cc: use '-x c*-header' for header files for C and C++ linters. (#4318)

When linting an header file in C or C++, `-x c-header` or
`-x c++-header` should be used instead of `-x c` or `-x c++`.

Using `-x c` or `-x c++` for headers files can lead to unused variables
and functions marked as static inlined as seen in #4096.

Using `-x c-header` or `-x c++-header` solve these issues.

The list of file extensions that are considered as header files can be
configured with the variables `g:ale_c_cc_header_exts` and
`g:ale_cpp_cc_header_exts`.

7 files changed, 73 insertions, 2 deletions

diff --git a/ale_linters/c/cc.vim b/ale_linters/c/cc.vim
index 5655fbf7..d0215041 100644
--- a/ale_linters/c/cc.vim
+++ b/ale_linters/c/cc.vim
@@ -3,6 +3,7 @@
 
 call ale#Set('c_cc_executable', '<auto>')
 call ale#Set('c_cc_options', '-std=c11 -Wall')
+call ale#Set('c_cc_header_exts', ['h'])
 
 function! ale_linters#c#cc#GetExecutable(buffer) abort
     let l:executable = ale#Var(a:buffer, 'c_cc_executable')
@@ -22,6 +23,8 @@ endfunction
 function! ale_linters#c#cc#GetCommand(buffer, output) abort
     let l:cflags = ale#c#GetCFlags(a:buffer, a:output)
     let l:ale_flags = ale#Var(a:buffer, 'c_cc_options')
+    let l:header_exts = ale#Var(a:buffer, 'c_cc_header_exts')
+    let l:lang_flag = ale#c#GetLanguageFlag(a:buffer, l:header_exts, 'c')
 
     if l:cflags =~# '-std='
         let l:ale_flags = substitute(
@@ -36,7 +39,7 @@ function! ale_linters#c#cc#GetCommand(buffer, output) abort
     "
     " `-o /dev/null` or `-o null` is needed to catch all errors,
     " -fsyntax-only doesn't catch everything.
-    return '%e -S -x c'
+    return '%e -S -x ' . l:lang_flag
     \   . ' -o ' . g:ale#util#nul_file
     \   . ' -iquote %s:h'
     \   . ale#Pad(l:cflags)
diff --git a/ale_linters/cpp/cc.vim b/ale_linters/cpp/cc.vim
index ffb8f068..f8021531 100644
--- a/ale_linters/cpp/cc.vim
+++ b/ale_linters/cpp/cc.vim
@@ -3,6 +3,7 @@
 
 call ale#Set('cpp_cc_executable', '<auto>')
 call ale#Set('cpp_cc_options', '-std=c++14 -Wall')
+call ale#Set('cpp_cc_header_exts', ['h', 'hpp'])
 
 function! ale_linters#cpp#cc#GetExecutable(buffer) abort
     let l:executable = ale#Var(a:buffer, 'cpp_cc_executable')
@@ -22,6 +23,8 @@ endfunction
 function! ale_linters#cpp#cc#GetCommand(buffer, output) abort
     let l:cflags = ale#c#GetCFlags(a:buffer, a:output)
     let l:ale_flags = ale#Var(a:buffer, 'cpp_cc_options')
+    let l:header_exts = ale#Var(a:buffer, 'cpp_cc_header_exts')
+    let l:lang_flag = ale#c#GetLanguageFlag(a:buffer, l:header_exts, 'c++')
 
     if l:cflags =~# '-std='
         let l:ale_flags = substitute(
@@ -36,7 +39,7 @@ function! ale_linters#cpp#cc#GetCommand(buffer, output) abort
     "
     " `-o /dev/null` or `-o null` is needed to catch all errors,
     " -fsyntax-only doesn't catch everything.
-    return '%e -S -x c++'
+    return '%e -S -x ' . l:lang_flag
     \   . ' -o ' . g:ale#util#nul_file
     \   . ' -iquote %s:h'
     \   . ale#Pad(l:cflags)
diff --git a/autoload/ale/c.vim b/autoload/ale/c.vim
index e729aec8..4a9b80fb 100644
--- a/autoload/ale/c.vim
+++ b/autoload/ale/c.vim
@@ -585,3 +585,14 @@ function! ale#c#IncludeOptions(include_paths) abort
 
     return join(l:option_list)
 endfunction
+
+" Get the language flag depending on if the file is a header or not.
+function! ale#c#GetLanguageFlag(buffer, header_exts, linter_lang) abort
+    let l:buf_ext = expand('#' . a:buffer . ':e')
+
+    if index(a:header_exts, l:buf_ext) >= 0
+        return a:linter_lang . '-header'
+    endif
+
+    return a:linter_lang
+endfunction
diff --git a/doc/ale-c.txt b/doc/ale-c.txt
index bd8c4937..c71a81de 100644
--- a/doc/ale-c.txt
+++ b/doc/ale-c.txt
@@ -136,6 +136,17 @@ g:ale_c_cc_options                                         *g:ale_c_cc_options*
   This variable can be change to modify flags given to the C compiler.
 
 
+g:ale_c_cc_header_exts                                 *g:ale_c_cc_header_exts*
+                                                       *b:ale_c_cc_header_exts*
+  Type: |List|
+  Default: `['h']`
+
+  This variable can be change to modify the list of extensions of the files
+  considered as header files.
+
+  ALE will use `'-x c-header'` instead of `'-x c'` for header files.
+
+
 ===============================================================================
 ccls                                                               *ale-c-ccls*
 
diff --git a/doc/ale-cpp.txt b/doc/ale-cpp.txt
index 1f079205..a09bc021 100644
--- a/doc/ale-cpp.txt
+++ b/doc/ale-cpp.txt
@@ -65,6 +65,17 @@ g:ale_cpp_cc_options                                     *g:ale_cpp_cc_options*
   This variable can be change to modify flags given to the C++ compiler.
 
 
+g:ale_cpp_cc_header_exts                             *g:ale_cpp_cc_header_exts*
+                                                     *b:ale_cpp_cc_header_exts*
+  Type: |List|
+  Default: `['h', 'hpp']`
+
+  This variable can be change to modify the list of extensions of the files
+  considered as header files.
+
+  ALE will use `'-x c++-header'` instead of `'-x c++'` for header files.
+
+
 ===============================================================================
 ccls                                                             *ale-cpp-ccls*
 
diff --git a/test/linter/test_c_cc.vader b/test/linter/test_c_cc.vader
index c8c2de7d..953aa238 100644
--- a/test/linter/test_c_cc.vader
+++ b/test/linter/test_c_cc.vader
@@ -53,3 +53,19 @@ Execute(The -std flag should be replaced by parsed C flags):
   let g:get_cflags_return_value = '-std=c99'
 
   AssertLinter 'gcc', ale#Escape('gcc') . b:command_tail
+
+Execute(The header files should use -x c-header):
+  let b:command_tail = substitute(b:command_tail, '-x c', '-x c-header', '')
+
+  call ale#test#SetFilename('../test-files/c/makefile_project/subdir/test.h')
+
+  AssertLinter 'gcc', ale#Escape('gcc') . b:command_tail
+
+Execute(The header file extensions should be configurable):
+  let b:command_tail = substitute(b:command_tail, '-x c', '-x c-header', '')
+
+  call ale#assert#SetUpLinterTest('c', 'cc')
+  let b:ale_c_cc_header_exts = ['json']
+  call ale#test#SetFilename('../test-files/c/json_project/build/compile_commands.json')
+
+  AssertLinter 'gcc', ale#Escape('gcc') . b:command_tail
diff --git a/test/linter/test_cpp_cc.vader b/test/linter/test_cpp_cc.vader
index dec3a07c..44fe4521 100644
--- a/test/linter/test_cpp_cc.vader
+++ b/test/linter/test_cpp_cc.vader
@@ -53,3 +53,19 @@ Execute(The -std flag should be replaced by parsed C flags):
   let g:get_cflags_return_value = '-std=c++11'
 
   AssertLinter 'gcc', ale#Escape('gcc') . b:command_tail
+
+Execute(The header files should use -x c++-header):
+  let b:command_tail = substitute(b:command_tail, '-x c++', '-x c++-header', '')
+
+  call ale#test#SetFilename('../test-files/c/hpp_file_project/test.hpp')
+
+  AssertLinter 'gcc', ale#Escape('gcc') . b:command_tail
+
+Execute(The header file extensions should be configurable):
+  let b:command_tail = substitute(b:command_tail, '-x c++', '-x c++-header', '')
+
+  call ale#assert#SetUpLinterTest('cpp', 'cc')
+  let b:ale_cpp_cc_header_exts = ['json']
+  call ale#test#SetFilename('../test-files/c/json_project/build/compile_commands.json')
+
+  AssertLinter 'gcc', ale#Escape('gcc') . b:command_tail