bootstrap: fix bug preventing the use of custom targets

the bug was caused by two factors:
1. only checking the RUST_TARGET_PATH form, not the full filepath form
2. indirectly trying to use the Debug presentation to get the file path

Author: lolbinarycat

src/bootstrap/src/core/config/config.rs

@@ -518,17 +518,21 @@ impl TargetSelection {
 
 impl fmt::Display for TargetSelection {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.triple)?;
         if let Some(file) = self.file {
-            write!(f, "({file})")?;
+            write!(f, "{file}")
+        } else {
+            write!(f, "{}", self.triple)
         }
-        Ok(())
     }
 }
 
 impl fmt::Debug for TargetSelection {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{self}")
+        write!(f, "{}", self.triple)?;
+        if let Some(file) = self.file {
+            write!(f, "({file})")?;
+        }
+        Ok(())
     }
 }
 

src/bootstrap/src/core/sanity.rs

@@ -12,7 +12,7 @@ use std::collections::HashMap;
 use std::env;
 use std::ffi::{OsStr, OsString};
 use std::fs;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 #[cfg(not(feature = "bootstrap-self-test"))]
 use crate::builder::Builder;
@@ -262,7 +262,9 @@ than building it.
 
             if !has_target {
                 // This might also be a custom target, so check the target file that could have been specified by the user.
-                if let Some(custom_target_path) = env::var_os("RUST_TARGET_PATH") {
+                if Path::new(&target_str).exists() {
+                    has_target = true;
+                } else if let Some(custom_target_path) = env::var_os("RUST_TARGET_PATH") {
                     let mut target_filename = OsString::from(&target_str);
                     // Target filename ends with `.json`.
                     target_filename.push(".json");
@@ -277,8 +279,12 @@ than building it.
 
             if !has_target {
                 panic!(
-                    "No such target exists in the target list,
-                specify a correct location of the JSON specification file for custom targets!"
+                    "No such target exists in the target list,\n\
+                     make sure to correctly specify the location \
+                     of the JSON specification file \
+                     for custom targets!\n\
+                     Use BOOTSTRAP_SKIP_TARGET_SANITY=1 to \
+                     bypass this check."
                 );
             }
         }