rustc_target: Update some old naming around self contained linking

The "fallback" naming pre-dates introduction of `-Clink-self-contained`

compiler/rustc_target/src/spec/crt_objects.rs

@@ -63,7 +63,7 @@ pub(super) fn all(obj: &'static str) -> CrtObjects {
     ])
 }
 
-pub(super) fn pre_musl_fallback() -> CrtObjects {
+pub(super) fn pre_musl_self_contained() -> CrtObjects {
     new(&[
         (LinkOutputKind::DynamicNoPicExe, &["crt1.o", "crti.o", "crtbegin.o"]),
         (LinkOutputKind::DynamicPicExe, &["Scrt1.o", "crti.o", "crtbeginS.o"]),
@@ -74,7 +74,7 @@ pub(super) fn pre_musl_fallback() -> CrtObjects {
     ])
 }
 
-pub(super) fn post_musl_fallback() -> CrtObjects {
+pub(super) fn post_musl_self_contained() -> CrtObjects {
     new(&[
         (LinkOutputKind::DynamicNoPicExe, &["crtend.o", "crtn.o"]),
         (LinkOutputKind::DynamicPicExe, &["crtendS.o", "crtn.o"]),
@@ -85,7 +85,7 @@ pub(super) fn post_musl_fallback() -> CrtObjects {
     ])
 }
 
-pub(super) fn pre_mingw_fallback() -> CrtObjects {
+pub(super) fn pre_mingw_self_contained() -> CrtObjects {
     new(&[
         (LinkOutputKind::DynamicNoPicExe, &["crt2.o", "rsbegin.o"]),
         (LinkOutputKind::DynamicPicExe, &["crt2.o", "rsbegin.o"]),
@@ -96,7 +96,7 @@ pub(super) fn pre_mingw_fallback() -> CrtObjects {
     ])
 }
 
-pub(super) fn post_mingw_fallback() -> CrtObjects {
+pub(super) fn post_mingw_self_contained() -> CrtObjects {
     all("rsend.o")
 }
 
@@ -108,7 +108,7 @@ pub(super) fn post_mingw() -> CrtObjects {
     all("rsend.o")
 }
 
-pub(super) fn pre_wasi_fallback() -> CrtObjects {
+pub(super) fn pre_wasi_self_contained() -> CrtObjects {
     // Use crt1-command.o instead of crt1.o to enable support for new-style
     // commands. See https://reviews.llvm.org/D81689 for more info.
     new(&[
@@ -120,37 +120,41 @@ pub(super) fn pre_wasi_fallback() -> CrtObjects {
     ])
 }
 
-pub(super) fn post_wasi_fallback() -> CrtObjects {
+pub(super) fn post_wasi_self_contained() -> CrtObjects {
     new(&[])
 }
 
-/// Which logic to use to determine whether to fall back to the "self-contained" mode or not.
+/// Which logic to use to determine whether to use self-contained linking mode
+/// if `-Clink-self-contained` is not specified explicitly.
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
-pub enum CrtObjectsFallback {
+pub enum LinkSelfContainedDefault {
+    False,
+    True,
     Musl,
     Mingw,
-    Wasm,
 }
 
-impl FromStr for CrtObjectsFallback {
+impl FromStr for LinkSelfContainedDefault {
     type Err = ();
 
-    fn from_str(s: &str) -> Result<CrtObjectsFallback, ()> {
+    fn from_str(s: &str) -> Result<LinkSelfContainedDefault, ()> {
         Ok(match s {
-            "musl" => CrtObjectsFallback::Musl,
-            "mingw" => CrtObjectsFallback::Mingw,
-            "wasm" => CrtObjectsFallback::Wasm,
+            "false" => LinkSelfContainedDefault::False,
+            "true" | "wasm" => LinkSelfContainedDefault::True,
+            "musl" => LinkSelfContainedDefault::Musl,
+            "mingw" => LinkSelfContainedDefault::Mingw,
             _ => return Err(()),
         })
     }
 }
 
-impl ToJson for CrtObjectsFallback {
+impl ToJson for LinkSelfContainedDefault {
     fn to_json(&self) -> Json {
         match *self {
-            CrtObjectsFallback::Musl => "musl",
-            CrtObjectsFallback::Mingw => "mingw",
-            CrtObjectsFallback::Wasm => "wasm",
+            LinkSelfContainedDefault::False => "false",
+            LinkSelfContainedDefault::True => "true",
+            LinkSelfContainedDefault::Musl => "musl",
+            LinkSelfContainedDefault::Mingw => "mingw",
         }
         .to_json()
     }