Merge from origin/emacs-27

4b9fbdb5a7 ; Update TODO item about ligature support
03d44acfdd * doc/lispref/control.texi (Processing of Errors): Improve...
b48ab743a8 Minor fixups for mutability doc
6ac2326e5b Don’t use “constant” for values you shouldn’t change

9 files changed, 116 insertions, 89 deletions

diff --git a/doc/lispintro/emacs-lisp-intro.texi b/doc/lispintro/emacs-lisp-intro.texi
index 7484ce57607..f6dd77a3d96 100644
--- a/doc/lispintro/emacs-lisp-intro.texi
+++ b/doc/lispintro/emacs-lisp-intro.texi
@@ -7317,8 +7317,6 @@ which leave the original list as it was.  One way to find out how this
 works is to experiment.  We will start with the @code{setcar} function.
 
 @need 1200
-@cindex constant lists
-@cindex mutable lists
 First, we can make a list and then set the value of a variable to the
 list, using the @code{setq} special form.  Because we intend to use
 @code{setcar} to change the list, this @code{setq} should not use the
@@ -7327,8 +7325,7 @@ a list that is part of the program and bad things could happen if we
 tried to change part of the program while running it.  Generally
 speaking an Emacs Lisp program's components should be constant (or
 unchanged) while the program is running.  So we instead construct an
-animal list that is @dfn{mutable} (or changeable) by using the
-@code{list} function, as follows:
+animal list by using the @code{list} function, as follows:
 
 @smallexample
 (setq animals (list 'antelope 'giraffe 'lion 'tiger))
diff --git a/doc/lispref/control.texi b/doc/lispref/control.texi
index c601e3af9bc..7755cbb5f25 100644
--- a/doc/lispref/control.texi
+++ b/doc/lispref/control.texi
@@ -1867,6 +1867,7 @@ concept of continuable errors.
 @node Processing of Errors
 @subsubsection How Emacs Processes Errors
 @cindex processing of errors
+@cindex handle errors
 
 When an error is signaled, @code{signal} searches for an active
 @dfn{handler} for the error.  A handler is a sequence of Lisp
diff --git a/doc/lispref/elisp.texi b/doc/lispref/elisp.texi
index bba1b63115f..9a6796790c4 100644
--- a/doc/lispref/elisp.texi
+++ b/doc/lispref/elisp.texi
@@ -297,7 +297,7 @@ Lisp Data Types
 * Circular Objects::        Read syntax for circular structure.
 * Type Predicates::         Tests related to types.
 * Equality Predicates::     Tests of equality between any two objects.
-* Constants and Mutability::  Whether an object's value can change.
+* Mutability::              Some objects should not be modified.
 
 Programming Types
 
diff --git a/doc/lispref/eval.texi b/doc/lispref/eval.texi
index baddce4d9c9..39f342a798b 100644
--- a/doc/lispref/eval.texi
+++ b/doc/lispref/eval.texi
@@ -158,11 +158,11 @@ contents unchanged.
 @end group
 @end example
 
-  A self-evaluating form yields constant conses, vectors and strings, and you
-should not attempt to modify their contents via @code{setcar}, @code{aset} or
+  A self-evaluating form yields a value that becomes part of the program,
+and you should not try to modify it via @code{setcar}, @code{aset} or
 similar operations.  The Lisp interpreter might unify the constants
 yielded by your program's self-evaluating forms, so that these
-constants might share structure.  @xref{Constants and Mutability}.
+constants might share structure.  @xref{Mutability}.
 
   It is common to write numbers, characters, strings, and even vectors
 in Lisp code, taking advantage of the fact that they self-evaluate.
@@ -564,8 +564,8 @@ and vectors.)
 
 @defspec quote object
 This special form returns @var{object}, without evaluating it.
-The returned value is a constant, and should not be modified.
-@xref{Constants and Mutability}.
+The returned value might be shared and should not be modified.
+@xref{Self-Evaluating Forms}.
 @end defspec
 
 @cindex @samp{'} for quoting
@@ -608,9 +608,9 @@ Here are some examples of expressions that use @code{quote}:
 
   Although the expressions @code{(list '+ 1 2)} and @code{'(+ 1 2)}
 both yield lists equal to @code{(+ 1 2)}, the former yields a
-freshly-minted mutable list whereas the latter yields a constant list
-built from conses that may be shared with other constants.
-@xref{Constants and Mutability}.
+freshly-minted mutable list whereas the latter yields a list
+built from conses that might be shared and should not be modified.
+@xref{Self-Evaluating Forms}.
 
   Other quoting constructs include @code{function} (@pxref{Anonymous
 Functions}), which causes an anonymous lambda expression written in Lisp
@@ -710,8 +710,9 @@ Here are some examples:
 @end example
 
 If a subexpression of a backquote construct has no substitutions or
-splices, it acts like @code{quote} in that it yields constant conses,
-vectors and strings that should not be modified.
+splices, it acts like @code{quote} in that it yields conses,
+vectors and strings that might be shared and should not be modified.
+@xref{Self-Evaluating Forms}.
 
 @node Eval
 @section Eval
diff --git a/doc/lispref/lists.texi b/doc/lispref/lists.texi
index fcaf4386b15..ae793d5e15e 100644
--- a/doc/lispref/lists.texi
+++ b/doc/lispref/lists.texi
@@ -873,8 +873,8 @@ primitives @code{setcar} and @code{setcdr}.  These are destructive
 operations because they change existing list structure.
 Destructive operations should be applied only to mutable lists,
 that is, lists constructed via @code{cons}, @code{list} or similar
-operations.  Lists created by quoting are constants and should not be
-changed by destructive operations.  @xref{Constants and Mutability}.
+operations.  Lists created by quoting are part of the program and
+should not be changed by destructive operations.  @xref{Mutability}.
 
 @cindex CL note---@code{rplaca} vs @code{setcar}
 @quotation
@@ -911,7 +911,7 @@ value @var{object}.  For example:
 
 @example
 @group
-(setq x (list 1 2))  ; @r{Create a mutable list.}
+(setq x (list 1 2))
      @result{} (1 2)
 @end group
 @group
@@ -931,7 +931,7 @@ these lists.  Here is an example:
 
 @example
 @group
-;; @r{Create two mutable lists that are partly shared.}
+;; @r{Create two lists that are partly shared.}
 (setq x1 (list 'a 'b 'c))
      @result{} (a b c)
 (setq x2 (cons 'z (cdr x1)))
@@ -1022,11 +1022,11 @@ reached via the @sc{cdr}.
 
 @example
 @group
-(setq x (list 1 2 3))  ; @r{Create a mutable list.}
+(setq x (list 1 2 3))
      @result{} (1 2 3)
 @end group
 @group
-(setcdr x '(4))  ; @r{Modify the list's tail to be a constant list.}
+(setcdr x '(4))
      @result{} (4)
 @end group
 @group
@@ -1135,11 +1135,11 @@ Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are
 
 @example
 @group
-(setq x (list 1 2 3))  ; @r{Create a mutable list.}
+(setq x (list 1 2 3))
      @result{} (1 2 3)
 @end group
 @group
-(nconc x '(4 5))  ; @r{Modify the list's tail to be a constant list.}
+(nconc x '(4 5))
      @result{} (1 2 3 4 5)
 @end group
 @group
diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index cd037d663da..83066744121 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -46,10 +46,6 @@ you store in it, type and all.  (Actually, a small number of Emacs
 Lisp variables can only take on values of a certain type.
 @xref{Variables with Restricted Values}.)
 
-  Some Lisp objects are @dfn{constant}: their values should never change.
-Others are @dfn{mutable}: their values can be changed via destructive
-operations that involve side effects.
-
   This chapter describes the purpose, printed representation, and read
 syntax of each of the standard types in GNU Emacs Lisp.  Details on how
 to use these types can be found in later chapters.
@@ -63,7 +59,7 @@ to use these types can be found in later chapters.
 * Circular Objects::            Read syntax for circular structure.
 * Type Predicates::             Tests related to types.
 * Equality Predicates::         Tests of equality between any two objects.
-* Constants and Mutability::    Whether an object's value can change.
+* Mutability::                  Some objects should not be modified.
 @end menu
 
 @node Printed Representation
@@ -2383,51 +2379,58 @@ that for two strings to be equal, they have the same text properties.
 @end example
 @end defun
 
-@node Constants and Mutability
-@section Constants and Mutability
-@cindex constants
+@node Mutability
+@section Mutability
 @cindex mutable objects
 
-  Some Lisp objects are constant: their values should never change
-during a single execution of Emacs running well-behaved Lisp code.
-For example, you can create a new integer by calculating one, but you
-cannot modify the value of an existing integer.
-
-  Other Lisp objects are mutable: it is safe to change their values
-via destructive operations involving side effects.  For example, an
-existing marker can be changed by moving the marker to point to
-somewhere else.
-
-  Although all numbers are constants and all markers are
-mutable, some types contain both constant and mutable members.  These
-types include conses, vectors, strings, and symbols.  For example, the string
-literal @code{"aaa"} yields a constant string, whereas the function
-call @code{(make-string 3 ?a)} yields a mutable string that can be
+  Some Lisp objects should never change.  For example, the Lisp
+expression @code{"aaa"} yields a string, but you should not change
+its contents.  And some objects cannot be changed; for example,
+although you can create a new number by calculating one, Lisp provides
+no operation to change the value of an existing number.
+
+  Other Lisp objects are @dfn{mutable}: it is safe to change their
+values via destructive operations involving side effects.  For
+example, an existing marker can be changed by moving the marker to
+point to somewhere else.
+
+  Although numbers never change and all markers are mutable,
+some types have members some of which are mutable and others not.  These
+types include conses, vectors, and strings.  For example,
+although @code{"cons"} and @code{(symbol-name 'cons)} both yield
+strings that should not be changed, @code{(copy-sequence "cons")} and
+@code{(make-string 3 ?a)} both yield mutable strings that can be
 changed via later calls to @code{aset}.
 
-  A mutable object can become constant if it is part of an expression
-that is evaluated.  The reverse does not occur: constant objects
-should stay constant.
-
-  Trying to modify a constant variable signals an error
-(@pxref{Constant Variables}).
-A program should not attempt to modify other types of constants because the
-resulting behavior is undefined: the Lisp interpreter might or might
-not detect the error, and if it does not detect the error the
-interpreter can behave unpredictably thereafter.  Another way to put
-this is that although mutable objects are safe to change and constant
-variables reliably prevent attempts to change them, other constants
-are not safely mutable: if a misbehaving program tries to change such a
-constant then the constant's value might actually change, or the
-program might crash or worse.  This problem occurs
-with types that have both constant and mutable members, and that have
-mutators like @code{setcar} and @code{aset} that are valid on mutable
-objects but hazardous on constants.
-
-  When the same constant occurs multiple times in a program, the Lisp
+  A mutable object stops being mutable if it is part of an expression
+that is evaluated.  For example:
+
+@example
+(let* ((x (list 0.5))
+       (y (eval (list 'quote x))))
+  (setcar x 1.5) ;; The program should not do this.
+  y)
+@end example
+
+@noindent
+Although the list @code{(0.5)} was mutable when it was created, it should not
+have been changed via @code{setcar} because it given to @code{eval}.  The
+reverse does not occur: an object that should not be changed never
+becomes mutable afterwards.
+
+  If a program attempts to change objects that should not be
+changed, the resulting behavior is undefined: the Lisp interpreter
+might signal an error, or it might crash or behave unpredictably in
+other ways.@footnote{This is the behavior specified for languages like
+Common Lisp and C for constants, and this differs from languages like
+JavaScript and Python where an interpreter is required to signal an
+error if a program attempts to change an immutable object.  Ideally the Emacs
+Lisp interpreter will evolve in latter direction.}
+
+  When similar constants occur as parts of a program, the Lisp
 interpreter might save time or space by reusing existing constants or
-constant components.  For example, @code{(eq "abc" "abc")} returns
+their components.  For example, @code{(eq "abc" "abc")} returns
 @code{t} if the interpreter creates only one instance of the string
-constant @code{"abc"}, and returns @code{nil} if it creates two
+literal @code{"abc"}, and returns @code{nil} if it creates two
 instances.  Lisp programs should be written so that they work
 regardless of whether this optimization is in use.
diff --git a/doc/lispref/sequences.texi b/doc/lispref/sequences.texi
index 1cb0d05cc7b..91c3049f875 100644
--- a/doc/lispref/sequences.texi
+++ b/doc/lispref/sequences.texi
@@ -183,11 +183,11 @@ for other ways to copy sequences.
 
 @example
 @group
-(setq bar (list 1 2))  ; @r{Create a mutable list.}
+(setq bar (list 1 2))
      @result{} (1 2)
 @end group
 @group
-(setq x (vector 'foo bar))  ; @r{Create a mutable vector.}
+(setq x (vector 'foo bar))
      @result{} [foo (1 2)]
 @end group
 @group
@@ -278,7 +278,7 @@ Unlike @code{reverse} the original @var{sequence} may be modified.
 
 @example
 @group
-(setq x (list 'a 'b 'c))  ; @r{Create a mutable list.}
+(setq x (list 'a 'b 'c))
      @result{} (a b c)
 @end group
 @group
@@ -320,7 +320,7 @@ presented graphically:
   For the vector, it is even simpler because you don't need setq:
 
 @example
-(setq x (copy-sequence [1 2 3 4]))  ; @r{Create a mutable vector.}
+(setq x (copy-sequence [1 2 3 4]))
      @result{} [1 2 3 4]
 (nreverse x)
      @result{} [4 3 2 1]
@@ -331,6 +331,7 @@ x
 Note that unlike @code{reverse}, this function doesn't work with strings.
 Although you can alter string data by using @code{aset}, it is strongly
 encouraged to treat strings as immutable even when they are mutable.
+@xref{Mutability}.
 
 @end defun
 
@@ -374,7 +375,7 @@ appears in a different position in the list due to the change of
 
 @example
 @group
-(setq nums (list 1 3 2 6 5 4 0))  ; @r{Create a mutable list.}
+(setq nums (list 1 3 2 6 5 4 0))
      @result{} (1 3 2 6 5 4 0)
 @end group
 @group
@@ -1228,7 +1229,7 @@ This function sets the @var{index}th element of @var{array} to be
 
 @example
 @group
-(setq w (vector 'foo 'bar 'baz))  ; @r{Create a mutable vector.}
+(setq w (vector 'foo 'bar 'baz))
      @result{} [foo bar baz]
 (aset w 0 'fu)
      @result{} fu
@@ -1237,7 +1238,7 @@ w
 @end group
 
 @group
-;; @r{@code{copy-sequence} creates a mutable string.}
+;; @r{@code{copy-sequence} copies the string to be modified later.}
 (setq x (copy-sequence "asdfasfd"))
      @result{} "asdfasfd"
 (aset x 3 ?Z)
@@ -1247,9 +1248,7 @@ x
 @end group
 @end example
 
-The @var{array} should be mutable; that is, it should not be a constant,
-such as the constants created via quoting or via self-evaluating forms.
-@xref{Constants and Mutability}.
+The @var{array} should be mutable.  @xref{Mutability}.
 
 If @var{array} is a string and @var{object} is not a character, a
 @code{wrong-type-argument} error results.  The function converts a
@@ -1262,7 +1261,6 @@ each element of @var{array} is @var{object}.  It returns @var{array}.
 
 @example
 @group
-;; @r{Create a mutable vector and then fill it with zeros.}
 (setq a (copy-sequence [a b c d e f g]))
      @result{} [a b c d e f g]
 (fillarray a 0)
@@ -1271,7 +1269,6 @@ a
      @result{} [0 0 0 0 0 0 0]
 @end group
 @group
-;; @r{Create a mutable string and then fill it with "-".}
 (setq s (copy-sequence "When in the course"))
      @result{} "When in the course"
 (fillarray s ?-)
@@ -1310,8 +1307,8 @@ same way in Lisp input.
 evaluation: the result of evaluating it is the same vector.  This does
 not evaluate or even examine the elements of the vector.
 @xref{Self-Evaluating Forms}.  Vectors written with square brackets
-are constants and should not be modified via @code{aset} or other
-destructive operations.  @xref{Constants and Mutability}.
+should not be modified via @code{aset} or other destructive
+operations.  @xref{Mutability}.
 
   Here are examples illustrating these principles:
 
diff --git a/doc/lispref/strings.texi b/doc/lispref/strings.texi
index a4c9c2549c5..70c3b3cf4be 100644
--- a/doc/lispref/strings.texi
+++ b/doc/lispref/strings.texi
@@ -49,10 +49,9 @@ by a distinguished character code.
 
   Since strings are arrays, and therefore sequences as well, you can
 operate on them with the general array and sequence functions documented
-in @ref{Sequences Arrays Vectors}.  For example, you can access or
-change individual characters in a string using the functions @code{aref}
-and @code{aset} (@pxref{Array Functions}).  However, you should not
-try to change the contents of constant strings (@pxref{Modifying Strings}).
+in @ref{Sequences Arrays Vectors}.  For example, you can access
+individual characters in a string using the function @code{aref}
+(@pxref{Array Functions}).
 
   There are two text representations for non-@acronym{ASCII}
 characters in Emacs strings (and in buffers): unibyte and multibyte.
@@ -382,9 +381,7 @@ usual value is @w{@code{"[ \f\t\n\r\v]+"}}.
 @cindex string modification
 
   You can alter the contents of a mutable string via operations
-described in this section.  However, you should not try to use these
-operations to alter the contents of a constant string.
-@xref{Constants and Mutability}.
+described in this section.  @xref{Mutability}.
 
   The most basic way to alter the contents of an existing string is with
 @code{aset} (@pxref{Array Functions}).  @code{(aset @var{string}
diff --git a/etc/TODO b/etc/TODO
index 20262a77e97..f983fa27d33 100644
--- a/etc/TODO
+++ b/etc/TODO
@@ -264,6 +264,37 @@ of the mode line.
 The prettify-symbols-mode should be deprecated once ligature support
 is in place.
 
+A related, but somewhat independent, feature is being able to move the
+cursor "into a ligature", whereby cursor motion commands shows some
+pseudo-cursor on some part of a ligature.  For example, if "ffi" is
+displayed as a ligature, then moving by one buffer position should
+show the middle part of the ligature's glyph similar to the cursor
+display: some special background and perhaps also a special
+foreground.  There are two possible ways of figuring out the offset at
+which to display the pseudo-cursor:
+
+  . Arbitrarily divide the ligature's glyph width W into N parts,
+    where N is the number of codepoints composed into the ligature, then
+    move that pseudo-cursor by W/N pixels each time a cursor-motion
+    command is invoked;
+  . Use the font information.  For example, HarfBuzz has the
+    hb_ot_layout_get_ligature_carets API for that purpose.  However,
+    it could be that few fonts actually have that information recorded
+    in them, in which case the previous heuristics will be needed as
+    fallback.
+
+One subtle issue needs to be resolved to have this feature of
+"sub-glyph" cursor movement inside composed characters.  The way Emacs
+currently displays the default block cursor is by simply redrawing the
+glyph at point in reverse video.  So Emacs currently doesn't have a
+way of displaying a cursor that "covers" only part of a glyph.  To
+make this happen, the display code will probably need to be changed to
+draw the cursor as part of drawing the foreground and/or background of
+the corresponding glyph, which is against the current flow of the
+display code: it generally first completely draws the background and
+foreground of the entire text that needs to be redrawn, and only then
+draws the cursor where it should be placed.
+
 ** Support for Stylistic Sets
 This will allow using "alternate glyphs" supported by modern fonts.
 For an overview of this feature, see