rust: Rebuild targets when compiler got updated

rustc updates are usually incompatible and requires recompiling of all
targets. This adds the compiler exe as dependency of every rust
ninja targets. Also try to resolve the real rustc exe being used when
the toolchain is wrapped by rustup.

Fixes: mesonbuild#10706

mesonbuild/backend/ninjabackend.py

@@ -1877,7 +1877,7 @@ def generate_rust_target(self, target: build.BuildTarget) -> None:
         self.generate_generator_list_rules(target)
 
         # dependencies need to cause a relink, they're not just for ordering
-        deps: T.List[str] = []
+        deps: T.List[str] = [rustc.get_exe_file()]
 
         # Dependencies for rust-project.json
         project_deps: T.List[RustDep] = []

mesonbuild/compilers/rust.py

@@ -81,6 +81,19 @@ def __init__(self, exelist: T.List[str], version: str, for_machine: MachineChoic
         if 'link' in self.linker.id:
             self.base_options.add(OptionKey('b_vscrt'))
         self.native_static_libs: T.List[str] = []
+        # Resolve the real rustc executable. When using rustup, "rustc" in PATH
+        # is a wrapper that won't change when updating the toolchain, which
+        # means ninja would not rebuild rust targets after "rustup update". That
+        # can cause build issues because different rustc versions are generally
+        # uncompatible. This also means that once a Meson project has been
+        # configured, changing the default toolchain with e.g.
+        # "rustup default nightly" won't have any effect.
+        sysroot = self.get_sysroot()
+        real_rustc = os.path.join(sysroot, 'bin', 'rustc')
+        if os.path.exists(real_rustc):
+            exelist = [real_rustc] + exelist[1:]
+            self.exelist = exelist
+            self.exelist_no_ccache = exelist
 
     def needs_static_linker(self) -> bool:
         return False