fix(path): path_is_url returns false for "foo:/" #19797

Problem:
path_to_url() returns false for single-slash URIs ("foo:/" vs "foo://").
This is not compliant with the URI spec. https://url.spec.whatwg.org/#url-representation
LSP in particular allows single-slash URIs.

Solution:
Relax path_to_url() to accept single-slash URIs. This is not fully
compliant (only ":" is required by the spec), but it is hopefully good
enough without causing false-positives in typical text files.

ref https://url.spec.whatwg.org/#windows-drive-letter
ref https://github.com/neovim/neovim/pull/19773
ref https://github.com/neovim/neovim/pull/19773#issuecomment-1214763769

3 files changed, 27 insertions, 5 deletions

diff --git a/src/nvim/path.c b/src/nvim/path.c
index d50ef87b62..ce0a7fb281 100644
--- a/src/nvim/path.c
+++ b/src/nvim/path.c
@@ -33,7 +33,7 @@
 #include "nvim/vim.h"
 #include "nvim/window.h"
 
-#define URL_SLASH       1               // path_is_url() has found "://"
+#define URL_SLASH       1               // path_is_url() has found ":/"
 #define URL_BACKSLASH   2               // path_is_url() has found ":\\"
 
 #ifdef gen_expand_wildcards
@@ -1751,12 +1751,26 @@ char_u *find_file_name_in_path(char_u *ptr, size_t len, int options, long count,
   return file_name;
 }
 
-// Check if the "://" of a URL is at the pointer, return URL_SLASH.
+/// Checks for a Windows drive letter ("C:/") at the start of the path.
+///
+/// @see https://url.spec.whatwg.org/#start-with-a-windows-drive-letter
+bool path_has_drive_letter(const char *p)
+  FUNC_ATTR_NONNULL_ALL
+{
+  return strlen(p) >= 2
+         && ASCII_ISALPHA(p[0])
+         && (p[1] == ':' || p[1] == '|')
+         && (strlen(p) == 2 || ((p[2] == '/') | (p[2] == '\\') | (p[2] == '?') | (p[2] == '#')));
+}
+
+// Check if the ":/" of a URL is at the pointer, return URL_SLASH.
 // Also check for ":\\", which MS Internet Explorer accepts, return
 // URL_BACKSLASH.
 int path_is_url(const char *p)
 {
-  if (strncmp(p, "://", 3) == 0) {
+  // In the spec ':' is enough to recognize a scheme
+  // https://url.spec.whatwg.org/#scheme-state
+  if (strncmp(p, ":/", 2) == 0) {
     return URL_SLASH;
   } else if (strncmp(p, ":\\\\", 3) == 0) {
     return URL_BACKSLASH;
@@ -1781,6 +1795,10 @@ int path_with_url(const char *fname)
     return 0;
   }
 
+  if (path_has_drive_letter(fname)) {
+    return 0;
+  }
+
   // check body: alpha or dash
   for (p = fname + 1; (isalpha(*p) || (*p == '-')); p++) {}
 
@@ -1789,7 +1807,7 @@ int path_with_url(const char *fname)
     return 0;
   }
 
-  // "://" or ":\\" must follow
+  // ":/" or ":\\" must follow
   return path_is_url(p);
 }
 
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 89611177f9..7ad5e49d2f 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -6582,7 +6582,7 @@ char_u *file_name_in_line(char_u *line, int col, int options, long count, char_u
 
   /*
    * Search forward for the last char of the file name.
-   * Also allow "://" when ':' is not in 'isfname'.
+   * Also allow ":/" when ':' is not in 'isfname'.
    */
   len = 0;
   while (vim_isfilec(ptr[len]) || (ptr[len] == '\\' && ptr[len + 1] == ' ')
diff --git a/test/unit/path_spec.lua b/test/unit/path_spec.lua
index fb476397e6..eb23a3cff1 100644
--- a/test/unit/path_spec.lua
+++ b/test/unit/path_spec.lua
@@ -640,6 +640,10 @@ describe('path.c', function()
       eq(2, path_with_url([[test-abc:\\xyz\foo\b3]]))
       eq(0, path_with_url([[-test://xyz/foo/b4]]))
       eq(0, path_with_url([[test-://xyz/foo/b5]]))
+      eq(1, path_with_url([[test-C:/xyz/foo/b5]]))
+      eq(1, path_with_url([[test-custom:/xyz/foo/b5]]))
+      eq(0, path_with_url([[c:/xyz/foo/b5]]))
+      eq(0, path_with_url([[C:/xyz/foo/b5]]))
     end)
   end)
 end)