Add Rust support to default_target_compile

This adds support for the Rust language to default_target_compile.
This comes from a gdb patch:

    commit 67218854b1987d89593ccaf5feaf5b29b1b976f2
    Author: Tom Tromey <tom@tromey.com>
    Date:   Tue Apr 26 19:38:43 2016 -0600

	Update gdb test suite for Rust
    [...]
	2016-05-17  Tom Tromey  <tom@tromey.com>
		    Manish Goregaokar <manishsmail@gmail.com>

4 files changed, 52 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index 216b4ec..b775c8f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2020-06-19  Tom Tromey  <tromey@adacore.com>
 
+	* doc/dejagnu.texi (target_compile procedure): Document rust
+	additions.
+	* lib/target.exp (default_target_compile): Handle rust.
+	* lib/libgloss.exp (find_rustc): New proc.
+
+2020-06-19  Tom Tromey  <tromey@adacore.com>
+
 	* lib/target.exp (default_target_compile): Add early_flags.
 	* doc/dejagnu.texi (target_compile procedure): Document
 	early_flags.
diff --git a/doc/dejagnu.texi b/doc/dejagnu.texi
index fd7f16f..5052497 100644
--- a/doc/dejagnu.texi
+++ b/doc/dejagnu.texi
@@ -4564,6 +4564,8 @@ Use a compiler for the D language.
 Use a compiler for Fortran 77.
 @item f90
 Use a compiler for Fortran 90.
+@item rust
+Use a compiler for Rust.
 @end table
 If none of these options are given, the C compiler is used by default.
 Giving multiple language-selection options is an error.
@@ -4670,6 +4672,9 @@ compiler will be used and the @code{compiler} option ignored.
 @item F90_FOR_TARGET
 Override Fortran 90 compiler.  If the @code{f90} option is given, this
 compiler will be used and the @code{compiler} option ignored.
+@item RUSTC_FOR_TARGET
+Override Rust compiler.  If the @code{rust} option is given, this
+compiler will be used and the @code{compiler} option ignored.
 @item GNATMAKE_FOR_TARGET
 Override Ada compiler.  If the @code{ada} option is given, this
 compiler will be used and the @code{compiler} option ignored.
diff --git a/lib/libgloss.exp b/lib/libgloss.exp
index 56a9728..522e864 100644
--- a/lib/libgloss.exp
+++ b/lib/libgloss.exp
@@ -765,6 +765,22 @@ proc find_gnatmake {} {
     return $CC
 }
 
+proc find_rustc {} {
+    global tool_root_dir
+    if {![is_remote host]} {
+	set rustc [lookfor_file $tool_root_dir rustc]
+	if {$rustc eq ""} {
+	    set rustc rustc
+	}
+    } else {
+	set rustc ""
+    }
+    if {$rustc ne ""} {
+	append rustc " --color never"
+    }
+    return $rustc
+}
+
 proc find_nm {} {
     global tool_root_dir
 
diff --git a/lib/target.exp b/lib/target.exp
index c98fbd0..132c1a3 100644
--- a/lib/target.exp
+++ b/lib/target.exp
@@ -400,6 +400,18 @@ proc default_target_compile {source destfile type options} {
 	    }
 	}
 
+	if { $i eq "rust" } {
+	    set compiler_type "rust"
+	    if {[board_info $dest exists rustflags]} {
+		append add_flags " [board_info $dest rustflags]"
+	    }
+	    if {[board_info $dest exists rustcompiler]} {
+		set compiler [board_info $dest rustcompiler]
+	    } else {
+		set compiler [find_rustc]
+	    }
+	}
+
 	if {[regexp "^dest=" $i]} {
 	    regsub "^dest=" $i "" tmp
 	    if {[board_info $tmp exists name]} {
@@ -460,6 +472,7 @@ proc default_target_compile {source destfile type options} {
     global F77_FOR_TARGET
     global F90_FOR_TARGET
     global GNATMAKE_FOR_TARGET
+    global RUSTC_FOR_TARGET
 
     if {[info exists GNATMAKE_FOR_TARGET]} {
 	if { $compiler_type eq "ada" } {
@@ -497,6 +510,12 @@ proc default_target_compile {source destfile type options} {
 	}
     }
 
+    if {[info exists RUSTC_FOR_TARGET]} {
+	if {$compiler_type eq "rust"} {
+	    set compiler $RUSTC_FOR_TARGET
+	}
+    }
+
     if { $compiler eq "" } {
 	if { [board_info $dest exists compiler] } {
 	    set compiler [board_info $dest compiler]
@@ -515,7 +534,11 @@ proc default_target_compile {source destfile type options} {
     }
 
     if {$type eq "object"} {
-	append add_flags " -c"
+	if {$compiler_type eq "rust"} {
+	    append add_flags "--emit obj"
+	} else {
+	    append add_flags " -c"
+	}
     }
 
     if { $type eq "preprocess" } {