Some documentation language adjustments & improved error messages

docs/markdown/Syntax.md

@@ -198,7 +198,15 @@ s = f'int: @n@, string: @m@'
 ```
 
 Currently only identity-expressions are supported inside of format
-strings.
+strings, meaning you cannot use arbitrary Meson expressions inside of them.
+
+```meson
+n = 10
+m = 5
+
+# The following is not a valid format string
+s = f'result: @n + m@'
+```
 
 ### String methods
 

docs/markdown/snippets/fstrings.md

@@ -1,7 +1,7 @@
 ## Introducing format strings to the Meson language
 
 In addition to the conventional `'A string @0@ to be formatted @1@'.format(n, m)`
-method of formatting strings in the Meson DSL, there's now the additional
+method of formatting strings in the Meson language, there's now the additional
 `f'A string @n@ to be formatted @m@'` notation that provides a non-positional
 and clearer alternative. Meson's format strings are currently restricted to
 identity-expressions, meaning `f'format @'m' + 'e'@'` will not parse.

mesonbuild/interpreterbase.py

@@ -937,7 +937,8 @@ The result of this is undefined and will become a hard error in a future Meson r
             try:
                 val = self.variables[var]
                 if not isinstance(val, (str, int, float, bool)):
-                    raise InvalidCode(f'Identifier "{var}" does not name a formattable variable.')
+                    raise InvalidCode(f'Identifier "{var}" does not name a formattable variable ' +
+                        '(has to be an integer, a string, a floating point number or a boolean).')
 
                 return str(val)
             except KeyError:

test cases/failing/113 invalid fstring/test.json

@@ -1,7 +1,7 @@
 {
     "stdout": [
         {
-            "line": "test cases/failing/113 invalid fstring/meson.build:4:0: ERROR: Identifier \"dict\" does not name a formattable variable."
+            "line": "test cases/failing/113 invalid fstring/meson.build:4:0: ERROR: Identifier \"dict\" does not name a formattable variable (has to be an integer, a string, a floating point number or a boolean)."
         }
     ]
 }