifest(
         )));
     }
 
-    let rust_version = if let Some(rust_version) = &package.rust_version {
-        let rust_version = field_inherit_with(rust_version.clone(), "rust_version", || {
-            inherit()?.rust_version()
-        })?;
-        let req = rust_version.to_caret_req();
-        if let Some(first_version) = edition.first_version() {
-            let unsupported =
-                semver::Version::new(first_version.major, first_version.minor - 1, 9999);
-            if req.matches(&unsupported) {
-                bail!(
-                    "rust-version {} is older than first version ({}) required by \
-                            the specified edition ({})",
-                    rust_version,
-                    first_version,
-                    edition,
-                )
-            }
-        }
-        Some(rust_version)
-    } else {
-        None
-    };
-
     if package.metabuild.is_some() {
         features.require(Feature::metabuild())?;
     }

tests/testsuite/bench.rs

@@ -331,6 +331,7 @@ fn bench_with_lib_dep() {
                 [package]
                 name = "foo"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [[bin]]
@@ -396,6 +397,7 @@ fn bench_with_deep_lib_dep() {
                 [package]
                 name = "bar"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [dependencies.foo]
@@ -454,6 +456,7 @@ fn external_bench_explicit() {
                 [package]
                 name = "foo"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [[bench]]
@@ -697,6 +700,7 @@ fn lib_bin_same_name() {
                 [package]
                 name = "foo"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [lib]
@@ -796,6 +800,7 @@ fn lib_with_standard_name2() {
                 [package]
                 name = "syntax"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [lib]
@@ -842,6 +847,7 @@ fn bench_dylib() {
                 [package]
                 name = "foo"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [lib]
@@ -883,6 +889,7 @@ fn bench_dylib() {
                 [package]
                 name = "bar"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
 
                 [lib]
@@ -932,6 +939,7 @@ fn bench_twice_with_build_cmd() {
                 [package]
                 name = "foo"
                 version = "0.0.1"
+                edition = "2015"
                 authors = []
                 build = "build.rs"
             "#,
@@ -977,6 +985,7 @@ fn bench_with_examples() {
                 [package]
                 name = "foo"
                 version = "6.6.6"
+                edition = "2015"
                 authors = []
 
                 [[example]]
@@ -1061,6 +1070,7 @@ fn test_a_bench() {
                 name = "foo"
                 authors = []
                 version = "0.1.0"
+                edition = "2015"
 
                 [lib]
                 name = "foo"
@@ -1219,6 +1229,7 @@ fn test_bench_multiple_packages() {
                 name = "foo"
                 authors = []
                 version = "0.1.0"
+                edition = "2015"
 
                 [dependencies.bar]
                 path = "../bar"
@@ -1239,6 +1250,7 @@ fn test_bench_multiple_packages() {
                 name = "bar"
                 authors = []
                 version = "0.1.0"
+                edition = "2015"
 
                 [[bench]]
                 name = "bbar"
@@ -1269,6 +1281,7 @@ fn test_bench_multiple_packages() {
                 name = "baz"
                 authors = []
                 version = "0.1.0"
+                edition = "2015"
 
                 [[bench]]
                 name = "bbaz"
@@ -1307,6 +1320,7 @@ fn bench_all_workspace() {
                 [package]
                 name = "foo"
                 version = "0.1.0"
+                edition = "2015"
 
                 [dependencies]
                 bar = { path = "bar" }
@@ -1360,6 +1374,7 @@ fn bench_all_exclude() {
                 [package]
                 name = "foo"
                 version = "0.1.0"
+                edition = "2015"
 
                 [workspace]
                 members = ["bar", "baz"]
@@ -1405,6 +1420,7 @@ fn bench_all_exclude_glob() {
                 [package]
                 name = "foo"
                 version = "0.1.0"
+                edition = "2015"
 
                 [workspace]
                 members = ["bar", "baz"]
@@ -1549,6 +1565,7 @@ fn legacy_bench_name() {
                 [package]
                 name = "foo"
                 version = "0.1.0"
+                edition = "2015"
 
                 [[bench]]
                 name = "bench"