return dummy cc and friends during dry runs

Some targets are added to these hashmaps at runtime, and are not present
during dry runs. To avoid errors, this commit changes all the related
functions to always return empty strings/paths during dry runs.

Author: pietroalbini

src/bootstrap/builder.rs

@@ -1650,7 +1650,7 @@ impl<'a> Builder<'a> {
             }
         };
         cargo.env(profile_var("DEBUG"), debuginfo_level.to_string());
-        if self.cc.borrow()[&target].args().iter().any(|arg| arg == "-gz") {
+        if !self.config.dry_run() && self.cc.borrow()[&target].args().iter().any(|arg| arg == "-gz") {
             rustflags.arg("-Clink-arg=-gz");
         }
         cargo.env(

src/bootstrap/compile.rs

@@ -1272,6 +1272,9 @@ pub fn compiler_file(
     c: CLang,
     file: &str,
 ) -> PathBuf {
+    if builder.config.dry_run() {
+        return PathBuf::new();
+    }
     let mut cmd = Command::new(compiler);
     cmd.args(builder.cflags(target, GitRepo::Rustc, c));
     cmd.arg(format!("-print-file-name={}", file));

src/bootstrap/lib.rs

@@ -1104,12 +1104,18 @@ impl Build {
 
     /// Returns the path to the C compiler for the target specified.
     fn cc(&self, target: TargetSelection) -> PathBuf {
+        if self.config.dry_run() {
+            return PathBuf::new();
+        }
         self.cc.borrow()[&target].path().into()
     }
 
     /// Returns a list of flags to pass to the C compiler for the target
     /// specified.
     fn cflags(&self, target: TargetSelection, which: GitRepo, c: CLang) -> Vec<String> {
+        if self.config.dry_run() {
+            return Vec::new();
+        }
         let base = match c {
             CLang::C => self.cc.borrow()[&target].clone(),
             CLang::Cxx => self.cxx.borrow()[&target].clone(),
@@ -1154,16 +1160,25 @@ impl Build {
 
     /// Returns the path to the `ar` archive utility for the target specified.
     fn ar(&self, target: TargetSelection) -> Option<PathBuf> {
+        if self.config.dry_run() {
+            return None;
+        }
         self.ar.borrow().get(&target).cloned()
     }
 
     /// Returns the path to the `ranlib` utility for the target specified.
     fn ranlib(&self, target: TargetSelection) -> Option<PathBuf> {
+        if self.config.dry_run() {
+            return None;
+        }
         self.ranlib.borrow().get(&target).cloned()
     }
 
     /// Returns the path to the C++ compiler for the target specified.
     fn cxx(&self, target: TargetSelection) -> Result<PathBuf, String> {
+        if self.config.dry_run() {
+            return Ok(PathBuf::new());
+        }
         match self.cxx.borrow().get(&target) {
             Some(p) => Ok(p.path().into()),
             None => {
@@ -1174,6 +1189,9 @@ impl Build {
 
     /// Returns the path to the linker for the given target if it needs to be overridden.
     fn linker(&self, target: TargetSelection) -> Option<PathBuf> {
+        if self.config.dry_run() {
+            return Some(PathBuf::new());
+        }
         if let Some(linker) = self.config.target_config.get(&target).and_then(|c| c.linker.clone())
         {
             Some(linker)