Remove dynamic detection of pipelining support

Pipelining has made its way to stable so no need to check that any
more.

src/cargo/core/compiler/build_context/target_info.rs

@@ -34,7 +34,6 @@ pub struct TargetInfo {
     pub rustflags: Vec<String>,
     /// Extra flags to pass to `rustdoc`, see `env_args`.
     pub rustdocflags: Vec<String>,
-    pub supports_pipelining: Option<bool>,
 }
 
 /// Kind of each file generated by a Unit, part of `FileType`.
@@ -102,18 +101,6 @@ impl TargetInfo {
             .args(&rustflags)
             .env_remove("RUSTC_LOG");
 
-        // NOTE: set this unconditionally to `true` once support for `--json`
-        // rides to stable.
-        //
-        // Also note that we only learn about this functionality for the host
-        // compiler since the host/target rustc are always the same.
-        let mut pipelining_test = process.clone();
-        pipelining_test.args(&["--error-format=json", "--json=artifacts"]);
-        let supports_pipelining = match kind {
-            Kind::Host => Some(rustc.cached_output(&pipelining_test).is_ok()),
-            Kind::Target => None,
-        };
-
         let target_triple = requested_target
             .as_ref()
             .map(|s| s.as_str())
@@ -203,7 +190,6 @@ impl TargetInfo {
                 "RUSTDOCFLAGS",
             )?,
             cfg,
-            supports_pipelining,
         })
     }
 

src/cargo/core/compiler/context/mod.rs

@@ -99,9 +99,8 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
 
         let pipelining = bcx
             .config
-            .get_bool("build.pipelining")?
-            .map(|t| t.val)
-            .unwrap_or(bcx.host_info.supports_pipelining.unwrap());
+            .get::<Option<bool>>("build.pipelining")?
+            .unwrap_or(true);
 
         Ok(Self {
             bcx,