Conditionally mark the test cfg as a well known cfg (#15007)

### What does this PR try to resolve?

This PR conditionally mark the `test` cfg as a well known cfg depending
on the target unit "test" field (ie `lib.test = false`, `[[bin]] test =
false` and others).

This is related to rust-lang/rust#117778 and
https://users.rust-lang.org/t/cargo-what-is-the-purpose-of-lib-test-false/102361.

When defining `lib.test = false` (and others), any use of `cfg(test)`
will trigger the `unexpected_cfgs` lint.
```toml
[lib]
test = false  # will now warn on cfg(test)
```

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

Best reviewed commit by commit. Second commit removes the `test` cfg
from the `--check-cfg` args.

### Additional information

T-compiler
[MCP#785](rust-lang/compiler-team#785) and
#14963 were of preparatory work.

r? @epage

src/cargo/core/compiler/mod.rs

@@ -1391,17 +1391,26 @@ fn check_cfg_args(unit: &Unit) -> Vec<OsString> {
     }
     arg_feature.push("))");
 
-    // In addition to the package features, we also include the `test` cfg (since
-    // compiler-team#785, as to be able to someday apply yt conditionaly), as well
-    // the `docsrs` cfg from the docs.rs service.
+    // In addition to the package features, we also conditionaly include the `test` cfg
+    // based on the unit target "test" field (ie `lib.test = false`, `[[bin]] test = false` and
+    // others).
     //
-    // We include `docsrs` here (in Cargo) instead of rustc, since there is a much closer
+    // We also include `docsrs` here (in Cargo) instead of rustc, since there is a much closer
     // relationship between Cargo and docs.rs than rustc and docs.rs. In particular, all
     // users of docs.rs use Cargo, but not all users of rustc (like Rust-for-Linux) use docs.rs.
+    let arg_extra = if unit.target.tested()
+        // Benchmarks default to `test = false` but most of them still use the test crate
+        // and the `#[test]` attribute, so for now always mark `test` as well known for them.
+        || unit.target.is_bench()
+    {
+        OsString::from("cfg(docsrs,test)")
+    } else {
+        OsString::from("cfg(docsrs)")
+    };
 
     vec![
         OsString::from("--check-cfg"),
-        OsString::from("cfg(docsrs,test)"),
+        arg_extra,
         OsString::from("--check-cfg"),
         arg_feature,
     ]

src/doc/src/reference/cargo-targets.md

@@ -216,7 +216,10 @@ the target name.
 ### The `test` field
 
 The `test` field indicates whether or not the target is tested by default by
-[`cargo test`]. The default is `true` for lib, bins, and tests.
+[`cargo test`], and whenever the target is expected to have tests. Warnings
+may be reported when tests are unexpected (i.e., `test = false`).
+
+The default is `true` for lib, bins, and tests.
 
 > **Note**: Examples are built by [`cargo test`] by default to ensure they
 > continue to compile, but they are not *tested* by default. Setting `test =

tests/testsuite/check_cfg.rs

@@ -317,6 +317,193 @@ fn well_known_names_values_doctest() {
         .run();
 }
 
+#[cargo_test(nightly, reason = "warning currently only on nightly")]
+fn test_false_lib() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+
+                [lib]
+                test = false
+            "#,
+        )
+        .file("src/lib.rs", "#[cfg(test)] mod tests {}")
+        .build();
+
+    p.cargo("check -v")
+        .with_stderr_does_not_contain(x!("rustc" => "cfg" of "docsrs,test"))
+        .with_stderr_contains(x!("rustc" => "cfg" of "docsrs"))
+        .with_stderr_data(str![[r#"
+...
+[WARNING] unexpected `cfg` condition name: `test`
+...
+
+[WARNING] `foo` (lib) generated 1 warning
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+
+    p.cargo("clean").run();
+    p.cargo("test -v")
+        .with_stderr_contains(x!("rustc" => "cfg" of "docsrs"))
+        .with_stderr_data(str![[r#"
+...
+[WARNING] unexpected `cfg` condition name: `test`
+...
+
+[WARNING] `foo` (lib) generated 1 warning
+[FINISHED] `test` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[DOCTEST] foo
+[RUNNING] [..]
+
+"#]])
+        .run();
+
+    p.cargo("clean").run();
+    p.cargo("test --lib -v")
+        .with_stderr_contains(x!("rustc" => "cfg" of "docsrs"))
+        .with_stderr_data(str![[r#"
+...
+[WARNING] unexpected `cfg` condition name: `test`
+ --> src/lib.rs:1:7
+...
+
+[WARNING] `foo` (lib test) generated 1 warning
+[FINISHED] `test` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[RUNNING] `[ROOT]/foo/target/debug/deps/foo-[HASH][EXE]`
+
+"#]])
+        .run();
+}
+
+#[cargo_test(nightly, reason = "warning currently only on nightly")]
+fn test_false_bins() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+
+                [[bin]]
+                name = "daemon"
+                test = false
+                path = "src/deamon.rs"
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}\n#[cfg(test)] mod tests {}")
+        .file("src/deamon.rs", "fn main() {}\n#[cfg(test)] mod tests {}")
+        .build();
+
+    p.cargo("check -v")
+        .with_stderr_contains(x!("rustc" => "cfg" of "docsrs,test")) // for foo
+        .with_stderr_contains(x!("rustc" => "cfg" of "docsrs")) // for deamon
+        .with_stderr_data(str![[r#"
+...
+[WARNING] unexpected `cfg` condition name: `test`
+...
+
+[WARNING] `foo` (bin "daemon") generated 1 warning
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
+#[cargo_test(nightly, reason = "warning currently only on nightly")]
+fn test_false_examples() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+
+                [lib]
+                test = false
+
+                [[example]]
+                name = "daemon"
+                test = false
+                path = "src/deamon.rs"
+            "#,
+        )
+        .file("src/lib.rs", "#[cfg(test)] mod tests {}")
+        .file("src/deamon.rs", "fn main() {}\n#[cfg(test)] mod tests {}")
+        .build();
+
+    p.cargo("check --examples -v")
+        .with_stderr_does_not_contain(x!("rustc" => "cfg" of "docsrs,test"))
+        .with_stderr_contains(x!("rustc" => "cfg" of "docsrs"))
+        .with_stderr_data(str![[r#"
+...
+[WARNING] unexpected `cfg` condition name: `test`
+...
+
+[WARNING] `foo` (lib) generated 1 warning
+...
+[WARNING] unexpected `cfg` condition name: `test`
+...
+
+[WARNING] `foo` (example "daemon") generated 1 warning
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
+#[cargo_test(
+    nightly,
+    reason = "bench is nightly & warning currently only on nightly"
+)]
+fn test_false_benches() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.0"
+                edition = "2018"
+
+                [[bench]]
+                name = "ben1"
+                test = false
+                path = "benches/ben1.rs"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "benches/ben1.rs",
+            r#"
+            #![feature(test)]
+            extern crate test;
+            #[bench] fn run1(_ben: &mut test::Bencher) { }
+            "#,
+        )
+        .build();
+
+    // Benches always require the `test` cfg, there should be no warning.
+    p.cargo("bench --bench ben1")
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[FINISHED] `bench` profile [optimized] target(s) in [ELAPSED]s
+[RUNNING] benches/ben1.rs (target/release/deps/ben1-[HASH][EXE])
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn features_doc() {
     let p = project()

tests/testsuite/test.rs

@@ -4191,7 +4191,19 @@ fn test_hint_workspace_virtual() {
         .file("a/src/lib.rs", "#[test] fn t1() {}")
         .file("b/Cargo.toml", &basic_manifest("b", "0.1.0"))
         .file("b/src/lib.rs", "#[test] fn t1() {assert!(false)}")
-        .file("c/Cargo.toml", &basic_manifest("c", "0.1.0"))
+        .file(
+            "c/Cargo.toml",
+            r#"
+                [package]
+                name = "c"
+                version = "0.1.0"
+                edition = "2015"
+
+                [[example]]
+                name = "ex1"
+                test = true
+            "#,
+        )
         .file(
             "c/src/lib.rs",
             r#"
@@ -4275,14 +4287,17 @@ fn test_hint_workspace_virtual() {
 [ERROR] test failed, to rerun pass `-p c --bin c`
 [RUNNING] tests/t1.rs (target/debug/deps/t1-[HASH][EXE])
 [ERROR] test failed, to rerun pass `-p c --test t1`
+[RUNNING] unittests examples/ex1.rs (target/debug/examples/ex1-[HASH][EXE])
+[ERROR] test failed, to rerun pass `-p c --example ex1`
 [DOCTEST] a
 [DOCTEST] b
 [DOCTEST] c
 [ERROR] doctest failed, to rerun pass `-p c --doc`
-[ERROR] 4 targets failed:
+[ERROR] 5 targets failed:
     `-p b --lib`
     `-p c --bin c`
     `-p c --test t1`
+    `-p c --example ex1`
     `-p c --doc`
 
 "#]])