Auto merge of #13874 - torhovland:old-syntax-suggestion, r=weihanglo

Old syntax suggestion

Fixes #13868.

The build error in the issue will now include a suggestion:

```
   Compiling zerocopy v0.8.0-alpha.9
error: the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, but the minimum supported Rust version of `zerocopy v0.8.0-alpha.9` is 1.56.0.
Consider using the old `cargo:` syntax in front of `rustc-check-cfg=`.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.
```

The suggestion is only included for reserved prefixes.

A test has been added.

Author: bors

src/cargo/core/compiler/custom_build.rs

@@ -722,16 +722,35 @@ impl BuildOutput {
         const DOCS_LINK_SUGGESTION: &str = "See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
                 for more information about build script outputs.";
 
+        fn has_reserved_prefix(flag: &str) -> bool {
+            RESERVED_PREFIXES
+                .iter()
+                .any(|reserved_prefix| flag.starts_with(reserved_prefix))
+        }
+
         fn check_minimum_supported_rust_version_for_new_syntax(
             pkg_descr: &str,
             msrv: &Option<RustVersion>,
+            flag: &str,
         ) -> CargoResult<()> {
             if let Some(msrv) = msrv {
                 let new_syntax_added_in = RustVersion::from_str("1.77.0")?;
                 if !new_syntax_added_in.is_compatible_with(msrv.as_partial()) {
+                    let old_syntax_suggestion = if has_reserved_prefix(flag) {
+                        format!(
+                            "Switch to the old `cargo:{flag}` syntax (note the single colon).\n"
+                        )
+                    } else if flag.starts_with("metadata=") {
+                        let old_format_flag = flag.strip_prefix("metadata=").unwrap();
+                        format!("Switch to the old `cargo:{old_format_flag}` syntax instead of `cargo::{flag}` (note the single colon).\n")
+                    } else {
+                        String::new()
+                    };
+
                     bail!(
                         "the `cargo::` syntax for build script output instructions was added in \
                         Rust 1.77.0, but the minimum supported Rust version of `{pkg_descr}` is {msrv}.\n\
+                        {old_syntax_suggestion}\
                         {DOCS_LINK_SUGGESTION}"
                     );
                 }
@@ -793,16 +812,13 @@ impl BuildOutput {
             };
             let mut old_syntax = false;
             let (key, value) = if let Some(data) = line.strip_prefix("cargo::") {
-                check_minimum_supported_rust_version_for_new_syntax(pkg_descr, msrv)?;
+                check_minimum_supported_rust_version_for_new_syntax(pkg_descr, msrv, data)?;
                 // For instance, `cargo::rustc-flags=foo` or `cargo::metadata=foo=bar`.
                 parse_directive(whence.as_str(), line, data, old_syntax)?
             } else if let Some(data) = line.strip_prefix("cargo:") {
                 old_syntax = true;
                 // For instance, `cargo:rustc-flags=foo`.
-                if RESERVED_PREFIXES
-                    .iter()
-                    .any(|prefix| data.starts_with(prefix))
-                {
+                if has_reserved_prefix(data) {
                     parse_directive(whence.as_str(), line, data, old_syntax)?
                 } else {
                     // For instance, `cargo:foo=bar`.

tests/testsuite/build_script.rs

@@ -625,7 +625,7 @@ fn custom_build_invalid_host_config_feature_flag() {
         .with_status(101)
         .with_stderr_contains(
             "\
-error: the -Zhost-config flag requires the -Ztarget-applies-to-host flag to be set
+[ERROR] the -Zhost-config flag requires the -Ztarget-applies-to-host flag to be set
 ",
         )
         .run();
@@ -1038,7 +1038,7 @@ fn links_duplicates() {
 
     p.cargo("build").with_status(101)
                        .with_stderr("\
-error: failed to select a version for `a-sys`.
+[ERROR] failed to select a version for `a-sys`.
     ... required by package `foo v0.5.0 ([..])`
 versions that meet the requirements `*` are: 0.5.0
 
@@ -1163,7 +1163,7 @@ fn links_duplicates_deep_dependency() {
 
     p.cargo("build").with_status(101)
                        .with_stderr("\
-error: failed to select a version for `a-sys`.
+[ERROR] failed to select a version for `a-sys`.
     ... required by package `a v0.5.0 ([..])`
     ... which satisfies path dependency `a` of package `foo v0.5.0 ([..])`
 versions that meet the requirements `*` are: 0.5.0
@@ -4508,7 +4508,7 @@ fn links_duplicates_with_cycle() {
 
     p.cargo("build").with_status(101)
                        .with_stderr("\
-error: failed to select a version for `a`.
+[ERROR] failed to select a version for `a`.
     ... required by package `foo v0.5.0 ([..])`
 versions that meet the requirements `*` are: 0.5.0
 
@@ -5315,7 +5315,7 @@ fn wrong_output() {
         .with_stderr(
             "\
 [COMPILING] foo [..]
-error: invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo::example`
+[ERROR] invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo::example`
 Expected a line with `cargo::KEY=VALUE` with an `=` character, but none was found.
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.
@@ -5405,7 +5405,7 @@ fn test_invalid_old_syntax() {
         .with_stderr(
             "\
 [COMPILING] foo [..]
-error: invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo:foo`
+[ERROR] invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo:foo`
 Expected a line with `cargo:KEY=VALUE` with an `=` character, but none was found.
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.
@@ -5434,7 +5434,7 @@ fn test_invalid_new_syntax() {
         .with_stderr(
             "\
 [COMPILING] foo [..]
-error: invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo::metadata=foo`
+[ERROR] invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo::metadata=foo`
 Expected a line with `cargo::metadata=KEY=VALUE` with an `=` character, but none was found.
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.
@@ -5459,7 +5459,7 @@ for more information about build script outputs.
         .with_stderr(
             "\
 [COMPILING] foo [..]
-error: invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo::foo=bar`
+[ERROR] invalid output in build script of `foo v0.0.1 ([ROOT]/foo)`: `cargo::foo=bar`
 Unknown key: `foo`.
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.
@@ -5499,7 +5499,89 @@ fn test_new_syntax_with_old_msrv() {
         .with_stderr_contains(
             "\
 [COMPILING] foo [..]
-error: the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, \
+[ERROR] the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, \
+but the minimum supported Rust version of `foo v0.5.0 ([ROOT]/foo)` is 1.60.0.
+Switch to the old `cargo:foo=bar` syntax instead of `cargo::metadata=foo=bar` (note the single colon).
+See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
+for more information about build script outputs.
+",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn test_new_syntax_with_old_msrv_and_reserved_prefix() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+                rust-version = "1.60.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                fn main() {
+                    println!("cargo::rustc-check-cfg=cfg(foo)");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("build")
+        .with_status(101)
+        .with_stderr_contains(
+            "\
+[COMPILING] foo [..]
+[ERROR] the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, \
+but the minimum supported Rust version of `foo v0.5.0 ([ROOT]/foo)` is 1.60.0.
+Switch to the old `cargo:rustc-check-cfg=cfg(foo)` syntax (note the single colon).
+See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
+for more information about build script outputs.
+",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn test_new_syntax_with_old_msrv_and_unknown_prefix() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+                rust-version = "1.60.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                fn main() {
+                    println!("cargo::foo=bar");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("build")
+        .with_status(101)
+        .with_stderr_contains(
+            "\
+[COMPILING] foo [..]
+[ERROR] the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, \
 but the minimum supported Rust version of `foo v0.5.0 ([ROOT]/foo)` is 1.60.0.
 See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
 for more information about build script outputs.