perf(build-rs): Always emit :: directives

Our MSRV is much higher than 1.77.  Also, as time goes on, there is less
incentive to drop it below 1.77, especially with the MSRV-aware resolver
in 1.84

Author: epage

crates/build-rs/src/allow_use.rs

@@ -1,14 +1,5 @@
 use std::{process::Command, sync::OnceLock};
 
-fn rust_version_minor() -> u32 {
-    static VERSION_MINOR: OnceLock<u32> = OnceLock::new();
-    *VERSION_MINOR.get_or_init(|| {
-        version_minor(&crate::input::cargo_pkg_rust_version().unwrap_or_default())
-            // assume build-rs's MSRV if none specified for the current package
-            .unwrap_or_else(|| version_minor(env!("CARGO_PKG_RUST_VERSION")).unwrap())
-    })
-}
-
 fn cargo_version_minor() -> u32 {
     static VERSION_MINOR: OnceLock<u32> = OnceLock::new();
     *VERSION_MINOR.get_or_init(|| {
@@ -33,11 +24,6 @@ fn version_minor(version: &str) -> Option<u32> {
     Some(minor)
 }
 
-pub(crate) fn double_colon_directives() -> bool {
-    // cargo errors on `cargo::` directives with insufficient package.rust-version
-    rust_version_minor() >= 77
-}
-
 pub(crate) fn check_cfg() -> bool {
     // emit check-cfg if the toolchain being used supports it
     cargo_version_minor() >= 80

crates/build-rs/src/output.rs

@@ -13,11 +13,7 @@ use crate::{
 use std::{ffi::OsStr, fmt::Display, fmt::Write, path::Path, str};
 
 fn emit(directive: &str, value: impl Display) {
-    if allow_use::double_colon_directives() {
-        println!("cargo::{}={}", directive, value);
-    } else {
-        println!("cargo:{}={}", directive, value);
-    }
+    println!("cargo::{}={}", directive, value);
 }
 
 /// The `rerun-if-changed` instruction tells Cargo to re-run the build script if the
@@ -421,9 +417,5 @@ pub fn metadata(key: &str, val: &str) {
         panic!("cannot emit metadata: invalid value {val:?}");
     }
 
-    if allow_use::double_colon_directives() {
-        emit("metadata", format_args!("{}={}", key, val));
-    } else {
-        emit(key, val);
-    }
+    emit("metadata", format_args!("{}={}", key, val));
 }