add -Z linker-features to toggle lld on the CLI

but don't expose `+/-cc` yet

compiler/rustc_session/src/config.rs

@@ -18,7 +18,7 @@ use rustc_feature::UnstableFeatures;
 use rustc_span::edition::{Edition, DEFAULT_EDITION, EDITION_NAME_LIST, LATEST_STABLE_EDITION};
 use rustc_span::source_map::FilePathMapping;
 use rustc_span::{FileName, FileNameDisplayPreference, RealFileName, SourceFileHashAlgorithm};
-use rustc_target::spec::LinkSelfContainedComponents;
+use rustc_target::spec::{LinkSelfContainedComponents, LinkerFeatures};
 use rustc_target::spec::{SplitDebuginfo, Target, TargetTriple};
 use std::collections::btree_map::{
     Iter as BTreeMapIter, Keys as BTreeMapKeysIter, Values as BTreeMapValuesIter,
@@ -292,6 +292,48 @@ impl LinkSelfContained {
     }
 }
 
+/// The different values that `-Z linker-features` can take on the CLI: a list of individually
+/// enabled or disabled features used during linking.
+///
+/// There is no need to enable or disable them in bulk. Each feature is fine-grained, and can be
+/// used to turn `LinkerFeatures` on or off, without needing to change the linker flavor:
+/// - using the system lld, or the self-contained `rust-lld` linker
+/// - using a C/C++ compiler to drive the linker (not yet exposed on the CLI)
+/// - etc.
+#[derive(Default, Copy, Clone, PartialEq, Debug)]
+pub struct LinkerFeaturesCli {
+    /// The linker features that are enabled on the CLI, using the `+feature` syntax.
+    pub enabled: LinkerFeatures,
+
+    /// The linker features that are disabled on the CLI, using the `-feature` syntax.
+    pub disabled: LinkerFeatures,
+}
+
+impl LinkerFeaturesCli {
+    /// Accumulates an enabled or disabled feature as specified on the CLI, if possible.
+    /// For example: `+lld`, and `-lld`.
+    pub(crate) fn handle_cli_feature(&mut self, feature: &str) -> Option<()> {
+        // Duplicate flags are reduced as we go, the last occurrence wins:
+        // `+feature,-feature,+feature` only enables the feature, and does not record it as both
+        // enabled and disabled on the CLI.
+        // We also only expose `+/-lld` at the moment, as it's currenty the only implemented linker
+        // feature and toggling `LinkerFeatures::CC` would be a noop.
+        match feature {
+            "+lld" => {
+                self.enabled.insert(LinkerFeatures::LLD);
+                self.disabled.remove(LinkerFeatures::LLD);
+                Some(())
+            }
+            "-lld" => {
+                self.disabled.insert(LinkerFeatures::LLD);
+                self.enabled.remove(LinkerFeatures::LLD);
+                Some(())
+            }
+            _ => None,
+        }
+    }
+}
+
 /// Used with `-Z assert-incr-state`.
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
 pub enum IncrementalStateAssertion {

compiler/rustc_session/src/options.rs

@@ -426,6 +426,8 @@ mod desc {
         "one of supported split dwarf modes (`split` or `single`)";
     pub const parse_link_self_contained: &str = "one of: `y`, `yes`, `on`, `n`, `no`, `off`, or a list of enabled (`+` prefix) and disabled (`-` prefix) \
         components: `crto`, `libc`, `unwind`, `linker`, `sanitizers`, `mingw`";
+    pub const parse_linker_features: &str =
+        "a list of enabled (`+` prefix) and disabled (`-` prefix) features: `lld`";
     pub const parse_polonius: &str = "either no value or `legacy` (the default), or `next`";
     pub const parse_stack_protector: &str =
         "one of (`none` (default), `basic`, `strong`, or `all`)";
@@ -1269,6 +1271,22 @@ mod parse {
         true
     }
 
+    /// Parse a comma-separated list of enabled and disabled linker features.
+    pub(crate) fn parse_linker_features(slot: &mut LinkerFeaturesCli, v: Option<&str>) -> bool {
+        match v {
+            Some(s) => {
+                for feature in s.split(',') {
+                    if slot.handle_cli_feature(feature).is_none() {
+                        return false;
+                    }
+                }
+
+                true
+            }
+            None => false,
+        }
+    }
+
     pub(crate) fn parse_wasi_exec_model(slot: &mut Option<WasiExecModel>, v: Option<&str>) -> bool {
         match v {
             Some("command") => *slot = Some(WasiExecModel::Command),
@@ -1721,6 +1739,8 @@ options! {
         "link native libraries in the linker invocation (default: yes)"),
     link_only: bool = (false, parse_bool, [TRACKED],
         "link the `.rlink` file generated by `-Z no-link` (default: no)"),
+    linker_features: LinkerFeaturesCli = (LinkerFeaturesCli::default(), parse_linker_features, [UNTRACKED],
+        "a comma-separated list of linker features to enable (+) or disable (-): `lld`"),
     lint_mir: bool = (false, parse_bool, [UNTRACKED],
         "lint MIR before and after each transformation"),
     llvm_module_flag: Vec<(String, u32, String)> = (Vec::new(), parse_llvm_module_flag, [TRACKED],