How to create a custom decorator symbol in math mode (like tilde)

Question

I need some custom decorator symbols, like tilde in math mode.

The package MnSymbol has some neat symbols so I figured I would import these as suggested here:

documentclass{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
%usepackage{accents}
usepackage{bm}

% import certain symbols from MnSymbol,
% https://tex.stackexchange.com/questions/450691/create-diamond-with-a-dash-in-it/450703#450703
defImportFromMnSymbol#1{%
    DeclareFontFamily{U} {MnSymbol#1}{}
    DeclareFontShape{U}{MnSymbol#1}{m}{n}{ <-6> MnSymbol#15 <6-7> MnSymbol#16 <7-8> MnSymbol#17 <8-9> MnSymbol#18 <9-10> MnSymbol#19 <10-12> MnSymbol#110 <12-> MnSymbol#112 }{}
    DeclareFontShape{U}{MnSymbol#1}{b}{n}{ <-6> MnSymbol#1-Bold5 <6-7> MnSymbol#1-Bold6 <7-8> MnSymbol#1-Bold7 <8-9> MnSymbol#1-Bold8 <9-10> MnSymbol#1-Bold9 <10-12> MnSymbol#1-Bold10 <12-> MnSymbol#1-Bold12 }{}
    DeclareSymbolFont{MnSy#1} {U} {MnSymbol#1}{m}{n} }
newcommandDeclareMnSymbol[4]{DeclareMathSymbol{#1}{#2}{MnSy#3}{#4}}
ImportFromMnSymbol{C}
DeclareMnSymbol{smalltriangleup}{mathbin}{C}{73}

begin{document}

$
      tilde{f} 
quad tilde{bm{f}}
quad overset{smalltriangleup}{f}
quad stackrel{smalltriangleup}{f}
$

end{document}

My issue is that I’m clueless how to get the symbol positioned correctly:

The last attempt is in the right direction visually, but the method does not seem to be right, because when highlighting and copying the text from the pdf one gets a triangle symbol followed by an f, instead of a plain f.

So how is it done properly?
It is nice that Amsmath figures out how to position the tilde symbol pleasingly, is there a way to take advantage of that?

I remember the accents package being recommended, but with that package loaded not even the tilde is properly positioned any more:

accents math mode

egreg · Accepted Answer

Why not declaring the symbol as an accent?

documentclass{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
%usepackage{accents}
usepackage{bm}

% import certain symbols from MnSymbol,
% https://tex.stackexchange.com/questions/450691/create-diamond-with-a-dash-in-it/450703#450703
newcommandImportFromMnSymbol[1]{%
  DeclareFontFamily{U}{MnSymbol#1}{}
  DeclareFontShape{U}{MnSymbol#1}{m}{n}
    { <-6>    MnSymbol#15
      <6-7>   MnSymbol#16
      <7-8>   MnSymbol#17
      <8-9>   MnSymbol#18
      <9-10>  MnSymbol#19
      <10-12> MnSymbol#110
      <12->   MnSymbol#112
    }{}
  DeclareFontShape{U}{MnSymbol#1}{b}{n}
    { <-6>    MnSymbol#1-Bold5
      <6-7>   MnSymbol#1-Bold6
      <7-8>   MnSymbol#1-Bold7
      <8-9>   MnSymbol#1-Bold8
      <9-10>  MnSymbol#1-Bold9
      <10-12> MnSymbol#1-Bold10
      <12->   MnSymbol#1-Bold12
    }{}
  DeclareSymbolFont{MnSy#1}{U}{MnSymbol#1}{m}{n}
  SetSymbolFont{MnSy#1}{bold}{U}{MnSymbol#1}{b}{n}
}
newcommandDeclareMnSymbol[4]{DeclareMathSymbol{#1}{#2}{MnSy#3}{#4}}
newcommandDeclareMnAccent[4]{DeclareMathAccent{#1}{#2}{MnSy#3}{#4}}

ImportFromMnSymbol{C}
DeclareMnAccent{acctriangleup}{mathord}{C}{73}

begin{document}

$acctriangleup{f}+tilde{f}$

{boldmath$acctriangleup{f}+tilde{f}$}

end{document}

I reformatted the code for readability (I'm not sure about the intervals, though, but it's not a big deal) and added the setup for the bold math version.

Here's how you can scale down the requested font. I added some arguments in order to allow for loading the same font at different scale factors.

documentclass{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
%usepackage{accents}
usepackage{bm}

% import certain symbols from MnSymbol,
% https://tex.stackexchange.com/questions/450691/create-diamond-with-a-dash-in-it/450703#450703
begingroupmakeatletternfss@catcodes
gdefImportFromMnSymbolaux#1#2#3{%
  DeclareFontFamily{U}{MnSymbol#2#3}{}
  DeclareFontShape{U}{MnSymbol#2#3}{m}{n}
    { <-6>    s*[#1] MnSymbol#25
      <6-7>   s*[#1] MnSymbol#26
      <7-8>   s*[#1] MnSymbol#27
      <8-9>   s*[#1] MnSymbol#28
      <9-10>  s*[#1] MnSymbol#29
      <10-12> s*[#1] MnSymbol#210
      <12->   s*[#1] MnSymbol#212
    }{}
  DeclareFontShape{U}{MnSymbol#2#3}{b}{n}
    { <-6>    s*[#1] MnSymbol#2-Bold5
      <6-7>   s*[#1] MnSymbol#2-Bold6
      <7-8>   s*[#1] MnSymbol#2-Bold7
      <8-9>   s*[#1] MnSymbol#2-Bold8
      <9-10>  s*[#1] MnSymbol#2-Bold9
      <10-12> s*[#1] MnSymbol#2-Bold10
      <12->   s*[#1] MnSymbol#2-Bold12
    }{}
  DeclareSymbolFont{MnSy#2#3}{U}{MnSymbol#2#3}{m}{n}
  SetSymbolFont{MnSy#2#3}{bold}{U}{MnSymbol#2#3}{b}{n}
}
endgroup
newcommand{ImportFromMnSymbol}[3][1]{ImportFromMnSymbolaux{#1}{#2}{#3}}

newcommandDeclareMnSymbol[4]{DeclareMathSymbol{#1}{#2}{MnSy#3}{#4}}
newcommandDeclareMnAccent[4]{DeclareMathAccent{#1}{#2}{MnSy#3}{#4}}

ImportFromMnSymbol[0.7]{C}{70}
DeclareMnAccent{acctriangleup}{mathord}{C70}{73}

begin{document}

$acctriangleup{f}+tilde{f}$

{boldmath$acctriangleup{f}+tilde{f}$}

end{document}

Bastian · Answer

So, the accents package can be used and it provides a nice method for placing that smalltringleup-Symbol from my question obove the letter.
The two things one needs to bear in mind are:

bm{...} must be the outermost command enclosing tilde, etc.
There must be an extra set of braces inside bm if an accent is used: bm{{tilde{...}}} (Otherwise the compiling process will fail)

With usepackage{accents}:
    $
          tilde{f}
    quad tilde{bm{f}}
    quad bm{{tilde{f}}}
    quad accentset{smalltriangleup}{f}
    quad accentset{smalltriangleup}{bm{f}}
    quad bm{{accentset{smalltriangleup}{f}}}
    $

As campa pointed out in a comment below my question, one of the two remaining issues can be fixed with the single option, i.e., usepackage[single]{accents}:

How to create a custom decorator symbol in math mode (like tilde)

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